Lake Sedimentary DNA Research on Past Terrestrial and Aquatic Biodiversity: Overview and Recommendations

Abstract

The use of lake sedimentary DNA to track the long-term changes in both terrestrial and aquatic biota is a rapidly advancing field in paleoecological research. Although largely applied nowadays, knowledge gaps remain in this field and there is therefore still research to be conducted to ensure the reliability of the sedimentary DNA signal. Building on the most recent literature and seven original case studies, we synthesize the state-of-the-art analytical procedures for effective sampling, extraction, amplification, quantification and/or generation of DNA inventories from sedimentary ancient DNA (sedaDNA) via high-throughput sequencing technologies. We provide recommendations based on current knowledge and best practises.

1. Tracking Past Ecological Changes from Lakes and Catchments with Sedimentary DNA

1.1. Sedimentary DNA, a Powerful Proxy to Track Past Biodiversity Changes

Lake sediments consist of both autochthonous (in-lake) and allochthonous (from the catchment and beyond) organic and inorganic matter. Paleoecological inquiries using biological archives stored in lake sediments have been largely dominated by microscopic analyses of the relatively limited number of aquatic and terrestrial groups that leave well-preserved and readily identifiable morphological remains in the sediment (e.g., silicified diatoms, calcified nannofossils, organic walled or calcified dinoflagellates, chrysophytes cysts, Cladocera remains, chironomid head capsules, fungal spores, pollen and plant macrofossils) [1,2,3,4,5,6,7,8,9], keeping the remaining biological diversity out of reach. Where these morphological remains are not well preserved or cannot be taxonomically identified to the species level, alternative proxies have been sought to build a more detailed and comprehensive understanding of past biological diversity in a broader range of environments [10,11]. In particular, the use of biomarkers, such as pigments and lipids, have emerged as reliable alternatives for the study of past ecosystems functioning [12]. However, their taxonomic specificity remains limited.

A more recent alternative that is now gaining in popularity is to target nucleic acids (DNA) preserved in the sediment archive. The sequencing of targeted genetic regions or the metagenome (all DNA present) extracted from sedimentary DNA can be used to reconstruct changes in biodiversity i.e., distribution of species based on the genetic diversity—over a range of temporal and spatio-ecological scales [13,14,15,16]. Depending on the state of degradation—affected by age, temperature and other environmental variables [17]—sedDNA can be regarded as sedimentary DNA (younger and better preserved) or as sedimentary ancient DNA (sedaDNA; often older, and more poorly preserved). For simplicity, we define sedaDNA as the fraction of environmental DNA buried in subsurface sediment that originates from organisms that are no longer physiologically active. To the best of our knowledge, the first study to report the presence of ancient DNA in lake sediments was performed by Coolen and Overmann [18], who demonstrated that Holocene changes in water stratification and euxinia (i.e., a water column that is both anoxic and sulfidic) can be inferred from downcore changes in sulfur-reducing bacteria, based on analyses of sedimentary 16S rRNA amplicons. Since then, the number of publications in the emerging field of sedaDNA has increased dramatically, particularly in the last five years (Figure 1, Table S1).

In recent years, sedaDNA has shown a great potential for reconstructing past biological assemblages within lakes and their terrestrial catchments (for review see [13,14,15,19,20,21,22]). Following extraction of sedaDNA, several analytical methods have been used (Table S1) including: (i) methods aiming to detect or quantify the occurrence of specific target organisms on the basis of endpoint PCR assays, quantitative real-time qPCR and special applications such as droplet digital ddPCR; (ii) DNA metabarcoding-based methods combining PCR amplification of marker loci (often described as “barcodes”) with high-throughput sequencing; (iii) metagenomic approaches based on untargeted shotgun sequencing of the total pool of DNA recovered from the sediment; (iv) hybridization-based target enrichment methods to recover DNA fragments of interest from the sediment metagenome (see Section 3.8).

Because sedaDNA captures biological changes in both the aquatic and terrestrial ecosystems, past genetic signals have the potential to provide insights about landscape development (including terrestrial floral and faunal changes and anthropogenic impacts) and the lake ecology itself (Table S1). Using sedaDNA to reconstruct changes in local vegetation has become a particularly well-established method as a complement to traditional microscopic analyses in lake sediment (e.g., pollen) e.g., [23,24,25,26,27,28,29,30]. It also offers the opportunity of studying the response of organisms to disturbances of natural and/or anthropogenic origin [31,32,33,34,35], the interactions of species at different trophic levels [36,37,38] and between native and introduced species [35,39], or species recovery and ecosystem restoration [40]. Lake sedaDNA is not only of interest to paleoecologists but also to archaeologists, because the recovered data can provide insights into human history and interactions with the environment such as agriculture and urbanisation e.g., [41,42]. Lake sedaDNA can also be used by geomorphologists to trace sediment sources or by evolutionary biologists to study population changes through time.

In the present work, we aim to provide an overview of the most recent advances in sedaDNA research on lakes. We also identify the major methodological knowledge gaps that remain and present original data—in the form of seven case studies—that tackle these challenges. Finally, we recommend a series of current best laboratory practices to successfully and robustly reconstruct past environmental change from lake sediment DNA archives, but acknowledge that this is a rapidly advancing field, which requires continuous update.

1.2. sedaDNA to Study Past Vegetation Changes from Lake Catchment

DNA-based studies of past vegetation have mainly focused on arctic, boreal, and alpine regions because of their high sensitivity to climate change, providing new insights on past vegetation dynamics and species distributions e.g., [16,26,27,43,44,45,46,47,48,49]. At high-latitude regions, sedaDNA has for example contributed to increased knowledge on the occurrence of insect-pollinated plants, which are typically underestimated in pollen analyses [24,27,50]. Some recent examples of studies are described below.

An exceptionally rich sedaDNA record that covers 24,000 years of vegetation dynamics in the Polar Urals, provides empirical evidence that arctic-alpine species survived early Holocene forest expansion in this heterogeneous landscape [27,47]. A study of 10 lakes in northern Fennoscandia show that there was a continuous increase in the regional species pool from the onset of the Holocene until the last two millennia, suggesting a severe time lag in colonization [16]. In the European Alps, sedaDNA studies have provided new knowledge about the history of agricultural activities; for instance, fruit trees were detected by sedaDNA but were not detected by microscopic pollen analyses [51]. Other sedaDNA studies have focused on the effects of soil evolution, climate change and pastoral activities on plant communities e.g., [32]. Based on the current body of literature, we know that sedaDNA can also trace vegetation changes over millennia in environments at cold high-latitude regions, including tropical regions from high-altitude sites with lower temperatures and warmer lowland sites [52,53,54]. At the same time, some of the above-cited works highlighted confounding methodological issues related to accelerated degradation of sedaDNA with temperature, which may result in less informative historical DNA signals in lake sediments from moderate to hot temperature environments e.g., [53].

1.3. sedaDNA to Detect Human and Animal Presence in the Lake Catchment

Human activities have been inferred from microscopic analyses of pollen from cultivated plants, native plants that are favored by livestock grazing or other disturbances and coprophilous fungal spores associated with livestock [55]. The first studies that aimed to track the presence of humans and domesticated animals in a lake catchment with sedimentary DNA-based methods used bacterial DNA indicative of past human and animal faecal waste [56,57]. In contrast to pollen, which normally gives both a local and regional signal in a lake, bacterial DNA provides only a local and catchment specific signal, although it does not allow identification of which livestock species were present. The metabarcoding approach applied to a high-altitude lake sediment record in the Alps revealed the first history of livestock compositional dynamics, including Bos (cow), Ovis (sheep), and Equus (horse/donkey) [31]. Other studies of lakes in the northwestern Alps have similarly provided new detailed insights into the temporal patterns of pastoral activities and changes in livestock species composition, especially during the Medieval Period [58,59,60,61]. Even in the very remote site of Kerguelen Island, an ongoing rabbit invasion could be successfully traced using sedaDNA, elucidating the impact that the invasion had on plant community and landscape erosion [35]. Other studies have used shotgun metagenomics to detect the presence of mammalian megafauna and other animals formerly present in lake catchments. Such approaches have contributed to constraining the timing of an island woolly mammoth population extinction in Alaska during the Holocene [33] and understanding when newly deglaciated landscapes became biologically viable in North America at the end of the last ice age [62]. However, due to methodological limitations and/or lack of animal sedaDNA preservation in lakes, recent studies set out to target mammalian DNA from lake sediments have not been particularly successful [63] and resulted instead in by-catching different animal groups [64] (see Section 1.5).

1.4. sedaDNA to Unravel Past Diversity and Composition of Lake Biota

The use of long-term sedaDNA now offers the possibility to study the impact of climatic, environmental, and anthropogenic perturbations on aquatic biota e.g., [11,34,65]. This approach provides information about Holocene and Late Pleistocene biota, including taxa that leave no morphologically identifiable remains preserved in lakes (see [13] for review). Thus far, the most common investigated aquatic organisms in sedaDNA studies have been photosynthetic bacteria and phytoplankton [18,34,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81]. Although macrophytes and some algal groups are typically well detected in sedaDNA plant surveys [24,25,26,81,82], there are few papers reporting the presence of zooplanktonic DNA [83,84] and fish DNA from lake sediments [85,86,87].

Most of these studies describe the long-term responses of communities to environmental perturbations (see [13]), while others investigate ecological interactions through time, such as the co-existence of parasitic groups and their phytoplanktonic hosts [36,37] or the role of double-stranded DNA viruses in terminating past algal blooms [88]. A study succeeded in tracing within-population genetic variation in an algae population during the Last Glacial Maximum [81]. The presence of key functional sets of genes from aquatic organisms can also be traced using the sedaDNA approach, like in the case of mcy genes responsible for producing microcystin by specific cyanobacteria [89,90], amoA genes associated with ammonia oxidation [91] and merA genes involved in mercury detoxification [92,93]. Finally, the research works showed how it is possible to study past activity of methanotrophs in lake systems [66,94,95,96,97].

1.5. Influence of Taphonomic Processes on the Burial and Persistence of sedaDNA

The studies presented above show the utility of sedaDNA methods to unravel biodiversity changes in lakes and their catchments but several concerns remain about the interpretation of such data due to the limited knowledge we have on the taphonomic processes occurring in lakes (i.e., origin, transport, and preservation of genetic material under prevailing environmental conditions). While issues related to taphonomy in lakes have been reviewed elsewhere [21,39], we provide here a brief update.

From source environment to sediment: For aquatic organisms living in the lake, several factors can influence the processes of incorporation and burial of DNA in the sediments, e.g., organism abundance, its spatial distribution, the ability to form cysts, and edibility of the focal taxon [98,99]). For terrestrial organisms living in the lake catchment, it is important to consider also the transport of material and the DNA from the catchment to the lake, which likely depends on multiple factors including soil erosion and hydrological connectivity [14,30,39,64,82].

Degradation and adsorption of DNA molecules at the water-sediment interface and in sediments: Environmental conditions in water and at the water–sediment interface and within the sediment column—such as temperature, redox state, conductivity and pH—influence the rates and extent of both abiotic and biotic DNA degradation e.g., [66,67,100]. The adsorption of DNA to mineral particles was recently reported to be a strong factor controlling the persistence of DNA molecules in sedimentary archives [101,102]. Desorption of adsorbed DNA from the particulate phase in sediments can also occur and is dependent on the mineralogic composition, pore-water pH or the valence and concentrations of cations in the sediment [103]. Changes in environmental conditions at the water–sediment interface are thus likely to be of crucial importance for the long-term preservation of DNA molecules during burial into sediments. Meanwhile, sedaDNA has also been successfully recovered and analyzed from aquatic systems that did not provide ideal preservation conditions, including sediments from warm, tropical lakes [52,83,104,105] and oxygenated deep-sea sediments [11,106]. However, the state of such preservation was found to be poor with the exception of the upper sediments corresponding to the last ~200 years, as found by Bremond et al. [53] in tropical Lake Sele (Africa). This may be due to accelerated DNA degradation rates related to higher temperatures (i.e., average annual air temperature of 28 °C over the course of the year) and the associated higher bacterial activity at their study site. As such, there is still much to be learned regarding the conditions that promote and compromise sedaDNA preservation.

Early diagenesis of DNA molecules during burial in sediments: Experimental evidence from lake sediments suggest that there is only a limited effect of early diagenesis on the DNA signal of microbial eukaryotic communities [107]. However, it is possible that DNA is degraded or damaged during early diagenesis, e.g., by microbes using extracellular DNA as energy sources [108] or by environmentally-induced strand breakage [109]. In contrast, DNA from organisms that produce resting stages (cyanobacterial akinetes), Cladocera resting eggs (ephippia) or resting propagules (protists), is likely better protected than extracellular DNA or intracellular DNA from organisms with more fragile outer membranes (see Section 2.3). Similarly, recalcitrant structural elements such as lignin in terrestrial plants are likely to protect cellular DNA against microbial degradation more efficiently than for most of the aquatic primary producers (algae and macrophytes).

Long-term persistence of DNA molecules in sediments: Overall, the processes involved in the preservation of DNA on long time scales in sediments are not yet fully understood, but DNA preservation still seems to be affected mainly by environmental conditions (temperature and anoxia) during incorporation at the bottom of the lake. Nevertheless, sedaDNA molecules have been reported from more than 270,000-year-old sediments of Lake Van in Turkey [72], and from marine sediments nearly a million years old [110]. Such reports however, are very close to the theoretical limit of ancient DNA preservation [111] and should be therefore treated with caution until they are repeated using methods that can properly authenticated ancient DNA from these substrates [112]. Today we know that strand breakage, miscoding lesions’ crosslinks may heavily compromise the analysis of sedaDNA molecules [109,113,114,115]. Thus, the quality of DNA molecules needs to be carefully inspected to avoid confounding temporal biodiversity change with changes in DNA quality over time, particularly when inferring long-term patterns on species persistence and richness e.g., [16,26,39,47,116]. Authenticity of sedaDNA older than 10,000 years and/or from poorer preservation settings can be demonstrated by identifying characteristic DNA damage patterns in metagenomic data and supported by corroborating proxy data whenever possible [33].

2. To What Degree does the sedaDNA Signal Represent Past Communities?

Success in recovering targeted ancient DNA from sedimentary archives depends on the taphonomy and preservation of DNA molecules within the sediments (as described in Section 1.5) and the ability to extract, sequence and taxonomically identify the DNA molecules [82,117]. The recovery success can be evaluated by comparing plant DNA signals with flora surveys in the lake catchments and signals from other sedimentary proxies like macrofossils, pollen, biomarkers or historical records. Yet, it has now become evident that the taxonomic breadth of environmental reconstructions based on metagenomics approaches (e.g., [62,81,118]) can now potentially expand to all groups of organisms and is therefore too vast to be systematically “validated” by other proxies. The degradation of DNA molecules over time in sediments and the presence of active microbes that compromise or obscure the archived ancient signal should also be considered carefully when doing sedaDNA analysis.

2.1. sedaDNA Data Compared to Historical, Archaeological and Monitoring Data

Terrestrial organisms: For the DNA molecules of an organism that lived in a lake’s catchment, the chances of reaching the lake bottom depend on several factors [39]. One of these factors is organism biomass, which affects DNA production at the source [14,30,64,82]. A study comparing sedaDNA from surface samples with vegetation surveys from eleven lakes in the boreal/subarctic ecozone showed that the effective detection of plant taxa mainly depends on the abundance of the taxa in the vegetation and distance from the lake [82]. Additionally, there were differences among the lakes studied and among plant families. Overall, the sedaDNA signal was strong enough to reconstruct near-shore vegetation types, and the detection of aquatic species was particularly good [82]. When compared to historical maps, sedaDNA accurately tracked exotic conifer plantations, and in contrast to pollen analyses from the same site, it did not show any signal from taxa dispersed over longer distances [119]. Thus, the DNA signal from lake sediment represents the vegetation within the catchment, in particular that in close hydrologic connection to the lake (e.g., shoreline, streams).

Meanwhile, studies assessing the reliability of mammal DNA records from lake sediments in comparison to historical or archaeological data are rare. This is because most sedaDNA studies to date have focused on areas where such data are lacking, leaving this gap as yet unfilled. However, the first study reconstructing livestock composition has been validated by archaeological remains and historical data [31]. Finally, the temporal occurrence of sedaDNA from rabbits, which had invaded an island in the 19th Century, was consistent with known historical records [35].

Plankton DNA compared to water monitoring data: Several studies confirmed that DNA from most aquatic planktonic organisms—from photosynthetic microbes to fish—can be detected in lake sediments [11,18,73,85,120]. Recently, few studies provided first insights into the reliability of DNA signals preserved in sediments to represent lacustrine communities. For example, Capo et al. [120] revealed that 70% of the microbial eukaryotic taxa (phylogenetic units in this work) living in the water column were retrieved in the sedimentary archives of a temperate lake. However, a comparison of the protist composition in the water column and in the underlying surface sediments showed that some groups, including cryptophytes, were underrepresented in sediments [120]. This is thought to be related to their high nutritional value, which makes cryptophyte cells likely to be preferentially grazed by herbivorous zooplankton [121,122], explaining therefore why their DNA does not reach the sediment.

Processes involved during the transport of DNA molecules or dead cells in the water column are still poorly understood [99], especially for smaller sized aquatic prokaryotes and protists with poorly discernible morphological traits. Few bacterial taxa can be identified using traditional microscopy cell counts, with some exceptions such as (large-size fraction) cyanobacteria. The work of Monchamp et al. [73] revealed indeed a correlation between the taxonomy of pelagic cyanobacteria morphology identified in the water column and the genetic information obtained from sediment. Similarly, Garner et al. [80] identified DNA from heterotrophic bacteria and viruses in sediment using the contemporary microbial diversity in surface-water metagenomes from the corresponding lakes as references. The presence of DNA from pelagic zooplankton in lake sediments confirms the potential using the sedaDNA approach to study temporal community changes in larger taxa, including rotifers, copepods, and Cladocera e.g., [11,45,83,123]. Altogether, as recently stated by Armbrecht et al. [22], it is “reasonable to assume that obligate photosynthetic plankton and/or zooplankton do not survive and reproduce after burial”. While there is relatively limited evidence to date of how the active growth of modern-day microbial communities affects the ancient DNA signal at depth (i.e., below the first few centimeters in the sediment column), sedaDNA studies investigating past bacteria and archaeal communities can be strengthened by the authentication of ancient DNA signals with procedures to map DNA damage patterns such as those described in Pedersen et al. [62] and Lammers et al. [81].

2.2. sedaDNA Data Compared to Other Sediment Proxies

Molecular reconstructions of past flora and fauna based on sedaDNA have been compared against a range of biological and geochemical sediment proxies, often showing complementarity between the different approaches [27,28,29,33,48]. In particular, comparisons with well-established proxies such as plant macrofossils, pollen and diatom enumeration, coprophilous fungal spores, or specific biomarker identification, have provided unique opportunities to evaluate the nature and reliability of the DNA signal obtained from sediments.

Plant DNA compared to pollen and macrofossil analyses: The first studies comparing pollen and sedaDNA showed rather limited overlap, therefore only partly confirming the reliability of the taxa detected by DNA signatures [23,28,124]. However, the detection of taxa in sedaDNA has greatly increased in recent years with expanded reference libraries and improved molecular methods, and we see now higher taxonomic overlap between DNA and pollen [25,27]. A major issue for interpreting sedaDNA data, also applying to pollen data, is the source area. Plant abundances inferred from metabarcoding reads seem to decline with distance from the lake, suggesting that sedaDNA provides a local signal [82]. Pollen is likely not a major source of DNA in sediment records, as pollen from taxa typically dispersed over long distances is poorly recorded in sedaDNA [25,27,28,125,126,127]. A metagenomic study focusing on woody taxa from a Beringian island also found that sedaDNA from spruce (Picea) could not be detected, although it was present in the pollen record, whereas willow could be detected in both records, with the sedaDNA data authenticated [127]. Compared to pollen, sedaDNA provide a more detailed account of agricultural activities due to its higher taxonomic resolution (e.g., identification of fruit or vegetable plant species; In [39,58,128]. Thus, sedaDNA provides a more detailed description of the catchment area, particularly for the areas with close connectivity to the lake such as the shoreline and inflowing streams and for the areas impacted by erosion processes.

Fewer studies have compared sedaDNA with macrofossils [23,24,25,29,125] and the expectations are that plant species detected with macrofossil can also be detected with sedaDNA. However, some taxa detected as macrofossils may not be observed in the DNA record and vice versa. Which proxy is more sensitive may vary according to the nature and conditions of the site, the taxonomic group and the amount of biomass produced [24,25]. Nevertheless, the majority of the taxa detected are similar in comparative studies where macrofossil preservation is good.

In conclusion, comparison with pollen and macrofossils, as well as vegetation surveys and historical maps, confirms that sedaDNA detects the common species and has sufficient detectability and resolution to determine the vegetation types.

Mammal DNA compared to fossil records: The presence of herbivorous animal herds in a lake catchment can be inferred from sediments by using other proxies than sedaDNA. The most established approach is based on the analysis of pollen and the detection of nitrophilous and ruderal taxa favored by animal faeces, trampling and selection due to plant consumption e.g., Rumex sp., Urtica sp., Chenopodium sp., Plantago sp.; In [55]. Some studies also focused on the presence of DNA from bacterial lineages specific to the gut microbiota [56,57,129,130,131]. Other approaches involve the use of spores of coprophilous fungi, which develop on herbivorous faeces (Sporormiella sp., [33,35,130], lipid biomarkers [132], or corroborating data from radiocarbon dated bones [33]. All these approaches can be used to assess the robustness of mammalian DNA records [33,35,39,130]. Indeed, several of these studies showed that the detection rate of mammalian DNA is lower than rates estimated via other inferred methods. A good detection of mammals has been observed at sites where there was only one source of drinking water [33] or where the concentration of mammals was high due to pastoral practices (e.g., presence of stabling areas; In [31,39]) or migration routes [62]. In contrast, the detection of mammals in tundra sites is poorer, likely because individuals are scattered and the sources of drinking water are multiple [64]. At a more southern site, mammal sedaDNA has not been detected even in the presence of abundant coprophilous fungal spores [63]. Nevertheless, sedaDNA has the advantage of allowing identification at the species level, which is not possible using pollen, Sporormiella spores or specific bacteroidales. While currently no mammalian DNA and faecal DNA datasets have been compared, this is, nevertheless, a promising complementary approach. Indeed, ratios between different stanols and bile acids can be used to distinguish between omnivore and ruminant species and between humans and pigs [131,133,134,135]. Though limited, evidence from archaeological sites suggests that shotgun metagenomic reads assigned to a range of different animal taxa, mirror their respective biomass estimated using classical analyses of bone remains [136]. Nevertheless, some work remains to establish whether this quantitative relationship for sedaDNA might hold across different types of archives.

DNA from aquatic biota compared to lipids, pigments and subfossils: The sedaDNA signal of past aquatic biota has mainly been compared to lipid, pigment, or subfossil (such as diatom frustules) records. The current view is that the DNA information in sediments degrades faster than pigments or lipids, although significant positive correlations have been observed between these different proxies [18,72,77,137,138,139,140,141]. Studies comparing diatom diversity retrieved from sediment using morphological and genetic approaches consistently show that beta-diversity values obtained from both methods were highly comparable (i.e., the turnover is similar), whereas alpha-diversity values and taxonomic assignment datasets were not [45,142,143,144,145,146,147]. This discrepancy in the reconstructions is at least in part attributable to very sparsely populated genetic databases available for diatoms, which can be tested in the near-future thanks to the exponential growth of genomic databases and initiatives such as the Earth Biogenome Project [148]. Heinecke et al. [149] demonstrated that the detection of sedaDNA identified as Potamogetonaceae was consistent with the recovery of subfossil remains from a species within this family of hydrophytes, whereas Clarke et al. [26] recently detected Callitriche and Sparganium in both pollen and sedaDNA, whereas each of the proxies detected an additional two and five taxa of aquatic macrophytes.

2.3. Dead or Alive: What Makes Up the sedaDNA Pool?

DNA from animals, land plants, zooplankton, and from many photosynthetic bacteria and protists can be preserved in sediments. These data provide information on past ecosystems prevailing at the time of deposition. This is because the pool of DNA is unlikely to originate from organisms living in dark and/or anoxic conditions in the sediments upon burial. In contrast, subsurface microbial communities (notably facultative and obligate anaerobic microorganisms) are generally thought to be structured through in situ environmental conditions such as the availability of electron acceptors and donors, porosity, and sediment lithology e.g., [150,151]. However, recent studies suggest that subsurface microbial taxa were present at the time of deposition and that their vertical distribution in the sedimentary record was shaped by the paleoenvironmental conditions that prevailed at the time of deposition [152,153,154,155,156]. For example, downcore sedimentary 16S rRNA gene profiling revealed that Holocene sediments of Laguna Potrok Aike in Argentina reflected a vertical stratification linked to electron acceptors availability while in the Late Pleistocene samples, up to 50,000 years in age, salinity, organic matter-type and the depositional conditions over the Last Glacial–Interglacial cycle being the most important selective stressors [153]. Analogously, shotgun metagenomic analyses of sediments from the Arabian Sea revealed subseafloor bacteria that were involved in denitrification processes during the formation of an extensive oxygen minimum zone [154]. A switch to fermentation is a likely explanation for their subsequent long-term post-depositional survival. However, none of these examples determined to what extent the identified communities represented dead, dormant, or metabolically active communities. Conversely, a recent study of Dead Sea sediments [157] illustrated a new pathway of carbon transformation in the subsurface and demonstrated how life can be maintained in extreme environments characterized by long-term isolation and minimal energetic resources.

Besides revival and cultivation of the living subset of the community or methods based on metabolic probing, several indirect approaches have been used to test for microbial viability (live/dead) and/or activity (see [158] for a review). Here we describe the most feasible approaches that can be optimised for use with sedimentary records and which are also compatible with downstream DNA sequencing. It is generally accepted that a cell must be intact, capable of reproduction, and metabolically active to be considered alive. The separate extraction of intracellular vs. extracellular DNA and subsequent amplicon or shotgun metagenomic sequencing analysis can reveal the diversity and metabolic potential of intact living vs. dead subsurface bacteria. This approach was applied to shallow sediments of tropical Lake Towuti (Indonesia) to reveal which microbial populations grew, declined, or persisted at low density with sediment depth [159]. Sequencing analysis of reverse transcribed sedimentary RNA markers most likely reflects the activities of microbes that were alive at the time of sampling [160]. That is because transcription is among the first levels of cellular response to environmental stimuli and RNA has a much shorter average half-life than DNA, being in the order of hours or days for ribosomal RNA and hours to minutes for messenger RNA [161]. Viability PCR via propidium monoazide (PMA) is another promising live/dead approach for sedimentary bacteria [162]. This nucleic acid intercalating dye binds to extracellular DNA and DNA inside damaged cells whereas it cannot enter living cells with intact membranes. Upon exposure to a bright light source, photoactivation causes PMA to form covalent bonds so that the irreversibly damaged DNA cannot be amplified in PCR assays. A comparison of microbial communities between untreated and PMA treated samples will reveal, respectively, total vs. living bacteria. However, this approach needs to be performed on freshly collected sediments and efficient exposure to the light source is essential and requires optimisation ([158] and references therein).

Overall, short read lengths (<200 base pairs (bp)) have often been associated with the more damaged signatures of ancient DNA libraries (e.g., [33,62,104,118]) providing an idea about ancient and modern DNA in the sediment DNA pool. A few more sophisticated bioinformatic approaches have also been developed. For instance, the growth rate of environmental bacteria may be calculated by measuring genome replication rates from shotgun metagenomic data. The most promising of these approaches is the Growth Rate Index (GRiD) [163] because it can infer growth rates of specific microbial populations from complete or draft genomes as well as metagenomic bins at ultra-low sequence coverage (0.2x). If used in high throughput mode, prior knowledge of the microbial composition and coverage is not required [163]. Finally, the assessment of ancient DNA damage patterns, a bioinformatic method applied to metagenomic data (see Section 3.8), has recently been applied to sedaDNA to identify ancient DNA sequences with post-mortem damage [29,33,62,81,118,127]. Such a procedure is a powerful method to ensure the authenticity of the DNA fragments assigned as ancient in sedaDNA studies.

3. State of the Art Lake sedaDNA Analyses

Working on sensitive samples such as sedaDNA requires the application of strict sampling and laboratory protocols and prevents contamination by modern exogenous DNA. Lake-sediment cores should be opened and sub-sampled in clean, dedicated ancient DNA laboratories [164,165]. The DNA is extracted from the samples, and molecular methods are then applied using targeted approaches (PCR, qPCR, ddPCR, metabarcoding) or whole-(meta)genome shotgun sequencing (shotgun metagenomics, target enrichment through hybridization capture) (see Table S1 for detailed information about the methodology used in each study). Recovered DNA sequences are then taxonomically and/or functionally annotated using a suite of bioinformatic tools to answer paleoecological questions. The potential and limitations of DNA approaches have been largely discussed [117,166,167]. However, several considerations specifically related to the study of ancient DNA in sediments are addressed here [13,14,21,22,103]. The appropriate procedure for the analysis of sedaDNA will inevitably depend on an array of parameters, from the origin and distribution of studied organisms in their ecosystems to the factors influencing sedaDNA preservation, extractability and, in the case of PCR-directed approaches, the probability of amplifying authentic sedaDNA. In this section, we synthesize the most recent literature concerning the different steps of experimental design for sedaDNA work which we augment with seven original case studies described in detail in the Appendix A.

3.1. Criteria for the Selection of Lakes

Lake selection is first and foremost defined by the specific scientific aims and budgetary considerations. For many purposes, like archaeological studies, choice might be limited to natural archives close to the site of interest and conditions that lead to efficient burial and preservation of DNA in sediments. DNA paleo-reconstructions have mostly used sediments from alpine, boreal and arctic lakes, where ancient DNA is well preserved due, for instance, to low temperatures, bottom water anoxia, and little to no bioturbation [100]. Nevertheless, successful sedaDNA studies have also been conducted in temperate lakes, where conditions appear to be less suitable for DNA preservation. Thus, we likely do not yet have a full understanding of the factors controlling DNA preservation. For example, it has been suggested that faster sedimentation rates in temperate and tropical regions [168] result in the DNA being buried faster below the active surface sediment zone which would cause more rapid preservation and immobilization in anoxic conditions. This may in turn overshadow higher degradation rates that result from the comparatively high temperatures that prevail at the bottom of such systems. More work is needed to accurately define the environmental conditions where sedaDNA is most likely to be efficiently preserved.

When considering the effect of past environmental changes on terrestrial ecosystems, the topography and size of a lake catchment relative to the lake size can influence the prospects of successfully using a sedaDNA approach. For extracellular DNA, which is readily adsorbed to clay particles, the source of material transported to lake sediment can be strongly influenced by erosional processes [39]. For example, surface soil horizons would be expected to provide more plant DNA than deep mineral horizons, bare soil or glacial flours. In addition, a well-developed hydrological network and higher rates of topsoil erosion may transport and deposit DNA that provides a better representation of a catchment flora and thus of the different potential habitats than less hydrologically connected areas [39]. Thus, for lakes with low allochthonous sediment input from areas with low relief and/or no major inflow streams, the DNA signal will mostly represent the plant community in the riparian zone within a few hundred meters [82], or the lake itself. On the other hand, the DNA signal from lakes with a larger hydrological catchment and considerable riverine input may represent a much larger source area [27,47]. No sedaDNA research, to our knowledge, has focused on transport processes in arid zones, however it is likely that the intermittent hydrological connection to the catchment would also affect the representativity of the DNA signal towards a stronger riparian signature, in a similar way than what has been found for bulk organic carbon pools and biomarkers [169,170].

Additionally, different sediment lithologies, including those from within the same lake basin, pose varying challenges to successful PCR amplification of sedaDNA. This is the case for the presence of humic substances, co-extracted with DNA, that can act as inhibitors and have adverse effects on the performance of any PCR or other nucleic acid analysis (e.g., [171]). Depending on the concentration and type of inhibitors and the particular enzymes used for the PCR, effects can be highly variable, i.e., some enzymes being more sensitive than others. In our case study A1 (Appendix A), we evaluated to what extent PCR inhibition was found to be related to sediment quality by taking advantage of a minerogenic-rich to organic-rich sediment continuum. Analyzing PCR inhibition along a sediment profile (138 sediment samples), no clear relationships between inhibition and sediment type were observed. However, it was noted that minerogenic-rich sediments (~32,000–21,000 yr. BP) had little to no inhibition while organic-rich sediments (~21,000–14,000 yr. BP) resulted in stronger inhibition even if these effects were variable. A slow decrease in inhibition is observed towards the youngest organic part of the core, creating an overall V-shape pattern (Figure 2). The reason for this remains unclear. It might be related to community changes in or around the lake, or (bio)chemical changes in the sediments. One key result of this case study is the strong negative correlation (r² −0.66, p < 0.01) between PCR inhibition and the number of plant DNA sequences amplified and sequenced in our work (Figure A1 in Appendix A).

3.2. Number of Sediment Cores to Collect for sedaDNA

In molecular paleoecological studies, it is common practice to collect a single sediment core from the deepest part of a lake because we assume that the genetic information is homogeneously distributed across the sediments and that this in turn reflects the biodiversity in the catchment. This practice is largely based on the assumption that DNA is distributed in a manner similar to fine-grained material such as organic matter or pollen, where one core is typically representative for the entire studied lake basin. However, this assumption can be questioned because we know that micro and macrofossils and organic matter can have a patchy distribution in the lake basin [172,173,174]. Additionally, the transfer and deposition of organic matter—and therefore catchment-DNA—in lake sediments is not necessarily homogeneous and may depend on catchment features [39]. Indeed, while a single-core signal may be suitable for capturing the temporal dynamics of small planktonic organisms that are evenly distributed in the water mass, the detection of DNA from larger aquatic organisms (e.g., fish, hydrophytes, littoral mussel species) can be strongly influenced by their more heterogenous in-lake distributions, as shown previously in eDNA studies [99,175,176]. A complex lake topography (e.g., lakes with two distinct basins) may also cause spatial variation in the DNA signal. Thus, there is a need to assess spatial variability in sedaDNA signals.

The use of “field replicates” i.e., collection of several sediment cores within one lake basin, in sedaDNA studies may be used to assess (i) how consistent the signal is at a specific site (core-site replicates) and (ii) whether or not there is spatial variability in the sedaDNA signal. The work of Etienne et al. [177] showed that field replicates led to a high spatial heterogeneity on the signal of fungal spores. In contrast, the recent work of Weisbrod et al. [178] with surface sediment DNA showed that a single sediment core can capture the dominant microbial taxa when targeting toxin-producing cyanobacteria. Regarding aquatic plants, their dispersion potential in the water has been proposed as a factor that can influence their detection in sedaDNA studies, particularly in large and deep lakes (5.45 km² surface area, 71 m maximum depth; [179]). In this study, free floating-leaf plants that can be more easily dispersed, were also readily detected in the deepest part of the lake. In contrast, helophytes, which are rooted in the near-shore area (littoral zone), were less-well detected and submerged plants were in an intermediate position. These results contrast with the findings from a survey of 11 smaller and shallower lakes (0.04–27 ha; 1.7–20 m), where two samples taken 15 cm apart in the center of each lake allowed for the detection of 90% of the common and dominant and 30–60% of scattered and rare taxa of macrophytes [82].

Taking multiple sediment cores from the same site, i.e., “core-site replicates” may also facilitate the detection of organisms that are in relatively low abundance in aquatic systems or further away from the lake within the catchment. It may, for example, be necessary for detection of fish DNA in sediments. Indeed, despite numerous attempts by multiple authors involved in the present work, the detection of fish from sedaDNA archives has only been reported in a few lake systems to date [85,86,87,180]. Although there is still uncertainty about the reasons for this apparent failure, the low amounts of fish DNA, compared to microbial DNA, present in the sediment may be one explanation. The potential to increase the sensitivity of such analyses by capitalizing on the dramatic rise in sequencing capacity that we are currently experiencing may indeed help us tackle this problem.

3.3. Storage of Sediment Cores Prior to DNA Analysis

Inadequate handling during coring and subsequent storage of sediment samples can have unexpected consequences such as (i) degradation of DNA by fast-growing bacteria that use nucleic acids as a substrate or due to hydrolysis and oxidation and (ii) modification of the composition of mainly the microbial DNA pool by growth. For instance, even minimal exposure to oxygen results in rapid fungal and bacterial growth. Even storing sediment in well-sealed conditions at 4 °C can clearly affect the reliability of the sedaDNA signal, particularly when studying past microbial diversity with e.g., 16S or 18S rRNA metabarcoding. The 16S rRNA gene amplicon data from the case study A2 (Appendix A) highlight the effect of secondary anaerobic growth on the DNA signal observed from sedimentary archives. In particular, these data show that microbial seed banks can reactivate or alternatively, that new microbes can colonize the sediments shortly after exposure to oxygen (Figure 3). In the two cases reported here, freeze–thaw cycles (Figure 3A) and oxygen diffusion (Figure 3B) increased the proportion of extractable DNA that mainly originated from growth of facultative anaerobic and metabolically versatile Gamma proteobacteria. Therefore, in 16S rRNA amplicon DNA inventories, secondary growth can induce a significant bias in the signal from past and present bacterial communities in the sediment (Figure 3B, Appendix A).

Overall, it is likely desirable to split sediment cores lengthwise into two halves—one for other analyses (e.g., geochemistry, dating) and one for DNA analysis. Additionally, it is prudent to sub-sample sediments immediately after opening the core and then store subsamples frozen at −20 °C or below. However, freezing is not needed in all cases and storing cores at 4 °C under a protected, oxygen-free atmosphere in tightly sealed containers may improve their preservation by avoiding unnecessary freeze–thaw cycles. For example, the pristineness of DNA extracted two years after sediment core sub-sampling was validated by using qPCR assays targeting facultative anaerobes [154]. These results ensure that metagenomic information can be interpreted in terms of past microbial processes in the water column at the time of deposition. Similarly, a study of vascular plant sedaDNA from lake sediment cores that were retrieved in 2009, left untouched and stored at 4 °C until being sub-sampled in 2014 yielded a detailed floristic sedaDNA record [26,27]. Other cores stored for 5–10 years at 4 °C have been successfully used to recover plant DNA [16,181]. Some early studies [83,105,182] stored the sediment samples in Queen’s Tissue Buffer, but generally, the use of chemical DNA preservatives, such as ethanol, Longmire’s lysis buffer or RNA later for sediment subsamples is not common for studies of sedimentary ancient DNA. While a dedicated test has not been published to the best of our knowledge, the addition of chemical preservatives can potentially cause problems in DNA extraction, and would introduce a further potential source of contamination.

3.4. Number of Analytical Replicates to Perform for sedaDNA Research

Similar to field replicates, the use of analytical replicates for both DNA extraction and PCR amplification is required for detecting taxa with low detection probabilities like rare species (i.e., specific fish populations) or species that are remotely located (e.g., terrestrial mammals). In order to increase the probability of detection, processing a number of replicates has proven to be helpful [39,183,184]. Numerous sedaDNA studies have included replicate samples at the extraction and/or PCR steps [16,24,25,31,35,39,49]. In the context of studying taxa with low detection probabilities (e.g., mammals), Ficetola et al. [183] recommended the use of at least eight PCR replicates. For catchment vegetation, comparison to macrofossils [24] and vegetation surveys [24] have shown that one positive PCR out of four or eight PCR replicates, respectively, may represent true positives (i.e., the presence of the desired targeted DNA in the environmental sample). However, it was also mentioned that increasing the number of analytical replicates can increase the probability of false positives [185].

The use of analytical replicates to assess the diversity and composition of planktonic communities from sedaDNA has been shown to provide a highly consistent assemblage composition of dominant taxa e.g., [34,38,76,141]. For instance, Ibrahim et al. [38] revealed similarity between DNA inventories obtained from triplicate extraction replicates in terms of community structure of microbial eukaryotes, diatoms and cyanobacterial assemblages obtained from a sediment record covering the last 100 years. Although the relative abundances of dominant microbial molecular taxa do not vary much between analytical replicates, it is noticeable that the proportion of shared taxa can be relatively low (<40% shared taxa between extraction replicates; [34,107]). Such low levels of consistency can be caused by the detection of numerous rare taxa, illustrating the generic challenges involved in estimating absolute richness from DNA metabarcoding data [184].

3.5. Tracing Contamination of sedaDNA Samples

The process of recovering sedaDNA must be carried out following guidelines for ancient DNA, due to high contamination risks e.g., [186]. There are many ways in which sedaDNA samples can be contaminated, from the time of field sampling to sequencing. Contamination can originate from the equipment and consumables used during the sampling collection, sediment core extrusion and core splitting, but also derive from compromised cleanliness in the ancient DNA laboratory, non-sterile laboratory reagents and insufficient precautions taken by the operator(s) to avoid cross-contamination and introduction of exogenous DNA sources. Sterile tools, clean working environments, appropriate clothing (single-use suit, gloves, mask, hairnets) and molecular biology grade reagents that may need to be decontaminated with UV irradiation can help to minimize the introduction of exogenous modern DNA.

Because modern DNA molecules are intact and have typically limited post-mortem damage while are at the same time normally present in higher concentration than ancient DNA, there is always a risk that such “false targets” are amplified during PCR. It should also be noted that samples with the lowest concentrations of sedaDNA are more susceptible to contamination, because of lower competitiveness during PCR [39]. PCR amplicons generated by qPCR or metabarcoding are particularly insidious forms of contamination, as they are highly concentrated and indistinguishable from authentic results. For this reason, it is crucial that pre- and post-PCR facilities are physically separated and strict protocols are used in the way reagents and personnel transit between these areas [164]. Contamination by modern DNA is more likely when studying microbial, fungal, and human aDNA [113], or plant species that are widespread and/or used for furniture/building construction such as pine and spruce [25]. Even when all precautions are taken, and quality control procedures are followed (i.e., integrating all the required negative controls and taking multiple replicates), the authenticity of sedaDNA sequences can be difficult to demonstrate especially from metabarcoding data (see also Section 3.8), and the bioinformatic filtering procedure required is not always straightforward (see also Section 3.10).

One way to trace contamination that has occurred during coring activities is to spray or paint the coring equipment with an artificial DNA tracer like DNA extracts or amplicons of a plasmid or an exotic species not likely to be present in the original sample ensuring that only contaminant-free internal parts of the core are analyzed [16,62]. During subsampling, DNA extraction, PCR amplification and library preparation, negative controls are always necessary to track potential contamination and can be used to filter DNA inventories for potential contaminants [14,25,66,185,186]. Positive PCR controls are largely used in environmental DNA research to verify the success of the molecular biology procedure and evaluate the presence of sequencing errors. For ancient DNA analyses, they should be avoided or used in a laboratory physically separated from the ancient DNA areas, to avoid potential cross-contamination [16]. Finally, the use of occupancy-detection models is a new approach for estimating the frequency of false positives and can be informed by the results of negative controls [187,188].

3.6. DNA Extraction Methods for sedaDNA Research

The DNA extraction method used may influence the DNA signal obtained from sediments. The PowerSoil, PowerMax, and UltraClean DNA Isolation kits (Qiagen) have so far been widely used by terrestrial and aquatic molecular ecologists (Figure 4), while various other kits and custom protocols have also been used by a number of studies. Based on our review (Table S1), the choice of DNA extraction protocol appears to be driven largely by the prior success of the research group with a particular kit. As more studies comparing and/or optimizing extraction methods are published (case studies described in Box 1, In [189]), the importance of selecting extraction methods optimized for the different sediment types become apparent.

DNA deposited at the water–sediment interface can be either extracellular (exDNA) or intracellular (inDNA) [13,22,103] and different DNA extraction protocols can be used to preferentially extract the two fractions. InDNA is likely to be more protected inside protective resting stages such as cysts or spores, cell membranes, or lignins, but can also be attacked by nucleases present in the cells. In contrast, exDNA released into the environment after cell lysis can be quickly adsorbed on clay minerals, which significantly promotes preservation by decreasing chemical and physical degradation processes [101] and making its molecules less accessible as a food source for indigenous sediment bacteria [190]. One of the protocols that allows desorption of exDNA fragments bound to minerogenic particles is described in Taberlet et al. [191] named hereafter the Taberlet2012 protocol. This protocol relies on the use of a phosphate buffer and it is coupled to the NucleoSpin soil kit protocol (Macherey-Nagel). This approach has been successfully used in several recent lake sediments studies (e.g., [31,32,39,51] but see also [25]). However, no explicit comparison has been made between molecular inventories obtained from total DNA (inDNA and exDNA), inDNA and exDNA. Here, we provide three case studies—A3, A4 and A5 that highlight differences and similarities using extraction protocols targeting the different fractions of the DNA (see Box 1, Appendix A).

The physical and chemical properties of sediments can have strong effects on the efficiency of DNA extraction from sediments. For instance, clay minerals and calcite both bind tightly to DNA [101,102,192] but require different extraction approaches to maximize DNA recovery. In the case study A6, we show that carbonate-rich sediments yield lower amounts of sedaDNA than organic-rich sediments using a conventional extraction protocol (PowerSoil kit) and an optimized extraction protocol can greatly improve vascular plant DNA recovery from carbonate-rich samples. (Box 1, Appendix A). The co-extraction of inhibitors, particularly from organic-rich sediments, can cause major problems for downstream analyses. Although new DNA extraction methods are being developed to reduce the effects of inhibitors [189], further refinements and a better understanding of the nature of inhibition are still necessary. An alternative approach to reducing the concentration of inhibitors in organic-rich sediments is to include an additional clean-up step (e.g., [193]). This was shown in case study A1, using the OneStep PCR inhibitor removal kit (Zymo Research) after the PowerSoil kit, which successfully reduced the level of inhibition with only a limited loss of DNA (mean DNA recovery of 91%, see Appendix A).

To reduce reagent usage in downstream procedures, it may be desirable to concentrate the supernatant or eluate, either during or after DNA extraction, respectively. To achieve this, a concentration step using spin columns can be added, either just after the desorption when using the phosphate buffer or our carbonate-optimised approach or at the end of the extraction when using the PowerMax kit. This approach has been recently applied to sediments [29,35,42,53,62,81,154] and in the case study A7 we provide new data showing its efficiency (Box 1, Appendix A).

Altogether, the findings of our case studies allow us to provide recommendations about the type of DNA extraction protocols to use for specific ecosystems and target taxa. Ecosystems with different features—lake basin and catchment—can result in different relative abundances and richness because the studied biological communities and potentially sources of origin are different. Additionally, contrasted sediment lithologies (e.g., minerogenic vs. organic-rich sediments) can influence the extractability of sedaDNA and thus the DNA signal recovered from sedimentary archives. For instance, carbonated post-glacial lake sediments should be extracted with a DNA extraction protocol optimized for DNA extractability to maximize the recovery of plant richness from the sedimentary archives (case study A6). Furthermore, sediment records with changing lithologies over time can lead to different efficiency in DNA extractability and PCR inhibitions (case study A1). Conversely, different extraction protocols may yield significantly different relative abundances and richness in taxa and genes, and should therefore be selected following preliminary analyses for suitability, notably where the heterogeneity between sample types and reconstructed assemblages is expected to be important. In most cases, it is better to use a DNA extraction protocol that extracts both intracellular and extracellular DNA, such as NucleospinSoil, Powersoil and PowerMax kits, including a physical lysis step for improved detection of taxa, regardless of the targeted group (case studies A3, A4 and A5). While no test suggests the need for concentrating DNA to successfully recover the sedaDNA signal from relatively abundant and small organisms such as microbes (case studies A3 and A4), the optimization of DNA extractions protocol to increase DNA amounts is recommended when targeting remotely located and not abundant organisms such as terrestrial mammals (case study A7). While our case studies show that sediment type and extraction protocol may have a significant impact on taxonomic composition recovered from ancient sedaDNA, much like in the modern environmental DNA literature, advances in measuring and modelling the taxonomic bias inherent to the metagenomic experimental workflow towards more quantitative results are promising [194].

Box 1. Optimizing DNA extraction protocols for molecular paleoecology.

In this box we describe the main findings of five case studies and for each of them additional information is presented in Appendix A. The case study A3 compares eukaryotic inventories obtained using a protocol extracting both exDNA and inDNA (NucleoSpin protocol) and a protocol favoring exDNA extraction (Taberlet 2012 protocol). We showed that the composition of eukaryotes varies depending on the extraction protocol used, even when considering the high variability in the signal recovered from each lake (Figure 5A). This is also the case in terms of richness (Figure A3 in Appendix A). One striking finding is that the extraction of total DNA with the NucleoSpin protocol appeared to be more efficient for detecting rotifer DNA than the Tarberlet2012 protocol for extracting exDNA, most likely because of the improved extraction of DNA from rotifer eggs with the lysis buffer from the NucleoSpin protocol. The case study A4 revealed that the qPCR amplification of several aquatic and terrestrial taxa targeted (bacteria, diatoms, eukaryotes, plants, arthropods and vertebrates) led to similar amplification levels (low quantification cycle (Cq) values correspond to higher amplification success)—when comparing inDNA and exDNA fractions obtained from a modified Powersoil protocol, with the exception of arthropods that were found to be amplified preferentially from intracellular DNA (Figure 5B). In addition, the use of the unmodified Powersoil protocol for extracting total DNA was more efficient for detecting and amplifying sedaDNA from several biological groups (average Cq values lower). For plant DNA in surface sediments, similar results were obtained in the case study A5 across four different lakes, in which differences in the diversity retrieved from total DNA vs. exDNA (Taberlet2012) were investigated both for use with the NucleoSpin kit and the PowerMax Soil kit. For samples from lakes surrounded by a high taxonomic diversity of terrestrial plants, distinct differences in the number of plant molecular taxa retrieved were observed between extraction protocols, with the unmodified PowerMax kit revealing the highest number of Molecular Operational Taxonomic Units (MOTUs) (Figure 5C). In our case study A6, we show that a modified DNA extraction protocol designed to release the mineral-bound sedaDNA from calcitic minerals using EDTA-based chelation provide higher richness estimates of plant assemblages in calcite-rich sediments but was comparable for organic-rich sediments, though with a lower level of reproducibility (Figure 5D). Finally, while investigating the number of positive PCR replicates required for the analyses of domesticated mammal DNA (ovine and bovine) in the case study A7, we reported higher amplification success when using Amicon filters coupled with the Taberlet2012 protocol relative to using the standard approach (Taberlet2012 protocol) (Figure 5E).

Figure 5. (A) Case study A3. Proportion of reads within microbial eukaryotic and metazoan assemblages for DNA extracted with the NucleoSpin protocol (intracellular and extracellular DNA) and Taberlet 2012 protocol (extracellular DNA) from sediments collected in four lakes (Bourget (LDB), Geneva (LEM), Lauzanier (LAZ), Serre de l´Homme (SDH)). For each extraction protocol, two DNA extracts were obtained. (B) Case study A4. qPCR Amplification success (quantification cycle (Cq) value) of DNA extracts obtained with different extraction methods: the standard PowerSoil kit protocol (PS protocol), a Powersoil protocol coupled with Phosphate Buffer to extract the extracellular (exPS protocol) and intracellular DNA (inPS protocol) fractions. Lower Cq values imply a higher qPCR amplification success (i.e., a lower number of PCR cycles necessary to detect a signal above the fluorescence background). (C) Case study A5. Number of plant molecular taxa identified in six surface sediments after DNA extraction with four different protocols: FastDNA^® Spin Kit for soil (FD), Favor Prep Soil DNA Isolation Midi Kit (FP), Nucleospin Soil Kit (NS) and PowerMax Soil DNA Isolation Kit (PM). The samples were obtained from three small lakes from the Southern Taymyr Peninsula, Siberia, and from three locations within Lake Karakul, Pamir Mountains, Tajikistan. The molecular taxa were split into three categories: terrestrial plants, aquatic plants and bryophytes. (D) Case study A6. Detection of plant molecular taxa within six tested layers using an original protocol using Qiagen DNeasy PowerSoil kit (Powersoil protocol) and an optimized protocol for calcite-rich sediments (optimized protocol). M, minerogenic-rich; O: organic-rich; M-O: minerogenic–organic. Taxa are ordered and coloured by functional group as shown with the bars on the extreme right: brown, woody taxa; yellow, graminoids; dark green, forbs; light green, ferns/horsetails/club mosses; dark blue, aquatics; gray, non-vascular plants. (E) Case study A7. Detection of domestic mammals (Bos sp. and Ovis sp.) in positive PCRs in two sediment samples from the Anterne Lake (France) after DNA extraction with the Taberlet2012 protocol and a modified version of this protocol including a step with Amicon^®ultra-15 10k centrifugal filters (Millipore) to concentrate the DNA extract (AmTaberlet2012 protocol). The two samples were dated to the Roman age (2010 cal. years BP) and very recent years (–56 years BP). A total of 16 and 8 PCR replicates were performed for the Taberlet2012 protocol and AmTaberlet2012 protocol, respectively.

Figure 5. (A) Case study A3. Proportion of reads within microbial eukaryotic and metazoan assemblages for DNA extracted with the NucleoSpin protocol (intracellular and extracellular DNA) and Taberlet 2012 protocol (extracellular DNA) from sediments collected in four lakes (Bourget (LDB), Geneva (LEM), Lauzanier (LAZ), Serre de l´Homme (SDH)). For each extraction protocol, two DNA extracts were obtained. (B) Case study A4. qPCR Amplification success (quantification cycle (Cq) value) of DNA extracts obtained with different extraction methods: the standard PowerSoil kit protocol (PS protocol), a Powersoil protocol coupled with Phosphate Buffer to extract the extracellular (exPS protocol) and intracellular DNA (inPS protocol) fractions. Lower Cq values imply a higher qPCR amplification success (i.e., a lower number of PCR cycles necessary to detect a signal above the fluorescence background). (C) Case study A5. Number of plant molecular taxa identified in six surface sediments after DNA extraction with four different protocols: FastDNA^® Spin Kit for soil (FD), Favor Prep Soil DNA Isolation Midi Kit (FP), Nucleospin Soil Kit (NS) and PowerMax Soil DNA Isolation Kit (PM). The samples were obtained from three small lakes from the Southern Taymyr Peninsula, Siberia, and from three locations within Lake Karakul, Pamir Mountains, Tajikistan. The molecular taxa were split into three categories: terrestrial plants, aquatic plants and bryophytes. (D) Case study A6. Detection of plant molecular taxa within six tested layers using an original protocol using Qiagen DNeasy PowerSoil kit (Powersoil protocol) and an optimized protocol for calcite-rich sediments (optimized protocol). M, minerogenic-rich; O: organic-rich; M-O: minerogenic–organic. Taxa are ordered and coloured by functional group as shown with the bars on the extreme right: brown, woody taxa; yellow, graminoids; dark green, forbs; light green, ferns/horsetails/club mosses; dark blue, aquatics; gray, non-vascular plants. (E) Case study A7. Detection of domestic mammals (Bos sp. and Ovis sp.) in positive PCRs in two sediment samples from the Anterne Lake (France) after DNA extraction with the Taberlet2012 protocol and a modified version of this protocol including a step with Amicon^®ultra-15 10k centrifugal filters (Millipore) to concentrate the DNA extract (AmTaberlet2012 protocol). The two samples were dated to the Roman age (2010 cal. years BP) and very recent years (–56 years BP). A total of 16 and 8 PCR replicates were performed for the Taberlet2012 protocol and AmTaberlet2012 protocol, respectively.

3.7. Sediment Amount to Use for DNA Extraction

It makes sense to take as much sediment as possible to avoid a “nugget effect”, whereby DNA is heterogeneously distributed, and have a sample that best represents the variation present within a sample, although this depends on how heterogeneous the sediments are and if larger samples have been homogenized before subsampling for extraction. Furthermore, trade-offs need to be made with other complementary analyses that also require sediments. Many DNA extraction kits commonly used in sedaDNA studies (e.g., DNeasy PowerSoil kit, NucleoSpin soil kits) are limited to smaller amounts of sediment (<2 g wet sediment). However, one of the most widely used extraction kits (Figure 4) is the DNeasy PowerMax Soil DNA Isolation kit, which can use up to 10 g of starting material. By contrast, the Taberlet2012 protocol allows the extraction of DNA from a large amount of sediment (e.g., 15 g of wet sediment) to which phosphate buffer is added. A recent study by Kang et al. [195] showed that sediment mass input (0.5 g vs. 10 g) did not affect the resulting diatom richness or community structure inferred from metabarcoding. The case study A3 found differences in the total DNA concentration of eukaryotic sedaDNA extracted when applying the Taberlet2012 protocol with different sediment masses (0.75 and 4 g of wet sediment). Although only four lakes were examined, higher sediment masses did not consistently lead to higher total DNA concentrations in the extracts (Figure A3 in Appendix A). Secondly, the predominant OTUs differed strongly between the two sediment masses for both micro-eukaryotic and metazoan groups: in lake Lauzanier (LAZ), diatoms and rotifers were highly abundant in 0.75 g samples, whereas Cercozoa, nematodes, and unclassified Stramenopiles and Metazoa were predominant in 4 g of material. In contrast, extraction using phosphate buffer (Taberlet2012 protocol) resulted in a lower number of reads, poorer repeatability, and less diversity detected compared to PowerMax Soil kit both for minerogenic [24] and organic sediments [25].

3.8. Molecular Methods for Generating sedaDNA Data

The method used to generate sedaDNA data is first and foremost constrained by the ecological question of the specific project. We describe below the three main methods that are currently used in sedaDNA research.

Targeted quantitative analysis: is used to detect and/or quantify specific taxa through the use of methods such as qPCR and ddPCR. In qPCR, target relative abundance is quantified to provide information about the occurrence of historical taxa. However, inhibition during qPCR reactions can bias this quantification. Alternatively, this bias can be utilized to quantify inhibition (e.g., [189]). Unlike qPCR, the recently developed ddPCR does not require standard curves and inhibition assays, due to pre-amplification partitioning of target templates into thousands of droplets of defined minute volumes where individual PCR reactions will take place (see [196,197] for application to modern environmental samples). For this method, the detection limit is very low, which may be advantageous given the issues that can be present in sedaDNA extracts. Meanwhile, as in environmental DNA studies [167], the use of these methods should be validated by providing supporting data to confirm that the DNA amplified truly corresponds to the target. Two options include the use of TaqMan probes, increasing the specificity of binding to the targeted marker region, or the sequencing of qPCR products.

DNA metabarcoding: can be used to assess the diversity and composition of specific assemblages (e.g., plankton, vegetation, fish) [167]. This method is based on the barcoding principle, which consists of sequencing standardised markers that are conserved enough to be specific to the target higher taxonomic group but variable enough to contain enough information to discriminate lower taxonomic groups, such as species or genera [198]. Available reference sequences are compiled into databases to which the metabarcode sequences are compared for taxonomic assignment. Metabarcoding is a robust and powerful tool that has been widely applied in sedaDNA studies (Table S1) [11,13,74,181,199]. The power, but also limitation, of metabarcoding is the fact that it is PCR-based. This makes it possible to amplify minute quantities of template molecules, but also introduces PCR and amplification biases (e.g., [200]). Because the targeted, reference databases can often be relatively complete for specific biological groups, bioinformatics become easier to handle. This is achieved by adding unique combinations of forward and reverse primers that include short unique tags of 6–15 nucleotides, which enables pooling of a large number of PCR products that can be sequenced on the same run. After sequencing, the DNA reads obtained can be demultiplexed by subsetting the DNA reads based on the tag associated with each primer. Metabarcoding requires the template to be present in the sample, and, if the fragments in the sample are too degraded, then an intact template, including the primer binding sites, might no longer be present. Thus, generally short barcodes are targeted, which may have lower taxonomic resolution compared to longer ones [201]. Furthermore, this method does not retain the damage patterns found at or near the ends of aDNA molecules, which are typically used to authenticate ancient DNA. This is because primers, which are artificial constructs, would either bind to or exclude the ends of the template molecules where damage is present. The damage signal is therefore lost during PCR amplification. Nevertheless, it remains the most used approach to studies of many eukaryotes as large numbers of samples can be run with comparatively low processing efforts and costs (see [167]).

Shotgun and target-enriched metagenomics: represent nascent but promising approaches to reconstructing past biodiversity preserved in sedaDNA [29,33,62,202]. The immediate advantage of metagenomic over PCR-based approaches is that they can resolve the ultrashort DNA sequences that cannot be amplified by PCR but are characteristic of the vast majority of sedaDNA molecules. In particular, shotgun metagenomics does not have the taxonomic biases and blind-spots that are inherent to PCR approaches, and which may preclude this latter approach for certain ecological questions and/or taxonomic groups.

Unlike PCR-based approaches, metagenomic approaches allow for the ends of ancient DNA molecules to be sequenced. This allows one to differentiate between modern DNA (from indigenous sediment microbiota or exogenous contamination) and sedaDNA by identifying patterns of DNA damage that accumulate via age- and temperature-related hydrolytic and oxidative decay (e.g., cytosine deamination, depurination induced DNA strand breakage), [29,33,62,81,118,127]. However, to retain damage patterns in metagenomic libraries, it is necessary that the polymerase used during the indexing PCR step does not stall when amplifying these templates. Great care is also required when selecting a proofreading, high fidelity polymerase because they often cannot read through uracils. Notably, polymerases such as PfuTurbo Cx HotStart (Agilent) or AccuPrime polymerase (Thermo Fisher) are specially engineered to be able to amplify templates containing uracil bases with minimal bias [203].

The phylogenomic resolving power of metagenomics, either through non-targeted shotgun sequencing or targeted enrichment via hybridization-based capture, can be harnessed to reconstruct population-level diversity. Schulte et al. [202] demonstrated that ancient Siberian larch species can be resolved by designing hybridization probes based on contemporary chloroplast genomes. To reconstruct an ancient algal population, Lammers et al. [81] used an iterative-mapping approach to reconstruct full organellar genomes and were able to distinguish multiple haplogroups. Fragment recruitment strategies, as used by these studies, could also be deployed to reconstruct microbial ecotypes which, if matching with high similarity to contemporary ecotypes that are sufficiently described, can lend insights into the ecological niches of paleoenvironments [80].

Metagenomics is positioned to become a powerful approach to reconstructing historical lake systems through microbial functional analysis and may enable access to the functional diversity of past microbiomes. For example, a novel approach to describing paleo-oceanographic conditions through the functional analysis of Black Sea sediment metagenomes draws an interesting perspective on reconstructing microbial functions by leveraging the depositional-age functional traits that survive in the metabolisms of extant sediment microbiota [193].

When sufficiently comprehensive genomic reference databases are available, the detection of species may be dramatically enhanced [29]. With the growth of these databases the potential for identifying species using the shotgun metagenomics approach will greatly increase. One such promising initiative is the recently launched Earth Biogenome Project setting out to provide a high-quality genome inventory of eukaryotic life on earth [148]. Currently, there is a strong bias towards mammals, agricultural and medicinal plants, and pathogenic microbes in available databases. However, this will change as an increasing number of large-scale genome skims data—i.e., shallow shotgun sequencing of genomes to reconstruct multicopy markers and organellar genomes—become available [204,205]. Thus, we expect metagenomics-based approaches to be increasingly used in the near future.

Published comparisons between methods: In the wider molecular ecology literature, comparisons between these methods have been performed on environmental DNA samples and mock communities. For example, Wood et al. [206] found that ddPCR had the highest detection rate of the Mediterranean fanworm in water and biofouling samples when compared with qPCR and metabarcoding (which had the lowest detection rate) approaches. qPCR was also found to have a higher detection rate over metabarcoding by Harper et al. [207] in an eDNA study; however, other studies have emphasized the power of metabarcoding for discovering members of the community which are not anticipated at the stage of selecting more specific primers [208]. The choice between metabarcoding and metagenomics is equally blurred. Studies have generally shown that metabarcoding provides slightly more accurate assignment [209,210], with higher detection frequencies [211], but may also generate more spurious Molecular Operational Taxonomic Units (MOTUs) than metagenomic approaches. We note that the performance of metagenomics will almost certainly improve further as genomic reference databases become more complete. Differences in taxonomic composition can be observed between metabarcoding and metagenomics datasets, especially if the DNA is very degraded [189,212]. In all cases, comparisons will be context-dependent, relying on parameters such as DNA degradation, sequencing depth, primer selection, detection thresholds, available reference libraries, and appropriate use of controls.

3.9. DNA Markers and Reference Databases Used in Current sedaDNA Research

sedaDNA from lake sediments is degraded into short fragments. For instance, shotgun libraries from Holocene lake sediments consisted mainly of DNA reads between 30 and 100 bp [33,62], although fragments up to 560 bp have been found in 10,000-year-old sediments in the Black Sea ([11]. Thus, targeting long barcodes (typically >150 bp; [213]), which generally have higher taxonomic resolution [198,214,215] is not a viable option for sedaDNA approaches, even if shorter barcodes generally have lower taxonomic resolution [201,216,217]. An alternative is to use multiple barcodes and compare the results [48]. If shotgun metagenomics is used, then a representative sample of all DNA fragments present in an extract is sequenced, independent of the length.

Nevertheless, as mentioned in Section 3.8, there will be a bias in the data related to representation in the reference databases [29]. This bias increases the risk of random matches to model organisms that are well covered in reference libraries [81], although fortunately, methods have recently been developed to address this issue (e.g., [218]). For any of the methods, there is a real risk of false positive assignment due to incomplete reference libraries or inappropriate filtering (see Section 3.10). This risk is inversely related to the taxonomic resolution of the marker, as there is a higher chance of obtaining incorrect assignments for a conserved sequence than a highly specific one. An error in the reference library will furthermore have a greater effect the more conserved the marker is.

Here, we propose DNA markers that can be used to study sedaDNA for various types of organisms (

Table 1. Recommended DNA markers for sedaDNA research. For sedaDNA applications, we recommend that markers of <150 bp are targeted (see Section 3.9). Amplicon length includes primer binding sites. Microbial eukaryotes include unicellular fungi.

Organisms	Genic Regions	Length (bp)	References
plants	tnrL P6 loop	49–188	[201]
mammals	16S mtDNA	60–84	[31]
fish	12S gene	163–185	[221]
zooplankton	18S V7	100–110	[222]
zooplankton	COI gene	313	[223]
microbial eukaryotes	18S V1-V3	560	[11]
microbial eukaryotes	18S V4	300–540	[78]
microbial eukaryotes	18S V7	260	[76]
microbial eukaryotes	18S V9	130	[79]
fungi	ITS2 region	250–500	[224]
diatoms	rbcL gene	67–76	[105]
diatoms	rbcL gene	577	[142]
diatoms	18S V4	400	[225]
archaea	16S gene	220	[91]
bacteria	16S gene	194	[97]
ammonium-oxidizing bacteria	amoA gene	635	[91]
cyanobacteria	16S gene	400	[74]
methanotrophs	16S gene	111–200	[97]
dsDNA virus	mcp gene	260	[88]

). For each type of organism, we identify the current most-suitable DNA marker to use but acknowledge that as overall genomic resources increase [148,219,220], the potential for developing new markers, or using full organellar and/or nuclear genomes, also increases.

DNA markers to reconstruct past vegetation changes: Most environmental aDNA studies of plants target the short and variable P6 loop of trnL (UAA) intron [14,44,201], which is not a standard plant barcode (CBOL 2009, Ref. [214]). There is a well-curated trnL (UAA) intron reference library (ArctBorBryo) containing 2445 sequences of 815 arctic and 835 boreal vascular plants [50,226], as well as 455 low-arctic bryophytes [227]. Recent large-scale genome skimming reference libraries have been generated for Norway/Polar regions, the European Alps, the Carpathians (n = 6655, Ref. [25]), China (n = 1659, Ref. [204]) and Australia (n = 672, Ref. [205]). These new reference libraries give not only the full nuclear ribosomal DNA and chloroplast genome sequences (including the P6 loop), but allow for improved detection using shotgun metagenomics [29] and designing probes for target enrichment [189,202].

DNA markers to detect mammalian presence in a lake catchment: Several universal mammalian primer sets were initially designed for ancient DNA analyses from animal remains [228], which were then applied to environmental samples, such as coprolites, frozen soils, and cave sediments [43,229,230]. Ultimately, a large set of mitochondrial universal and species-specific markers were developed [43,229,231,232,233,234]. Because it is near-impossible to avoid modern human contamination, universal primers have to be combined with a human-blocking probe to inhibit the exponential amplification of human DNA templates (such as in [235]). In lake sedaDNA studies, a new universal primer, MamP007, leading to the amplification of a fragment of the mitochondrial 16S rRNA gene, was proposed and has been applied successfully [31,35,39,51,59,60]. Because of the low number of differences between this mammalian primer and the binding sites for avian species and clitellates worms, as well as the generally low mammal sedaDNA template content in lake sediments, MamP007 was also able to amplify these non-target groups of taxa [64], along with fish taxa and amphibians [179,236]. Consequently, the mammalian DNA of targeted species has to be considered as “rare” for lake sedaDNA applications. The scarcity of mammalian aDNA in lake sediment archives will also depend on biomass, behavior, human practices for domestic herbivores, and as explained in Section 2.1, the transfer capacity in the catchment–lake system [39,64]. The quantity of mammalian DNA can thus strongly vary from one site to another and according to the animals of interest.

DNA markers to study the past diversity of aquatic organisms: To date, 16S rRNA genes have been the most common target used to investigate ancient bacterial and archaeal diversity in metabarcoding studies [18,66,91,94,95,96,97,104]. For instance, 16S rRNA and amoA genes allowed for the detection of ammonium-oxidizing Archaea and inference of past variation in nutrient and salinity levels in lake sediments [91]. Total bacteria, type I methanotrophs, type II methanotrophs and the NC10 phylum were traced using primers for amplicons between 111 and 200 bp long encoding their 16S rRNA genes to study past methane oxidation across <2000 years freshwater sediment profiles [94,95,96,97]. For cyanobacterial assemblages, Monchamp et al. [73,74,84] used a 16S primer set (400 bp) specifically designed for cyanobacteria by Nübel et al. [237].

To track past changes in the diversity and composition of microbial eukaryotic communities (including phytoplankton and fungi), the 18S rRNA gene V7 region marker used in Capo et al. [76] offers a good tradeoff between taxonomic resolution and fragment length (260 bp). It has been proven to capture past modifications related to environmental change [34,38,76,141]. Coolen et al. [11] targeted the V1–V3 region of 18S rRNA genes while Kisand et al. [78] and More et al. [79] targeted 18S V4 and V9 region, respectively. Specific databases, such as PR2 [238] and SILVA [239], can be used to identify microbial eukaryotes from any of the 18S genetic regions. In addition, the ITS region has been proposed as a DNA marker for fungal specific barcoding from environmental samples [240]. Such DNA markers have been for instance used by Ortega-Arulú et al. [241] targeting ITS 1 and Tõnno et al. [242] ITS 2 to identify fungal taxonomy against the UNITE database [243].

The detection of diatom communities in environmental DNA can be traced by targeting the chloroplast rbcL [105,144,147,244] and the nuclear 18S V4 region [245] because both markers are well represented in current reference databases, which facilitates the assignment of sequences to lower taxonomic levels. The short rbcL metabarcode (67–76 bp) [105] has been shown to facilitate specific amplification of diatoms from tropical [105] and arctic lake sediments [143,146,147], as well as marine deposits [246]. Moreover, the amplification of larger (577 bp) rbcL fragments could be achieved from more recent sedimentary deposits [142,247], while the amplification of the hypervariable V4 region of the 18S results in the detection of additional taxa next to diatoms [248], which include other Stramenopiles, Alveolata and Rhizaria. A more specific diatom amplification with 18S V4 is enabled when applying the marker to filtered water DNA [249] or modern biofilm samples [250]. In addition, primers targeting a 230-bp region of the viral major capsid protein (mcp) gene have been used to study the diversity of dsDNA algal viruses (Phycodnaviridae) in Holocene sediments [88]

Complete zooplankton assemblages have not yet been extensively studied from sedimentary archives. Studies have targeted specific groups i.e., copepods [45,123] and rotifers [83], but studies employing universal primers are rare. A combination of both 18S and COI primers has been used to analyze eukaryotic organisms [251] but, to our knowledge, no work has been published to specifically study sedaDNA for zooplankton assemblages across multiple phylogenetic groups. One potential bias is that the V7 region of the 18S rRNA gene of Daphnia (and probably other cladoceran taxa) is longer than other zooplankton species [252] leading to a potential bias in the PCR amplification and/or sequencing of such DNA markers to reconstruct past zooplankton communities. Additionally, in the case study A3 (Box 1, Appendix A), 18S rRNA gene V7 region amplicon sequencing showed that microeukaryotes dominated in terms of taxonomic richness (76 to 96% of the MOTUs are represented by microeukaryotes).

There are few sedaDNA studies that have successfully applied metabarcoding techniques to fish. Published studies have focused on short single-marker PCR or qPCR approaches, designed for species such as Perca flavescens [253], Coregonus lavaretus [85], and Oncorhynchus sp. [86]. Evaluations of markers have been carried out in the eDNA literature for applications such as biomonitoring [254,255] and seafood identification [256]. Achieving a balance between specificity and breadth in a marker has proven challenging due to the diversity of fish taxa and ecotypes; however, several studies have highlighted the potential utility of the 12S ribosomal region. Collins et al. [254] found that a 12S rRNA gene marker achieved higher levels of universality and taxonomic discrimination, combined with lower levels of non-specific amplification when compared with COI, 16S rRNA, and cytb genes. Although COI has a much greater reference database coverage, due to being the universal metazoan barcode [198], many reads are assigned to non-target groups making it difficult to obtain reproducible signals for fish taxa, which will be especially problematic given the low amounts of fish DNA compared to bacterial or planktonic groups in sediments. The MiFish-U 12S primers, in combination with a high annealing temperature, have been used to successfully amplify a hypervariable 163–185 bp fragment of fish DNA in surface sediments [221], although applications to older sediments remain uncertain.

3.10. Bioinformatic Filtering and Analysis of sedaDNA Data

While a comprehensive review of bioinformatic methods for sedaDNA goes beyond the scope of this work, we provide here an overview of some of the main tools employed as background to our case studies and we discuss some of the specific implications related to study design and selection of laboratory methods recommended in Section 4. Currently, there is no standard procedure nor criteria proposed for data “filtering” or “cleaning”, neither for metabarcoding nor shotgun metagenomics data. This is because developments, and eventual standardization, of bioinformatic filtering criteria are still in their infancy. Generally, taxa that are more frequently detected in negative controls than in samples can be considered suspect and should be removed from the data set. Regardless of the procedure applied, “true” presences can be removed (type II error), and “false” presences retained (type I error) [82,183]. To limit these errors, sedaDNA data can be cross-validated by comparing it with independent proxies [22,24,39,82]. When no cross-validation is possible, one can set the threshold where the majority of retained taxa are biogeographically likely to be true positives whereas the taxa filtered out are not expected in the sample [27,47]. However, such an approach has the potential to be overly conservative and bias the taxonomic inventory toward the present. The detection of a given suspect taxon in the same sample as taxa with similar ecological preferences can also be used to validate the presence of this suspect taxon [39]. For metagenomic datasets, taxa with very low read counts (e.g., n = ~3–5 per sample or 1 count in each of 3–5 samples) are generally excluded as spurious and controls thoroughly screened [62]. In metagenomic data sets, it is also imperative that a strict minimum fragment length cutoff (e.g., 35 bp) is applied to reduce spurious alignments [257]. Where doubt remains, it is important that the filtering procedure does not impact the conclusions of the study [25].

A major issue with the lack of standard bioinformatic pipelines to analyze DNA metabarcoding or shotgun metagenomics data is that slightly different procedures may lead to contrasting results [167,258,259,260]. Many sedaDNA works use OBITOOLs [261] to analyze DNA metabarcoding data (e.g., [16,31,35,199]), using mainly the EMBL nucleotide database or a custom reference database depending on the target taxonomic group or geographic region. This package, along with similar packages such as the Anacapa toolkit [262], contains various functions allowing for a comprehensive control of different sections of the data handling pipeline from read alignments, cleaning and filtering (i.e., removal of potential sequence variants generated by sequencing errors) to taxonomic classifications with simple user-customizable parameters. For shotgun metagenomics data obtained from sedaDNA, an example emerging procedure is the Holi pipeline [62]. By mapping shotgun reads to the full EMBL genomic and/or nucleotide database, which encompasses all realms of life, and subsequently assigning those matches to the lowest common ancestor taxa, Holi greatly diminishes the risk of misassignments [62]. In addition to the cleaning, merging, mapping, and annotation steps provided by the tools compiled in this pipeline, tools such as pmdtools [263] and mapDamage [264] can be used to identify ancient DNA present in alignments to ultimately differentiate between modern and ancient DNA molecules ensuring, therefore, the reliability of the sedaDNA signal obtained as per described in Section 2.3.

The choice of reference library used to annotate DNA sequences is a key component for obtaining reliable ecological information. Indeed, a major source of false positives may also be due to errors in reference libraries or sequence sharing combined with local species lacking in the reference library. Hence, the choice between the use of curated and non-curated reference databases (using more liberal alignment parameters) and the use of regionally local reference databases (like that made for the Norway/Polar regions and the European Alps) is of utmost importance to avoid misidentifications.

Details on statistical data analysis for processed sedaDNA data (univariate and multivariate) are beyond the scope of the present review. We instead direct readers to some key publications providing relevant recommendations about this step of the workflow [188,265,266,267].

4. Recommendations

In this section, we use state-of-the-art developments in lake sedaDNA research identified from our synthesis of previous studies and compiled case studies to provide methodological recommendations for future sedaDNA work. Because of the high variability in the sedaDNA signal from lakes with contrasting catchment morphologies as well as numerous other factors influencing the transport of DNA from sources to sediments, its extractability and amplification, it remains difficult to provide clear and concise guidelines about how to collect, analyze, and interpret sedaDNA data. However, we aim for this effort to further promote and guide future sedaDNA research, which will result in more robust reconstructions of past changes in aquatic and terrestrial biodiversity and offer predictions of the consequences of current and future climatic and environmental changes on biota.

Lake selection: Select the lake based on ecological questions and, if needed, adapt protocols to improve the ability to extract and recover DNA from the studied sediments. As a coring campaign is a major effort, especially in remote areas, we suggest, as a pilot study, analyses of surface sediments to test the amenability of the sedimentary record for sedaDNA analysis. If studying DNA from terrestrial organisms, it is important to first consider the size and topography of lake catchment and the hydrographic network to estimate the efficiency of DNA transfer to the lake following detailed recommendations from Giguet-Covex et al. [39]. Sediment lithology (e.g., clay mineral content) may also impact the preservation potential of DNA in lake sediment [101], but further study is required to create a predictive framework for lithology-based site selection.

Field replicates: The ideal number of sediment cores to sample depends on the target taxon and question. If one wants to assess the signal from unequally distributed large organisms (e.g., fish), the use of spatial field replicates may be beneficial for the detection of your target species. For investigation of terrestrial or more evenly distributed aquatic organisms such as plankton, a single core taken in the central part of the lake may suffice.

Core-site replicates: If the target organisms are rare or remotely-located, the use of multiple replicate cores from the same site is recommended to increase the probability of detection. In contrast, taking multiple cores is time-consuming and costly and implies the need of inter-calibration between cores for dating. Coring with tubes with broad diameter (e.g., 90 mm) may be an alternative to core-site replicates to increase the amount of material collected.

Analytical replicates: When enough material (i.e., sediment) is available, use multiple analytical replicates to ensure the reliability of the data obtained. When material is limited: if targeting abundant biological organisms (i.e., plankton), two to three analytical replicates (extraction and/or PCR replicates) should suffice to capture their sedaDNA signal; if targeting rare or remotely-located organisms, the use of at least six to eight analytical replicates is recommended. For further discussion about how to calculate the suitable number of replicates needed for statistical considerations, we direct readers to relevant publications [167,188].

Handling of analytical replicates: Extraction replicates can be pooled prior to PCR step and PCR replicates can be multiplexed with the same tagged primers to reduce costs and increase the number of reads per sample. However, such an approach will result only in presence–absence data and will not allow for assessments of replicability or semi-quantification for the results. If replicability data are desired, then it is essential that different tagged primer sets are used so that replicates can be separated and analyzed independently in silico.

Storage of cores and sediment subsampling: For core storage, recreate in situ conditions in the sediment column (cold, dark, anoxic) as much as possible. Perform sub-sampling immediately after core collection, or immediately after splitting the core in half. After the core liners are cut, a wire cutter is often used to split the core, which often results in contaminated surfaces of the core halves. It is therefore essential that these surfaces are removed using sterile implements, so that an unexposed layer is sampled, Sterile syringes with the ends cut off with sterilized scissors, blades or a heated wire can serve as mini corers to obtain subsamples (e.g., procedure outlined by Pedersen et al. 2016). The collected material can then be transferred to sterile tubes, while avoiding the potentially cross-contaminated smeared surface (e.g., [18]). Directly store the subsampled sediment at −20 °C or proceed to DNA extraction. For vascular plant and mammalian sedaDNA analysis, the core can be stored cool, ideally unopened, for several years prior to sampling, although secondary growth by fungi, for example, may lead to reduced efficiency of targeted sedaDNA recovery. To avoid freeze–thaw cycles of the sediment, sub-sample the sediment in multiple tubes for later extraction, if needed.

Monitoring of contamination: Consider using a synthetic DNA or exotic amplicon tracer during coring. Follow established protocols for minimizing contamination in ancient DNA laboratories [164]. Include and sequence negative controls from sampling, extraction, and downstream steps, as a minimum. Compare sequences/taxa that appear in both the controls and samples—these should, at a minimum, be treated with caution or removed entirely. Whenever possible, perform a multi-proxy approach with diagnostic macroscopic, microscopic, cellular, and/or DNA markers for specific taxa to cross validate the sedaDNA approach, especially if results are unexpected or ground-breaking. Use analytical methods that remove putative contaminants from the dataset.

Choice of DNA extraction protocol: In most cases, the following protocols should efficiently extract DNA from sediments: Powersoil (Pro) kit for small amounts of sediment (approx 0.25 g), PowerMax or FastDNA kit for larger sample sizes (up to 10 g), although the sediment type (organic-, clay-, carbonate-rich, etc.) should also inform the protocol used. Different extraction protocols may yield significantly different richness and relative abundances in taxa and genes, and should therefore be selected following preliminary analyses for suitability, notably where the heterogeneity between sample types and reconstructed assemblages is expected to be important. For shotgun sequencing approaches on very old sediments, it is better to use a protocol that retains ultrashort DNA fragments, such as the protocol from Pedersen et al. [62] or Murchie et al. [189]. Combine DNA inventories obtained from different DNA extraction protocols to maximize your chances to detect a specific target and/or robustly reconstruct the studied biological assemblage.

Choice of the data generation method: It depends on the question and target organism(s) of interest. Shotgun metagenomics can give an indication about the degradation state of the DNA molecules that can be necessary when data authentication is required (e.g., working with microbes that can live in, or contaminate, lake sediments), but may not be strictly necessary if interested in terrestrial organisms or aquatic macroorganisms. However, metabarcoding and quantitative methods (qPCR and ddPCR) can be used to process more samples at lower cost and are therefore recommended for long time series or large data sets, provided there is sufficient sedaDNA preservation. If DNA preservation is poor and molecules of the target taxa are likely to be rare in the sedaDNA mixture, then a target enrichment approach is recommended.