Cyanobacterial Communities of Carbonate Sediments and Biomineralization in Peterhof Fountains’ Water Supply System, Russia

: The role of cyanobacterial communities in the formation of carbonate sediments (an-cient and modern) is not completely clear. We studied the cyanobacterial communities connected with carbonate sediments of the freshwater bodies feeding the historical Peterhof fountains (Saint-Petersburg, Russia). Cyanobacterial communities were studied by metagenome analysis and optical microscopy. Carbonates associated with cyanobacterial communities (both in situ and in vitro) were studied by powder X-ray diffraction analysis, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and Raman spectroscopy. The interconnection between the mineral composition of carbonate sediments and inhabiting microorganism species was established. The leading role of cyanobacteria in carbonate biomineralization in fresh water of Peterhof fountains water supply system was shown. Cyanobacteria of 24 genera were revealed in sediments composed of calcite and aragonite. The crystallization of carbonates on the surface of 13 species of cyanobacteria was found. Using model experiments, a signiﬁcant contribution of cyanobacterial species of the Oscillatoriaceae family ( Phormidium spp., Lyngbya sp., Oscillatoria formosa ) to carbonate biomineralization is demonstrated. have agreed to published


Introduction
Many studies have been focused on factors involving in the development of the sedimentary record of modern fluvial carbonate systems [1]. Tufa is a category of freshwater carbonate sediment that occurs at ambient temperature (cold water) at springs, waterfalls, and in fast-flowing streams [2,3]. Tufa precipitation concept evolved from pure abiotic control (i.e., precipitation of carbonates due to carbon dioxide outgassing [4]) via biotic control (i.e., biologically induced mineralization [2]) to a combination of both (abiotic/biotic) variants [5]. Recently, it was shown that that the presence of a microbial biofilm, in which cyanobacteria are often dominant, strongly influences the precipitation of carbonates in

Study Area and Sample Collection
Our study is based on materials collected from the southern vicinity of Saint Petersburg in the Lomonosov district near Orzhitsy village, Glyadino village, and the city of Peterhof (Figure 2a,b). The climate of the territory is temperate, with mild winters. The average air temperature is 8.6 °C, and the air humidity is 70%. The main wind direction is south and southwest [21]. Neoproterozoic to Paleozoic sedimentary rocks are flat lying and overlap Archean to the Paleoproterozoic basement (granite and gneiss) with an angular unconformity [22]. Neoproterozoic conglomerate, gravel, sandstone, and clay

Study Area and Sample Collection
Our study is based on materials collected from the southern vicinity of Saint Petersburg in the Lomonosov district near Orzhitsy village, Glyadino village, and the city of Peterhof (Figure 2a,b). The climate of the territory is temperate, with mild winters. The average air temperature is 8.6 • C, and the air humidity is 70%. The main wind direction is south and southwest [21]. Neoproterozoic to Paleozoic sedimentary rocks are flat lying and overlap Archean to the Paleoproterozoic basement (granite and gneiss) with an angular unconformity [22]. Neoproterozoic conglomerate, gravel, sandstone, and clay (overall thickness is up to 300 m) are succeeded by Cambrian deposits (thickness is up to 150 m) composed of clay, siltstone, and sandstone; they are overlapped by the Lower Ordovician black shale and sandstone gradually replaced by limestone [23]. Middle-to-Upper Ordovician sediments are composed of limestones, partly dolomitized, and dolostones (the thickness of the Ordovician strata is up to 200 m). Lower to Middle Ordovician clayey bioclastic limestones are exposed across the Shingarka River. Different stratigraphical levels of the Paleozoic rocks are overlaid by the Quarternary glacial, limnoglacial, or bog sediments, and the thickness of the Quarternary deposits do not exceed 20 m [24].
as near the places where groundwater emerged to the surface ( Figure 2). Carbonate deposits are 1-3 m thick (rarely up to 7-8 m) and are usually confined to slopes or relief depressions. The area of carbonate sediments depositions varies from hundreds of square meters to several square km. Local foci of carbonate accumulation were found in the tributaries of the Shingarka River [25,26]. The largest deposits of calcareous tuffs in the area were developed until 1917 [18].
A total of 50 samples of carbonate sediments with biofilms were collected in 2018-2020 from water bodies of 6 different types (I-VI) connected with Peterhof fountains water supply systems: I-water reservoir (2 samples); II-river and streams (24 samples); IIIwaterfalls (8 samples); IV-splash zones (5 samples); V-springs (7 samples); VI-shallow artificial reservoirs (4 samples) ( Table 1). Samples were taken in sterile containers and craft envelopes, and water pH was measured at each sampling point using a portable pH meter. Samples with typical biofilms were selected for metagenomic study from non-anthropogenic habitats of the water bodies, which are feeding the Peterhof water supply system (Table 1).

Figure 2.
Characteristic of the studied area and sampling points: (a)-simplified geological map of the Leningrad region (following [24], with changes); (b)-schematic map of sampling points, numbers corresponding to Table 1.  Table 1. Most of the freshwater carbonate sediments were formed in rivers and lakes, as well as near the places where groundwater emerged to the surface ( Figure 2). Carbonate deposits are 1-3 m thick (rarely up to 7-8 m) and are usually confined to slopes or relief depressions. The area of carbonate sediments depositions varies from hundreds of square meters to several square km. Local foci of carbonate accumulation were found in the tributaries of the Shingarka River [25,26]. The largest deposits of calcareous tuffs in the area were developed until 1917 [18].
A total of 50 samples of carbonate sediments with biofilms were collected in 2018-2020 from water bodies of 6 different types (I-VI) connected with Peterhof fountains water supply systems: I-water reservoir (2 samples); II-river and streams (24 samples); IIIwaterfalls (8 samples); IV-splash zones (5 samples); V-springs (7 samples); VI-shallow artificial reservoirs (4 samples) ( Table 1). Samples were taken in sterile containers and craft envelopes, and water pH was measured at each sampling point using a portable pH meter. Samples with typical biofilms were selected for metagenomic study from nonanthropogenic habitats of the water bodies, which are feeding the Peterhof water supply system (Table 1).

Identification of Cyanobacteria
Species identification of cyanobacteria was carried out using direct microscopy (Leica DM 1000 microscope) by morphological characteristics, using the identification guides [27][28][29]. Natural samples and accumulative pure cultures isolated in distilled water and in various nutrient media-namely, Gromov-6, Z8, and BG-11 [30][31][32], were used. Data on the geographical distribution of the species and their ecological characteristics were provided in accordance with the CRIS database [33,34] and by Davydov [35].
Metagenomic analysis of the prokaryotic community was carried out for some samples (Table 1) based on the nucleotide sequence of gene 16S rRNA. DNA was isolated according to the method described by Vladimirov [36]. Sample preparation of the libraries was carried out according to the recommendations described in the Metagenomic Library Prep Guide (Illumina) [37]. For the detection of cyanobacteria, primers were developed using a working solution containing a forward CYA359F and an equimolar mixture of two reversals 1:1 (CYA781Ra and CYA781Rb). For the second stage, the F + 2R equimolar mixture was used. The analysis was conducted by BioBeagle Co (St. Petersburg, Russia).

Laboratory Experiments on Biomineral Precipitation
Four precipitation experiments were conducted in natural water with the participation of cyanobacteria isolated from natural communities in studied habitats (DCS/BF-24, DCS/BF-41, DCS/BF-42, DCS/BF-45; Table 1). Samples of natural water (the initial pH of the medium was 8.7-8.9), together with the community of microorganisms, were placed in sterile containers with a volume of 120 mL and were exposed for 6 months at room temperature and natural light. The assessment of precipitated phases was carried out immediately after collection and after 6 months of exposure. Such environments are best suited for experiments with natural communities since they represent a complex microbiome, which contains microorganisms of different trophic levels. The use of natural water makes it possible to preserve the functioning of the natural community under suitable trophic conditions.
In order to clarify the role of different cyanobacterial species in carbonate sedimentation, carbonate precipitation experiments were also carried out with collection strains of cyanobacteria. Experiments were carried out in the mineral medium Gromov-6. The initial medium pH was 7. The use of the mineral medium (Gromov-6) was due to the fact that the strains of cyanobacteria used are kept in such medium in the Collection of Algae of Leningrad University (CALU). It also allowed the experiment to be conducted under controlled conditions. Experiments were provided in a liquid mineral medium in two variants: The experiment was carried out in Petri dishes for 9 months. As a control, a piece of marble was exposed in Gromov-6 medium without cyanobacteria.

Mineral Identification
Identification of biominerals, along with bedrock samples, was accomplished by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and Raman spectroscopy. PXRD studies were carried out using a Miniflex II diffractometer (Rigaku, Japan), Cu ka radiation, 2-theta range 3-80 • , velocity of 2 • /min, and step size of 0.02 • . The PDXL II software (version 2.8.4.0, Rigaku, Japan) was used to identify the mineral phases, accessing the International Centre for Diffraction Data (ICDD) database. SEM studies were carried out using a TM 3000 (Hitachi, Japan) electron microscope with an EDS (Oxford, Great Britain). Raman spectra were obtained on the Senterra spectrometer (Bruker, MA, USA) equipped with a 785 nm solid-state laser with 1 mW power under the sample. The spectra were recorded in the range from 80 to 3700 cm −1 . The aperture was 25 µm × 1000 µm. The integration time was 100 s with 6 repetitions. The device is equipped with an Olympus BX-51 microscope. The 100× objective with 0.9 numerical aperture was used for better spatial resolution. This made it possible to compare photographs obtained using an optical and electron microscope.

Mineral Composition of Sediments in Peterhof Fountains' Water Supply System
The following minerals were found in the samples from the Peterhof water supply system: calcite, aragonite, dolomite, quartz, feldspar (albite, microcline), and mica ( Table 2). Silicates (quartz, feldspar, and mica) are parts of the underlying substrate material. Calcite and dolomite could be both parts of underlying bedrock (limestone/dolomite) and newly formed biominerals, whereas aragonite could be biomineral only, as no aragonite-bearing rocks are known in the studied region. Studied calcite contains a small admixture of magnesium (0.5-1.8 wt.% MgO). According to the mineral composition the studied carbonate sediments can be divided into two groups: (1) microorganism related: aragonite ± Mg-calcite ( Figure 3a) and (2) abiogenic: dolomite ± low-Mg calcite. Samples from model experiments were enriched in organic material with minor mineral components. Therefore, no reliable PXRD data were obtained, whereas Raman spectroscopy reveal the presence of calcite (based on lattice modes peaks at 155, 281 as soon as 710 and 1086 cm −1 originating from v 4 (bending mode) and v 1 (symmetric stretching mode) in carbonate ion typical for calcite [13] (Figure 3b). Note: pH was measured in the summer period.

Mineral Composition of Sediments in Peterhof Fountains' Water Supply System
The following minerals were found in the samples from the Peterhof water supply system: calcite, aragonite, dolomite, quartz, feldspar (albite, microcline), and mica ( Table  2). Silicates (quartz, feldspar, and mica) are parts of the underlying substrate material. Calcite and dolomite could be both parts of underlying bedrock (limestone/dolomite) and newly formed biominerals, whereas aragonite could be biomineral only, as no aragonitebearing rocks are known in the studied region. Studied calcite contains a small admixture of magnesium (0.5-1.8 wt.% MgO). According to the mineral composition the studied carbonate sediments can be divided into two groups: (1) microorganism related: aragonite ± Mg-calcite ( Figure 3a) and (2) abiogenic: dolomite ± low-Mg calcite. Samples from model experiments were enriched in organic material with minor mineral components. Therefore, no reliable PXRD data were obtained, whereas Raman spectroscopy reveal the presence of calcite (based on lattice modes peaks at 155, 281 as soon as 710 and 1086 cm −1 originating from v4 (bending mode) and v1 (symmetric stretching mode) in carbonate ion typical for calcite [13] (Figure 3b).

Composition of Microbial Communities in Peterhof Fountains' Water Supply System
The use of metagenomic analysis made it possible to identify cyanobacteria from 17 genera ( Table 3). The most abundant (more than 15% share in the community) are Phormidium (47%), Tychonema (27%), Chamaesiphon (16%), Leptolyngbya (15%), Calothrix (20%). In the splash zone, a community is represented by the genera Phormidium and Leptolyngbya. In riffle habitats Tychonema is dominant (22% and 27%). Additionally, a large share (15%) is Leptolyngbya in the community with carbonate sediment located in the zone of water discharge from the reservoir. Table 3. The list of cyanobacterial genera with participation in microbiome of studied habitats.

Species
Number of Sample, %
The proportion of cyanobacteria in the microbiome ranges from 93% to 100% according to the results of metagenomic analysis (Figure 4). The share of Leptolyngbyaceae is higher in relation to other families in the Glyadinskoe reservoir ( Figure 5). In places with a fast current (BF/DCS-4-water discharge, BF/DCS-25-waterfall), the proportion of Phormidiaceae is higher. Samples from the splash zones in Glyadino (BF/DCS-5) and at the waterfall (BF/DCS-29) are similar in the proportion of families in the microbiome. This is due to the fact that riffle habitats (fast water flow) and splash zones (variable humidity, temperature drops) are highly specific, and most often, such places are inhabited by species adapted to these habitat conditions [38,39]. A large amount of unclassified and unknown DNA is due to the fact that such communities remain poorly understood. The proportion of cyanobacteria in the microbiome ranges from 93% to 100% according to the results of metagenomic analysis (Figure 4). The share of Leptolyngbyaceae is higher in relation to other families in the Glyadinskoe reservoir ( Figure 5). In places with a fast current (BF/DCS-4-water discharge, BF/DCS-25waterfall), the proportion of Phormidiaceae is higher. Samples from the splash zones in Glyadino (BF/DCS-5) and at the waterfall (BF/DCS-29) are similar in the proportion of families in the microbiome. This is due to the fact that riffle habitats (fast water flow) and splash zones (variable humidity, temperature drops) are highly specific, and most often, such places are inhabited by species adapted to these habitat conditions [38,39]. A large amount of unclassified and unknown DNA is due to the fact that such communities remain poorly understood.     Table 1).
Unfortunately, it should be noted that the use of only metagenomic analysis may not reveal the full diversity of microorganisms in the study area. Several factors influence the DNA extraction processes (for example, the high amounts of EPS produced by Nostocales and Chroococcales [40] and a high concentration of Mg in nature samples [41,42]. A bias in relative abundances can be expected if only molecular methods are considered [43]. Therefore, it is necessary to use a set of methods to identify diversity. In total, 25 species of cyanobacteria belonging to 16 genera, 11 families, and 4 orders were identified morphologically in the samples of carbonate sediments and biofilms ( Table 4). The order Synechococcales is represented by the largest number of families (4). Filamentous forms of cyanobacteria are most widely represented in the order Oscillatoriales (10 species), while coccoid forms prevailed in the order Chroococcales (7 species). The genera Gloeocapsa and Phormidium turned out to be the richest in species diversity in comparison with other cyanobacteria. As a whole, 24 genera of cyanobacteria were identified as a result of morphological study and metagenomic analysis. The combination of the two approaches makes it possible to more fully assess the diversity of the studied microorganisms.  Most diverse in taxa (15 taxa) was the sample of loose carbonate sediment from the Olginsky channel (Table 5). This is due to the fact that the Olginsky channel and Dry pond are subject to a greater anthropogenic load than other study sites. The presence of pipes that ensure the flow of water into the fountains of Peterhof adds microhabitats for the species. Cosmopolitan species of cyanobacteria, as well as species of stony substrates [44,45] (for example, Gloeocapsa atrata, Microcoleus autumnalis), have been noted. The appearance in the list of species characteristic of lithobiontic communities is most likely due to the fact that the coast of the Olginsky channel is fortified with stone slabs. A significant diversity of cyanobacteria was also noted in the Simonovskiy stream and Fabrichnaya river (10 species for each location). These sites are larger than springs and provide a variety of microhabitats for cyanobacteria. Species of the genera Leptolyngbya and Phormidium were most frequently found in the studied samples (Figure 6a,b). Phormidium was previously reported as the dominant periphyton species in the Leningrad Region rivers [46]. Species of the genus Lyngbya were found in splash zones and in large bodies of water, streams, and a river but not found in the reservoir and springs, and in Peterhof. Nostoc sp. (Figure 6c) and Pseudanabaena sp. (Figure 6d) are found in the Simonovskiy stream, Fabrichnaya river, and springs. In the sample form Dry pond, only one species was identified-Schizothrix sp. (Figure 6e), which was also recorded in the Olginsky channel.
Our study expands the list of cyanobacteria living in the carbonate sediments formation places in the Leningrad Region from 5 genera [13] to 24. Species of Oscillatoriales (Phormidium) and Synechococcales (Lepyolyngbya) predominate in such places. The data of metagenomic analysis are in significant agreement with the results of morphological identification and characterize the high diversity of cyanobacteria in the communities associated with the processes of carbonate sediment formation. The species composition changes depending on habitat conditions (reservoir, waterfall, stream, splash zone). Among the identified cyanobacteria, species confinement to certain habitats can be traced: for example, the predominance of species of the genus Tychonema is characteristic for riffle habitats. The genus Tychonema inhabits cold-water and slightly eutrophicated lakes [28]. For still water, an interesting picture is obtained. This type of water body is represented by the sample with the greatest diversity (15 species, LCS-48) and the sample with one species (Schizothrix sp., sample LCS-50). This fact testifies to the uneven distribution of cyanobacteria in such habitats that can also influence the formation of carbonate sediments. Phormidium was previously reported as the dominant periphyton species in the Leningrad Region rivers [46]. Species of the genus Lyngbya were found in splash zones and in large bodies of water, streams, and a river but not found in the reservoir and springs, and in Peterhof. Nostoc sp. (Figure 6c) and Pseudanabaena sp. (Figure 6d) are found in the Simonovskiy stream, Fabrichnaya river, and springs. In the sample form Dry pond, only one species was identified-Schizothrix sp. (Figure 6e), which was also recorded in the Olginsky channel. Our study expands the list of cyanobacteria living in the carbonate sediments formation places in the Leningrad Region from 5 genera [13] to 24. Species of Oscillatoriales (Phormidium) and Synechococcales (Lepyolyngbya) predominate in such places. The data of metagenomic analysis are in significant agreement with the results of morphological identification and characterize the high diversity of cyanobacteria in the communities associated with the processes of carbonate sediment formation. The species composition changes depending on habitat conditions (reservoir, waterfall, stream, splash zone). Among the identified cyanobacteria, species confinement to certain habitats can be traced: for example, the predominance of species of the genus Tychonema is characteristic for riffle habitats. The genus Tychonema inhabits cold-water and slightly eutrophicated lakes [28]. For still water, an interesting picture is obtained. This type of water body is represented by the sample with the greatest diversity (15 species, LCS-48) and the sample with one species (Schizothrix sp., sample LCS-50). This fact testifies to the uneven distribution of cyanobacteria in such habitats that can also influence the formation of carbonate sediments.
The studied community of microorganisms in freshwater bodies feeding the historical fountains of Peterhof is not only composed of prokaryotes. Eukaryotic algae make up a noticeable share of the autotrophic component. In all studied habitats, green algae Chlorophyta (coccoid and filamentous, for example, Cladophora sp., Klebsormidium sp.), as well as diatoms and Hydrurus sp. (Chrysophyta), were found. The studied community of microorganisms in freshwater bodies feeding the historical fountains of Peterhof is not only composed of prokaryotes. Eukaryotic algae make up a noticeable share of the autotrophic component. In all studied habitats, green algae Chlorophyta (coccoid and filamentous, for example, Cladophora sp., Klebsormidium sp.), as well as diatoms and Hydrurus sp. (Chrysophyta), were found.

Carbonate Precipitation in Fresh Water with the Participation of Cyanobacterial Species In Situ and In Vitro
It is possible to trace the connection of cyanobacteria with certain types of sediments using scanning electron microscopy. In carbonate sediments from the Dry pond (Table 1), cavities are clearly visible passing through the dense carbonate sediment area (Figure 7a), corresponding to trichomes of cyanobacteria (possibly Shizothrix). In the sediment from the waterfall of Simonovskiy stream, single trichomes of cyanobacteria surrounded by thick carbonate covers are clearly visible (Figure 7b,c). Holes in the mass of sediments are also clearly visible, corresponding to cyanobacterial trichomes (Figure 7d).
In the 6 months course of the experiment on the precipitation of carbonates in laboratory conditions, the pH values of water decreased from 8.5-8.9 to 6.0, which could be connected with the binding of free Ca 2+ ions to the biofilm EPS, the mass of which increases during the experiment and with the precipitation of calcium carbonate. In the biofilm with the domination of the cyanobacteria Phormidium spp., Geitlelinema sp., Pseudanabaena sp. grown in natural weakly alkaline water from the Symonovsky stream and Fabrichnay river, the formation of dumbbell-shaped crystals of calcium carbonates with Mg/Ca ratio ≤ 0.008 (Figure 8a), and splices 30-45 µm long, 20 µm wide (Figure 8b). For communities dominated by Lyngbya sp. the formation of intergrowths of acicular crystals characteristic of aragonite (the size of intergrowths 15-40 µm) was recorded (Figure 8c). In addition, thick carbonate covers (~15 µm) of complex composition (Mg/Ca ratio ≤ 0.05) were observed around the Lyngbya trichomes (Figure 8d). Raman data showed the presence of calcite.
Calcite crystallization was also detected by Raman spectroscopy in experiments with monocultures of cyanobacteria. Calcite formation was obtained in experiments with 13 out of 16 strains (Synechococcus sp., Lyngbya aerugineocoerulca f. minor, Phormidium subfuscum, Phormidium favosum, Phormidium sp., Stigonema hormoides, Gloetrichia echinulate, Oscillatoria sp., Nostoc sp., Calothrix sp., Gloeocapsa sp.). Crystallization was not detected in the following cases: In the 6 months course of the experiment on the precipitation of carbonates in laboratory conditions, the pH values of water decreased from 8.5-8.9 to 6.0, which could be connected with the binding of free Ca 2+ ions to the biofilm EPS, the mass of which increases during the experiment and with the precipitation of calcium carbonate. In the biofilm with the domination of the cyanobacteria Phormidium spp., Geitlelinema sp., Pseudanabaena sp. grown in natural weakly alkaline water from the Symonovsky stream and Fabrichnay river, the formation of dumbbell-shaped crystals of calcium carbonates with Mg/Ca ratio ≤ 0.008 (Figure 8a), and splices 30-45 μm long, 20 μm wide (Figure 8b). For communities dominated by Lyngbya sp. the formation of intergrowths of acicular crystals characteristic of aragonite (the size of intergrowths 15-40 μm) was recorded (Figure 8c). In addition, thick carbonate covers (~15 μm) of complex composition (Mg/Ca ratio ≤ 0.05) were observed around the Lyngbya trichomes (Figure 8d). Raman data showed the presence of calcite. Only two species of cyanobacteria-Phormidium sp. and Leptolyngbya sp.-were found in all types of sediments. Sediments with magnesian calcite and aragonite were recorded in the presence of the 15 species of 12 genera (Table 5). Metagenomic and morphological analysis data revealed 17 of 24 genera in sediments with calcite. Calcite and dolomite precipitation is in close association with EPS of coccoid organisms and cyanobacteria in springs [47]. In our studies, dolomite was found in sediment in the presence of Leptolyngbya sp., Phormidium sp., Geitlerinema sp. However, it was not observed in experiments on the carbonate sedimentation with these cyanobacterial species; this confirms that the dolomite is terrigenous.  Lyngbya appears to be closely related to carbonate formation sites. Lyngbya species are characterized by the presence of a well-visible mucous cover, which is most likely associated with the processes of carbonate sedimentation. At the same time, aragonite was swollen in the Dry pond, which suggests that Schizothrix sp. affects the sedimentation of carbonates in the form of aragonite in the Dry pond and the Olginsky channel.
The genus Phormidium (in particular, Phormidium incrustatum) is the most frequently encountered and studied in carbonate sediments (Table 6) [6,7,20,48]. In our study, the species of the genus Phormidium are also among the frequently occurring species in biofilms associated with carbonate sediments in nature. Experiments in natural water and the Gromov-6 medium confirm the participation of Phormidium in calcite formation.
Obviously, the formation of carbonate sediments can be associated with differences in the cyanobacterial species but also with micro conditions that determine the development of cyanobacteria at a particular point. It was for still water that the formation of loose carbonate sediments was noted, in which the presence of aragonite was found. We also found that Schizothrix sp. is associated with the formation of aragonite in the studied water bodies. Earlier it was reported Schizothrix calcicola is involved in the deposition of carbonate sediments [49]. Only two species of cyanobacteria-Phormidium sp. and Leptolyngbya sp.-were found in all types of sediments. Sediments with magnesian calcite and aragonite were recorded in the presence of the 15 species of 12 genera (Table 5). Metagenomic and morphological analysis data revealed 17 of 24 genera in sediments with calcite. Calcite and dolomite precipitation is in close association with EPS of coccoid organisms and cyanobacteria in springs [47]. In our studies, dolomite was found in sediment in the presence of Leptolyngbya sp., Phormidium sp., Geitlerinema sp. However, it was not observed in experiments on the carbonate sedimentation with these cyanobacterial species; this confirms that the dolomite is terrigenous.
Lyngbya appears to be closely related to carbonate formation sites. Lyngbya species are characterized by the presence of a well-visible mucous cover, which is most likely associated with the processes of carbonate sedimentation. At the same time, aragonite was swollen in the Dry pond, which suggests that Schizothrix sp. affects the sedimentation of carbonates in the form of aragonite in the Dry pond and the Olginsky channel.
The genus Phormidium (in particular, Phormidium incrustatum) is the most frequently encountered and studied in carbonate sediments (Table 6) [6,7,20,48]. In our study, the species of the genus Phormidium are also among the frequently occurring species in biofilms associated with carbonate sediments in nature. Experiments in natural water and the Gromov-6 medium confirm the participation of Phormidium in calcite formation.
Obviously, the formation of carbonate sediments can be associated with differences in the cyanobacterial species but also with micro conditions that determine the development of cyanobacteria at a particular point. It was for still water that the formation of loose carbonate sediments was noted, in which the presence of aragonite was found. We also found that Schizothrix sp. is associated with the formation of aragonite in the studied water   Results of this work complement the information on the participation of different cyanobacteria in carbonate biomineralization. Based on the literature data and our research, the most promising in mineralization are filamentous nonheterocytic cyanobacteria from the families Phormidiaceae, Oscillatoriaceae, and Leptolyngbyacea. This has been noted both in nature and under experimental conditions. These organisms are capable of forming dense mats with high amounts of EPS. The presence of EPS controls the spatial distribution of carbonate crystals, being a low-energy substrate for crystal nucleation [54]. Additionally, organic compounds usually play a decisive role in carrying out precisely the initial stage of carbonate deposition into aqueous magnesium and aragonite carbonates [55].
Earlier, we carried out work on the determination of carbonate sediment in the fountains of Peterhof [19], the results of which revealed a greater variety of carbonates. At the same time, the diversity of cyanobacteria and other community members is higher in natural water bodies feeding the fountains. This is probably due to the anthropogenic load on the Peterhof fountains itself, reflected by the presence of heavy metals-related (Pb, Sr) carbonates on fountains sculptures [19]. Conversely, previous studies of water from the Peterhof water supply system showed low content of heavy metals [56]. As a result, calcium (monohydrocalcite, aragonite), magnesium (dypingite, nesquehonite), lead, and strontium carbonates (strontianite-cerussite series) crystallize in the fountains [13].
Thus, it can be concluded that cyanobacteria in fresh water affect the precipitation of carbonate phases. Experiments with monocultures of cyanobacteria show different calcite morphologies and suggest that the morphology of carbonate sediments depends on the species composition of cyanobacteria. On the other hand, the differences in phase composition (i.e., mineral species) are influenced by abiotic factors. Additionally, diatoms are able to secrete an EPS such as cyanobacteria, forming colonies with a mucous matrix. These organisms could be important contributors to the mineralization process [20]. This process can also be due to the activity of bacteria symbionts of diatoms [57]. However, aerobic organisms are not the only organisms noted for the processes of carbonate sedimentation. Anaerobic photosynthetic bacteria are also able to reduce the concentration of Ca and form CaCO 3 (mainly, calcite with some vaterite and monohydrocalcite) [49].
Despite the complexity of the microbial community at the sites of carbonate formation in fresh water, this study clearly showed that cyanobacteria play dominant roles in the biofilm community by forming their mucous matrix, thus affecting the carbonate sediment formation.

Conclusions
In the course of this work, carbonates sediments and cyanobacterial communities of the water supply system of Peterhof were studied. The interrelationships between the mineral composition of carbonate sediments and inhabiting microorganism species were established. It was shown that carbonate sediments are predominantly composed of calcite with a subordinate amount of aragonite and have a much lower mineral diversity in comparison with those studied by us earlier in the Peterhof fountains [19]. This fact can be associated with the following factors: (1) water circulation in closed systems of fountain bowls, which can locally lead to an increase in pH and magnesium concentration; (2) anthropogenic concentration of metals (e.g., Pb, Sr).
Despite the complexity of the microbial community at the sites of carbonate formation, the present study clearly showed that cyanobacteria play dominant roles in biomineralization in freshwater bodies. Using the results of model experiments and field observations, the crystallization of carbonates on the surface of 13 species of cyanobacteria was found. The significant contribution of cyanobacteria of the Oscillatoriaceae family (Phormidium spp., Lyngbya sp., Oscillatoria formosa) to carbonate crystallization was shown.