The McMurdo Dry Valleys in Antarctica cover about 4000 square km; free of snow and ice for the 30% of the surface, they represent the largest ice-free region of the continent. The landscape includes mountain ranges, nunataks, glaciers, ice-free valleys, frozen lakes, ponds, meltwater streams, arid soils and permafrost, sand dunes, and interconnected water systems. This region represents a nearly pristine environment, largely undisturbed and uncontaminated by humans, while hosts unusual microhabitats and biological communities and unique geological features and minerals. Due to the unique geological and biological characteristics, the McMurdo Dry Valleys, as a whole, are designated as an ASMA (Antarctic Specially Managed Area) to assist planning and coordination of activities to ensure the long-term protection of this unique environment and to safeguard its values for scientific research, education, and minimize environmental impacts [1
]. The McMurdo Dry Valleys include five different ASPA (Antarctic Specially Protected Areas); each ASPA has its own management plan and require specific permits for entry.
The area encompasses a cold and extremely arid desert with mean annual temperature of –20 °C, always below the freezing point [2
], and annual precipitation less than 100 mm water equivalent, strong winds, strict oligotrophy and strong UV irradiation. These remarkably extreme conditions make the region an important analogue for the conditions of ancient Earth and Mars and is the most investigated area as a model environment for astrobiological studies [3
]. Life in these regions, where present, is mostly dominated by endolithic microbes dwelling inside rocks [6
]. The low temperature and aridity are incompatible with active life on rock surfaces, and endolithic adaptation enables microbes to exploit a protected niche as the only survival [7
]. Antarctic microbial cryptoendoliths are complex and self-supporting assemblages of phototrophic and heterotrophic microorganisms, including Bacteria, Cyanobacteria, Chlorophyta and both free-living and lichen-forming fungi [10
]. They are among the most stress-resistant organisms known to date, living to the edge of their physiological adaptability [6
]. Being well-adapted and specialized, microbes of these communities are very sensitive to any external perturbation [13
]. This sensitivity makes them important part of the early detection and warning system for Climate Change. Antarctica is prone to the most rapid climate and has five times the mean rate of global warming in some areas over the past 50 years [15
]. Since this process is likely to intensify in the future, before irreversible changes of ecosystems occur, it is critical to gain a deep understanding of Antarctic terrestrial ecosystems and to develop tools and assays to monitor future changes [16
Recent next-generation sequencing based studies have brought new insights into the biodiversity and composition of Antarctic cryptoendolithic communities. This has helped to distinguish functional guilds of the fungal component in these communities [11
], but knowledge of the factors that structure communities remains patchy.
Understanding how biodiversity varies over an extended ice-free geographic area and in response to increasing environmental pressure, individuation of key and threatened species may give tools to understand evolutionary processes in the extremes and to model how life evolves in response to rapid environmental change [19
With this in mind, we planned this study with a focus on Fungi as they are pivotal organisms influencing the nutrient balance and functionality of these extreme and highly oligotrophic ecosystems. Fungi are important members of community participating in recycling of organic matter by facilitating nutrient liberation and uptake. The main aim was to determine the shifts of fungal community structure under different degrees of environmental pressure imposed by sun exposure since a reduced insolation means lower temperatures, lower water availability in addition to reduced incidence of photosynthetic active radiation. We also aimed to identify reliable fungal indicator taxa and determine individual taxa distribution in relation to different environmental pressure.
In this study, we performed a wide survey of rocks collected in 6 locations right over the McMurdo Dry Valley, the largest ice-free area in Antarctica, sampling the main accessible sandstone outcrops and considering opposite sun-exposed surfaces to test the effect of sunlight on fungal diversity.
Using ITS1 metabarcoding, we identified Lecanoromycetes, Tremellomycetes, Dothideomycetes and Eurotiomycetes as predominant classes. The majority of the Ascomycota were identified as members of families Caliciaceae, Acarosporaceae, Lecideaceae Lecanoraceae, and Teratosphaeriaceae. Even though members of Extremaceae, Taphrinaceae, and Herpotrichiellaceae were frequently isolated in the almost last 20 years cultivation-based analysis [31
], taxa belonging to these groups were almost absent in this study. Among the identified genera, species belonging to lichenized genera as Acarospora, Lecidea,
and dothideomycetous Friedmanniomyces
spp. were the most abundant, confirming previous molecular studies [13
When taxonomic composition for each fungal class was compared in respect to sun exposure, all phyla in fungal communities did not vary across northern and southern surfaces. Among the identified twenty-five families Caliciaceae, Acarosporaceae, Lecideaceae, Lecanoraceae, and Teratosphaeriaceae were predominant (Figure 2
C). Teratosphaeriaceae was the only with significantly higher relative abundance in the southern exposed samples across the twenty-five identified families (p
= 0.05). We also identified Friedmanniomyces
as the only fungal genus that contributed significantly to endolithic sun exposure patterning as shown in Figure 2
F. These results agree with previous findings reported in Coleine et al. [18
], where authors found all functional groups of fungi more abundant in communities sampled in north-exposed rocks, with the exception of Rock-Inhabiting Fungi (RIF) and black fungi, that predominated in southern expositions, where conditions are much more extreme. Black fungi, indeed, are well known to be particularly adapted to highly diverse stressing environments such as saltpans, hydrocarbon-contaminated sites, exposed bare rocks and monuments, icy habitats, deserts and solar panels and building roofs [36
]. Their extraordinary abilities to resurrect from dry conditions, e.g., [42
] and to tolerate almost chemical and physical stresses including extreme pH, high and low temperature, desiccation, UV and ionizing radiation and alpha particles [43
], allow these extremo-tolerant organisms to succeed when conditions are incompatible for the most [50
Due to the harshest conditions of the study area, the diversity (Shannon’s index ~ 2) and richness (~90 per sample) of the fungal community observed in this study were relatively lower compared to other more temperate habitats [51
]. Although biodiversity indices did not show significant differences across northern and southern sun-exposed communities, the effect of sun exposure was reported on community composition as revealed by NMDS/PERMANOVA analysis, showing that most samples clustered by sun exposure [18
]. The effect of sunlight was also recently tested by a preliminary molecular survey, based on DGGE and qPCR techniques, of 48 rocks with north and south sun exposure, collected in Victoria Land along an altitudinal transect from 834 to 3100 m a.s.l. [52
]. There, it has been found that differences in sun radiation influenced community composition and relative abundance of the three main biological compartments (fungi, algae and bacteria). In Coleine et al. [53
], a first untargeted metabolomics approach, based on ultra-high-performance liquid chromatography (UHPLC) and Mass Spectrometry has been performed to give insights on the functionality of Antarctic endoliths and demonstrate how the metabolic response shifts across variation due to sun exposure, detecting altered metabolites unique for north and south, respectively.
We also identified a core mycobiome composed of 101 OTUs that were shared across two exposures, as represented by the overlapping areas of circles in Venn diagram, suggesting that these taxa may play important roles in the function of the community and be critical to the function of that type of community [54
]. The primary effect of sun exposure concerned OTU presence/absence in north and south sun-exposed sites. We, indeed, found 83 taxa (35% of the total) unique for north (i.e., OTUs belonging to Acarosporaceae, Caliciaceae, Catillariaceae, and Trapeliaceae) and 49 (21%) for south (i.e., OTUs belonging to Didymellaceae, Pleosporaceae, and Teratospheriaceae).
In this study, using indicator species analysis, we were able to identify marker species that may serve as a measure of the environmental conditions that exist in a given sun exposure in McMurdo Dry Valleys. We showed that differences in sun-exposed communities were detected with indicator species, supporting hypothesis that Antarctic cryptoendolithic communities are mainly structured by sunlight. Among the identified marker species for northern samples, Lecanoromycetes predominated. Lichens are considered extraordinary well adapted to the lithic lifestyle, due to their low mineral nutrient demand, high freezing tolerance, and ability to be photosynthetically active at suboptimal temperatures [55
]. Moreover, due to the large range in growth rates coupled with the simplicity of measuring lichen growth, they are regarded as an excellent tool for the detection of climate change in continental Antarctica [56
The species Friedmanniomyces endolithicus
] was, instead, found as marker species to the harshest conditions occurring in the shady, south exposed rock surfaces. This species is the most widespread and frequently isolated fungus, retrieved over 20-years of Italian Antarctic campaigns, up to 3300 m asl and 96 km of sea distance [14
], suggesting a high degree of adaptation to the prohibitive environmental conditions of Antarctic desert. Proteomic studies have highlighted that responses to sub-optimal temperature are related to a downregulation of response rather than a heat-shock protein over-expression [58
] and the ability of this fungus to tolerate acute doses of gamma radiation (up to 400 Gy) was also demonstrated [59
]. Recently, the whole genome Friedmanniomyces endolithicus
CCFEE 5311 was sequenced and assembled, resulting in 46.75 Mbp and 18,027 predicted proteins; genomic traits in response to salt, X-rays, cold and DNA damage stresses have been identified, confirming exceptional poly-heterotolerance of this species to survive across a wide variety of stresses [60
In conclusion, in this study, based on the largest rocks survey in the McMurdo Dry Valleys, we reliably demonstrated that sun exposure has an extensive effect mainly on the fungal diversity and composition of the Antarctic cryptoendolithic communities and we were able to identify, to our knowledge, for the first time, the specific taxa that were associated with differently sun-exposed habitats.