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Impact of biological crusts on soil formation in polar ecosystems

We tested the impact of biological soil crusts (BSC) at different degrees of development on soil formation in Polar ecosystems, specifically in two Arctic sites, Svalbard Island (Norway) and Tarfala (Sweden), and in an Antarctic site, Apostrophe Island (Victoria Land). In each site, slightly developed BSC (thin and made by green algae and cyanobacteria associations – SD-BSC) highly developed BSC (thick and dominated by green algae and cyanobacteria associations with/without mosses and/or lichens – HD-BSC), and moderately developed BSC (MD-BSC) with intermediate characteristics between the slightly and the highly developed, were sampled together with the AC and A horizons immediately under the crust. After separating the organic residue of the biocrust from the mineral phase by density fractionation, in the heavy fractions obtained from HD-BSC, MD-BSC and SD-BSC we determined the amount of soil organic carbon and its radiocarbon natural abundance, total nitrogen content, mineralogical assemblage by x-ray diffraction, and quality of soil organic matter (SOM) by infrared photoacoustic spectroscopy (FTIR-PAS). We found that when BSC were able to develop on stable ice-free surfaces, they modified the soil by supplying new organic substances. These new substances, in addition to diluting the old C inherited from the substrate and darkening the upper mineral horizon, promoted acidification, which is responsible for mineral weathering and neogenesis of clay minerals. With their development, BSC act as ecosystem engineers and promote soil formation in Polar ecosystems by increasing soil stability, organic matter content and nutrient availability, which indirectly improve aggregation, water holding capacity, and soil heating.

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Highly differentiated soil bacterial communities in Victoria Land macro-areas (Antarctica).

Ice-free areas of Victoria Land, in Antarctica, are characterized by different terrestrial ecosystems, that are dominated by microorganisms supporting highly adapted communities. Despite the unique conditions of these ecosystems, reports on their bacterial diversity are still fragmentary. From this perspective, 60 samples from 14 localities were analyzed. These localities were distributed in coastal sites with differently developed biological soil crusts, inner sites in the McMurdo Dry Valleys with soils lacking of plant coverage, and a site called Icarus Camp, with a crust developed on a thin locally weathered substrate of the underlying parent granitic-rock. Bacterial diversity was studied through 16S rRNA metabarcoding sequencing. Communities diversity, composition and the abundance and composition of different taxonomic groups were correlated to soil physicochemical characteristics. Firmicutes, Bacteroidetes, Cyanobacteria and Proteobacteria dominated these communities. Most phyla were mainly driven by soil granulometry, an often disregarded parameter and other abiotic parameters. Bacterial composition differed greatly among the three macrohabitats, each having a distinct bacterial profile. Communities within the two main habitats (coastal and inner ones) were well differentiated from each other as well, therefore depending on site-specific physicochemical characteristics. A core community of the whole samples was observed, mainly represented by Firmicutes and Bacteroidetes.

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Fungal diversity and functionality are driven by soil texture in Taylor Valley, Antarctica

The McMurdo Dry Valleys surface is mainly constituted from unconsolidated permafrost. Despite the combination of cold and dry conditions, transiently wetted soils close to lake edges are hotspots of intense biological activity, that can support the surrounding soil ecosystems in such extreme environments. These soils host simple microbial communities that allow easy characterization of the parameters determining microbial establishment and diversification. Soil samples were collected close to three different glacial lakes (Lake Fryxell, Lake Hoare and Lake Joyce) located along a longitudinal gradient from the lower to the upper Taylor Valley. Fungal diversity and functionality of sampled soils were studied through ITS1 metabarcoding sequencing. The correlation between the parameters describing fungal diversity (i.e. total richness, relative richness of dominant taxonomic and functional groups, and community composition) and the edaphic physicochemical parameters (i.e. pH, moisture, C, N, P, Na + , K + , Mg 2+ and Ca 2+ , cation exchange capacity, and soil granulometry) was assessed. The fungal communities showed low richness (48 ± 32 OTUs per sample). Their composition was highly diversified even within different sites close to the same lake. The main parameters affecting the diversity and composition of fungal communities were soil texture, in turn influencing the retention of water and nutrients, and physicochemical properties. This is of particular concern for the survival of these communities, given the expected environmental changes due to global warming.

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Exchangeable cations and pH drive diversity and functionality of fungal communities in biological soil crusts from coastal sites of Victoria Land, Antarctica

Abstract Ice-free regions in coastal areas of Victoria Land, Antarctica, are patchily distributed, limited in extent and characterized by a simple vegetation of lichens and mosses, growing only for a short period during the austral summer. These organisms are associated with soil particles and microorganisms (e.g., algae, microfungi and bacteria) to make up biological soil crusts (BSCs), found worldwide in cold and/or arid and semi-arid regions, where plant growth is impaired. Despite BSCs being among the most widespread ecosystems throughout coastal ice-free areas of continental Antarctica, fungal components of these communities have received little focus. Through ITS1 DNA metabarcoding of samples from 17 sites of six different localities from 73 to 77°S, in a distance scale from 29 to 411 km among different sites, we provide insights into the diversity, community composition, and functionality of fungal communities of these peculiar ecosystems, deepening our knowledge on how they are related to different edaphic variables (i.e. chemical properties and texture). Although fungal richness was low (59 ± 27 OTUs per sample), we found numerous previously unsequenced, putatively unknown fungal species representing a great part of the sampled communities. Community composition was spatially auto-correlated and appeared to be driven by site-specific differences in environmental conditions, particularly edaphic factors, such as exchangeable cations and pH. These results are of particular interest, as they give a wide characterization of the parameters determining soil colonization in a such limiting environment, especially in the light of global changes that are expected to deeply modify the conditions of this environment.

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