Assessment of fungal diversity present in Arctic and Antarctic lakes and selection of heavy metal tolerant fungal isolates

Lakes are prominent features of the Arctic landscape and are also common in many parts of Antarctica. Various microbial communities (especially bacteria) have been characterized, including those in Antarctic and Arctic lakes, which face different extreme conditions such as low temperatures, high salinity, pH variation, seasonally high UV radiation, and low nutrient availability. However, the fungal component is often overlooked. Compared to other environments, studies on fungal communities in polar lakes are scarce, and most of them applied traditional culturing methods, which do not reveal the full diversity of the resident mycobiota. Furthermore, Arctic and Antarctic areas are generally considered pristine and unexplored environments, even if the presence of persistent pollutants, such as heavy metals, has been recorded many times. Lake ecosystems, especially small lakes and ponds are very sensitive to environmental perturbations. Although, microorganisms living in cold environments, including fungi, have been shown to cope with such pollutants. In this context we decided to assess the diversity of fungal communities present in water and sediments collected from Arctic lakes in Ny-Ålesund (Svalbard Island, High Arctic) and from Antarctic lakes in Deception and Livingston Island, using cultural methods and DNA metabarcoding. Furthermore, the study focused on the analysis of fungal cultivable fraction able to tolerate heavy metals (HMs) (iron, copper and mercury). Our results showed a total of 5980 amplicon sequence variants (ASVs), and only 102 (1.7 %) were shared between the two Polar regions. For Arctic lakes, unknown fungi dominated the sequence assemblages, suggesting the dominance of possibly undescribed fungi. The phylum Chytridiomycota was the most represented in all Arctic and Antarctic lakes, followed by Rozellomycota, Ascomycota, Basidiomycota, and the less frequent Monoblepharomycota, Aphelidiomycota, Mortierellomycota, Mucoromycota and, Neocallimastigomycota. At the genus level, the most abundant genera were psychrotolerant and cosmopolitan cold-adapted fungi including Alternaria, Cladosporium, Cadophora, Ulvella (Ascomycota), Leucosporidium and Vishniacozyma (Basidiomycota), and Betamyces (Chytridiomycota). The assemblages displayed high diversity and richness. The assigned diversity was composed mainly of taxa recognized as saprophytic fungi, followed by pathogenic and symbiotic fungi. A total of 51 fungal strains (31 filamentous fungi and 20 yeasts) were isolated from natural culture enrichments of sediment samples with 1000 ppm iron (Fe) and copper (Cu), and 100 ppm of mercury (Hg). Among them, 12 and 1 strains were able to grow up to 5000 ppm of Fe and Cu, respectively and 5 strains grew up to 500 ppm of Hg. A total of 7 strains were able to tolerate two different metals. Heavy metal tolerant isolates were assigned to the genera Pseuderotium sp., Rhodotorula sp., Cryptococcus sp. and to the species Glaciozyma antarctica, G. watsonii, G. martini, Cadophora fastigiata, Holtermanniella wattica and Mrakia robertii, already widely recognized as HMs tolerant, thus showing that fungi in these areas also have a significant role in environmental decontamination.