The structure and diversity of microbiomes of glacial cryoconites (North Caucasus Region)

Studying the diversity and abundance of cryoconite biota is relevant due to global climate warming, since organo-mineral particles in their composition have a significant impact on the ice albedo decrease and, thus, increase the rate of glacier melting. Since cryoconites are "hot spots" for biota development and the only loci where soil-like bodies can form on glaciers, they contribute significantly to the cycles of biogenic elements of ice and oligotrophic ecosystems.

Samples were collected from cryoconites from the Garabashi (GBg_c) and Shkhelda (SHKg_c) glaciers as well as from moraine (Garabashi); nearby soils (Chernozem, Forest-meadow, and organo-accumulative soil) were used as controls.

GBg_c samples were characterized by potentially higher values of microbial biomass (abundance of 16 S rRNA gene copies and ITS), with maximal values in samples taken from the cracked glacier. In contrast, minimal abundance values of the studied taxonomic markers in SHKg_c were determined. The values for the samples of nearby soils occupied an intermediate position. These results may be partially explained by different colors of cryoconites, determined by differences in their biochemical composition and origin: the GBg_c were represented by "black dust", with low values of albedo and, accordingly, higher values of temperature and moisture, apparently, more favorable for microbial activity compared to the "gray" dust of the SHKg_c.

Taxonomic structure analysis revealed a specific pattern of GBg_c samples– an oligotrophic psychrophilic community with a pronounced cyanobacterial dominance was detected. Despite significant differences between cryoconites and nearby moraine in the presence of major autotrophic representatives (cyanobacteria Tychonema, Phormidesmis), the heterotrophic component is similar and is represented by a very specific set of soil microorganisms of Bacteroides, Shingomonas, Burkholderiales groups, apparently, due to the flushing out of part of the microbiome from the autotrophic microbial consortia of the glacier, explaining, as well, the grouping of these samples in the Bray-Curtis NMDS ordination. No autotrophic microbiota predominance was detected in SHKg_c, these microbiomes were typical for soils without vegetative cover, as well as without biofilms on the surface (Verrucromicrobia, Sphingomonacia, Bacteroides). A low number of phylotypes was detected for the community of the GBg_c and Сhernozem. Moreover, the alpha-diversity indices were inversely proportional to the results of microbial biomass estimation, which can be explained by greater "homogeneity" (and, apparently, narrower functional specialization) of more numerous communities.

The metabolic profile of cryoconites (according to Picrust2) is characterized by the predominance of aerobic metabolic enzymes (cytochrome c) and proteins (amino acid synthesis), indicating a potentially high level of metabolic activity of the cryoconite microbial community. These results can be explained by the reparative needs of microbial cells under the conditions of oxygenic stress and extremely low temperatures. In contrast to the control soils (especially, Chernozem), relatively low levels of the catalytic pathway and carbon exchange were determined for the cryoconites’ metabolic pathways, possibly associated with both low available carbon stocks and supply of the glacier surface, as compared to soils with higher stocks of available forms of mineral nutrition.

The work is supported by RFBR  project No 19-05-50107.