Hot in Cold: Microbial Life in the Hottest Springs in Permafrost

Chukotka is an arctic region located in the continuous permafrost zone, but thermal springs are abundant there. In this study, for the first time, the microbial communities of the Chukotka hot springs (CHS) biofilms and sediments with temperatures 54-94 degrees C were investigated and analyzed by NGS sequencing of 16S rRNA gene amplicons. In microbial mats (54-75 degrees C), phototrophic bacteria of genusChloroflexusdominated (up to 89% of all prokaryotes), whileAquificaewere the most numerous at higher temperatures in Fe-rich sediments and filamentous streamers (up to 92%). The electron donors typical forAquificae, such as H2S and H-2, are absent or present only in trace amounts, and the prevalence ofAquificaemight be connected with their ability to oxidize the ferrous iron present in CHS sediments.Armatimonadetes,Proteobacteria,Deinococcus-Thermus,Dictyoglomi, andThermotogae,as well as uncultured bacteria (candidate divisions Oct-Spa1-106, GAL15, and OPB56), were numerous, andCyanobacteriawere present in low numbers. Archaea (less than 8% of the total community of each tested spring) belonged toBathyarchaeota,Aigarchaeota, andThaumarchaeota. The geographical location and the predominantly autotrophic microbial community, built on mechanisms other than the sulfur cycle-based ones, make CHS a special and unique terrestrial geothermal ecosystem.