Calcium carbonate precipitating extremophilic bacteria in an Alpine ice cave

Extensive research has provided a wealth of data on prokaryotes in caves and their role in biogeochemical cycles. Ice caves in carbonate rocks, however, remain enigmatic environments with limited knowledge of their microbial taxonomic composition. In this study, bacterial and archaeal communities of the Obstans Ice Cave (Carnic Alps, Southern Austria) were analyzed by next-generation amplicon sequencing and by cultivation of bacterial strains at 10 °C and studying their metabolism. The most abundant bacterial taxa were uncultured Burkholderiaceae and Brevundimonas spp. in the drip water, Flavobacterium, Alkanindiges and Polaromonas spp. in the ice, Pseudonocardia, Blastocatella spp., uncultured Pyrinomonadaceae and Sphingomonadaceae in carbonate precipitates, and uncultured Gemmatimonadaceae and Longimicrobiaceae in clastic cave sediments. These taxa are psychrotolerant/psychrophilic and chemoorganotrophic bacteria. On a medium with Mg²⁺/Ca²⁺ = 1 at 21 °C and 10 °C, 65% and 35% of the cultivated strains precipitated carbonates, respectively. The first ~ 200 µm-size crystals appeared 2 and 6 weeks after the start of the cultivation experiments at 21 °C and 10 °C, respectively. The crystal structure of these microbially induced carbonate precipitates and their Mg-content are strongly influenced by the Mg²⁺/Ca²⁺ ratio of the culture medium. These results suggest that the high diversity of prokaryotic communities detected in cryogenic subsurface environments actively contributes to carbonate precipitation, despite living at the physical limit of the presence of liquid water.