Diversity and functionality of archaeal, bacterial and fungal communities in deep Archaean bedrock groundwater

The diversity and metabolic functions of deep subsurface ecosystems remain relatively unexplored. Microbial communities in previously studied deep subsurface sites of the Fennoscandian Shield are distinctive to each site. Thus, we hypothesized that the microbial communities of the deep Archaean bedrock in Romuvaara, northern Finland, differ both in community composition and metabolic functionality from the younger sites of the Fennoscandian Shield. We characterized the composition, metabolic functionality and substrate preferences of the microbial communities at different depths in a 600-m deep borehole. In contrast to other Fennoscandian deep biosphere communities studied to date, iron-oxidizing Gallionella dominated the bacterial communities, while methanogenic and ammonia-oxidizing archaea were the most prominent archaeal type, and a diverse fungal community was also detected. Potential for methane cycling and sulphate and nitrate reduction was confirmed by detection of the functional genes of these metabolic pathways. Carbohydrates were the most preferred of the tested substrates, however organotrophs were less abundant in the microbial communities. The microbial communities shared features with those detected from other deep groundwaters with similar geochemistry and/or lithology, but the majority of taxa distinctive to Romuvaara are different from the taxa previously detected in saline deep groundwater in the Fennoscandian Shield.