Multi-Year Biofilm Formation on Granitic Surfaces Reveals Dynamic Microbial Communities in Fennoscandian Shield Deep Groundwaters.

The deep terrestrial biosphere is the vast biome beneath the soil layer that contains the majority of the Earth's prokaryotic biomass, yet it is one of the least investigated communities. Although, estimates of deep biosphere biomass suggest biofilm cells outnumber the planktonic biomass by several orders of magnitude, most investigations target planktonic communities captured from groundwaters. This multi-year study used 16S rRNA gene sequencing to compare planktonic and biofilm communities attached to natural granitic rock, demonstrating that biofilm formation selected for taxa with distinct relative abundances and exhibited temporal development. The biofilm communities also showed a decreasing influence of introduced populations on the natural rock surfaces (macadam) present at the onset of the incubations. After two- and four-years of biofilm incubation, a community developed that was dominated by sulfur/sulfate reducing Desulfocapsaceae, Desulfobacteraceae, and BM004 along with the families UBA5619, Rhodocyclaceae, Profunditerraquicolaceae, and UBA2206. This long-term community included populations predicted to be host-associated ultra-small cells. This contrasted with previous studies of early biofilm development in deep Fennoscandian Shield groundwaters that suggested biofilm initiation was mediated by lithotrophic carbon and nitrogen fixing populations. However, metabolic predictions based upon the 16S rRNA gene-based communities also showed an autotrophic and diazotrophic community including sulfur cycling in line with the previous studies. In conclusion, this study showed long-term biofilm composition to be dissimilar to the planktonic communities with a consistent strategy for energy conservation similar to previous studies of early biofilm formation from these groundwaters.