Fennoscandian Deep biosphere – similarities, differences and functions

The continental subsurface contains approximately 12 – 20% of Earth’s biomass. In deep rock environments this biomass dwells in aqueous spaces, fractures and pores of the rock, either attached in biofilms or free-living in the fluids. Meta-analyses have suggested a core deep biosphere microbiome from pre-collected data but face issues such as difference in sampling procedures, DNA extraction methods, negative control protocols, sequencing primers etc., which may introduce false variation. Addressing this issue and striving for consistent methodology, 7 sites of the Finnish Fennoscandian Shield was studied between 2009 – 2021 ranging in depth from 100 to 2300 m. Some of the sites have deep groundwater with more than 50 Ma residence times, whereas in other places the groundwater is considerably younger. All microbiomes contained bacteria, archaea and fungi, and the microbial community composition differed greatly between sites indicating that rock type and hydrogeochemistry play a great role in moulding the communities. Chemoheterotrophy was the universally dominant predicted metabolic strategy. The bacterial numbers in the groundwater were between < 1 to more than 5 x 10⁶ 16S rRNA gene copies mL^-1 but did not necessarily reflect sampling depth, but rather the concentration of DOC and DIC. In addition, dormant microbial communities activated within hours after introduction of CO₂ or methane. Finally, groundwater monitored at one site for 10 years showed that the microbial communities did dot remain static but varied in size (up to 100-fold) and composition over time in a cyclic manner, interchanging between mainly two distinct community compositions.