G03_01

Rock-leached organic carbon drives subsurface microbiomes

Heinze B^1,2, Schwab V², Trumbore S^2,3, Xu X³, Schroeter S², Chaudhari N^1,4, Küsel K^1,4

¹Aquatic Geomicrobiology, Institute of Biodiversity, Friedrich Schiller University, Jena, Germany, ²Department Biogeochemical Processes, Max-Planck-Institute for Biogeochemistry, Jena, Germany, ³Department of Earth System Science, University of California, Irvine, USA, ⁴German Center for Integrative Biodiversity Research (iDiv), Halle-Jena-Leipzig, Germany

More than 99% of global carbon is stored in sedimentary rocks, with an estimated 0.1 Gt CO₂ released every year due to weathering of rock organic carbon. However, little is known about the role of sedimentary rocks as source of energy and carbon in subsurface microbiomes. To study the extent to which rock-derived organic carbon is driving microbial activities in aquifers, we incubated crushed carbonate rocks rich in alkanes or aromatics with groundwaters under oxic and anoxic conditions. Rocks immediately leached aromatic and aliphatic substances into the groundwater, leading to a 50% increase in dissolved organic carbon (DOC). Interestingly, the Δ¹⁴C-signature of the DOC was in the range of -322 to -447‰ – ‘young’ compared to the ¹⁴C-dead rock-kerogen, suggesting the addition of younger, fresher organic materials along surfaces on rock fractures exposed to groundwater. The ¹⁴C-content of membrane lipids extracted from the POC of the incubations indicated that most of their C (96-100% in anoxic and 44-77% in oxic groundwaters) were derived from the younger rock leachate (i.e. DOC) rather than ¹⁴C-free sedimentary organic matter. Replicate incubations amended with ¹³C-labelled inorganic C showed little to no autotrophic C fixation over the time of incubation, excluding DIC as an important C source. Metagenomics identified potential bacterial degraders of rock-derived hydrocarbons within Desulfosporosinus and Dechloromonas (anoxic), and Rhodoferax, and Methyloversatilis (oxic). Our findings show that besides surface-derived or primarily produced C, organic C leached from sedimentary rocks can drive subsurface ecosystem metabolism, underlining the importance of microorganisms in geogenic C turnover.