In Situ Metabolic Rates of Alkane-Degrading Sulphate-Reducing Bacteria in Hydrocarbon Seep Sediments Revealed by Combining CARD-FISH, NanoSIMS, and Mathematical Modelling

Marine hydrocarbon seeps are hotspots for sulphate reduction coupled to hydrocarbon oxidation. In situ metabolic rates of sulphate-reducing bacteria (SRB) degrading hydrocarbons other than methane, however, remain poorly understood. Here, we assessed the environmental role of Desulfosarcinaceae clades SCA1, SCA2 for degradation of n-butane and clade LCA2 for n-dodecane. Quantification by CARD-FISH showed that SCA1 constituted up to 31%, SCA2 up to 9%, and LCA2 up to 6% of cells from the recently re-classified class Deltaproteobacteria across diverse hydrocarbon seeps. Cell-specific oxidation rates estimated by stable-isotope probing combined with NanoSIMS and modelling were ~0.73 and ~2.11 fmol butane cell⁻¹ d⁻¹ for SCA1 and SCA2, respectively, and ~0.023 fmol dodecane cell⁻¹ d⁻¹ for LCA2 in sediments from Amon Mud Volcano and Guaymas Basin sediments. Cellular carbon assimilation, dissolved inorganic carbon production, and sulphate reduction rates indicated that butane-degrading SRB have higher metabolic activity than those utilising dodecane. Estimates based on in situ cell abundances, biovolumes, and cellular activities suggest that at certain seeps, clades SCA1, SCA2 and LCA2 account for nearly all sulphate reduction not driven by methane oxidation. These findings highlight the important role of alkane-degrading SRB in influencing marine carbon and sulphur cycles, particularly at seeps emitting higher hydrocarbons.