Geomicrobiological linkages between short-chain alkane consumption and sulfate reduction rates in seep sediments

Marine hydrocarbon seeps are ecosystems that are rich in methane, and, in some cases, short-chain (C₂-C₅) and longer alkanes. C₂-C₄ alkanes such as ethane, propane, and butane can be significant components of seeping fluids. Some sulfate-reducing microbes oxidize short-chain alkanes anaerobically, and may play an important role in both the competition for sulfate and the local carbon budget. To better understand the anaerobic oxidation of short-chain n-alkanes coupled with sulfate-reduction, hydrocarbon-rich sediments from the Gulf of Mexico (GoM) were amended with artificial, sulfate-replete seawater and one of four n-alkanes (C₁-C₄) then incubated under strict anaerobic conditions. Measured rates of alkane oxidation and sulfate reduction closely follow stoichiometric predictions that assume the complete oxidation of alkanes to CO₂ (though other sinks for alkane carbon likely exist). Changes in the δ¹³C of all the alkanes in the reactors show enrichment over the course of the incubation, with the C₃ and C₄ incubations showing the greatest enrichment (4.4 and 4.5‰ respectively). The concurrent depletion in the d13C of dissolved inorganic carbon (DIC) implies a transfer of carbon from the alkane to the DIC pool (-3.5 and -6.7‰ for C₃ and C₄ incubations, respectively). Microbial community analyses reveal that certain members of the class Deltaproteobacteria are selectively enriched as the incubations degrade C₁-C₄ alkanes. Phylogenetic analyses indicate that distinct phylotypes are enriched in the ethane reactors, while phylotypes in the propane and butane reactors align with previously identified C₃-C₄ alkane-oxidizing sulfate-reducers. These data further constrain the potential influence of alkane oxidation on sulfate reduction rates (SRRs) in cold hydrocarbon-rich sediments, provide insight into their contribution to local carbon cycling, and illustrate the extent to which short-chain alkanes can serve as electron donors and govern microbial community composition and density.