Sulfate- and iron-dependent anaerobic methane oxidation occurring side-by-side in freshwater lake sediment

Anaerobic oxidation of methane (AOM) is a potentially important methane sink in lake sediments, but the biogeochemistry and microbial ecology of this process are understudied. Potential electron acceptors for AOM include Fe(III) and sulfate; however, it is not clear to which extent low sulfate concentrations constrain the coupling of AOM to sulfate reduction, nor if Fe(III) reduction drives AOM directly or via a cryptic sulfur cycle. We investigated AOM pathways in the sediment of iron-rich Danish Lake orn through anoxic sediment slurry incubations with additions of C-13-labeled methane as a substrate, sulfate and Fe(III) as potential electron acceptors, and molybdate as an inhibitor of sulfate reduction. The experiments demonstrated the co-occurrence of sulfate- and iron-dependent modes of AOM, with the former supported by recycling of sulfate coupled to iron reduction. Quantitative PCR analysis demonstrated the abundance of archaea of the ANME-2d clade (Ca. Methanoperedenaceae) as likely drivers of AOM. Our study demonstrates that sulfate-dependent AOM can consume methane at sulfate concentrations typical for freshwater systems and emphasizes the importance of sulfur and iron cycling in the regulation of methane emission from freshwater sediments.