Abstract
Anaerobic oxidation of methane (AOM) is a potentially important sink for methane in freshwater sediments. Assessing the contribution of AOM to methane budgets requires an understanding of AOM process kinetics, yet such information remains scarce. In the present study, we experimentally quantified the kinetics of AOM in the sediments of the Danish Lake Ørn, where sulfate‐ and iron‐dependent AOM occur side by side. The process exhibited saturation kinetics for methane, sulfate, and Fe(III), and the derived half‐saturation constants ( K m ) implied that these substrates co‐limit in situ AOM rates across concentration ranges typical for the study site and many other freshwater systems. Nevertheless, values of K m for methane (414 ± 79 μ M) and sulfate (31 ± 29 μ M) were low relative to marine systems, indicating an adaptation of the microbial community to the limnic environment. Experiments involving a synthetic analog of humic substances, anthraquinone‐2,6‐disulfonate (AQDS), demonstrated that sedimentary organic matter could play a role in alleviating restrictive Fe‐AOM kinetics. Incorporation of the obtained kinetic parameters into predictions of AOM rate profiles based on methane, sulfate, and Fe(III) concentrations revealed similar contributions of sulfate‐ and Fe(III)‐AOM pathways in the sulfate–methane transition zone. Our study expands the limited knowledge on the kinetics of microbial methane oxidation in freshwater systems and highlights the potential importance of AOM in regulating methane emissions from inland waters.