Deciphering cryptic methane cycling: Coupling of methylotrophic methanogenesis and anaerobic oxidation of methane in hypersaline coastal wetland sediment

Methanogenesis has recently been shown to fuel anaerobic oxidation of methane (AOM) within the sulfate-reducing zone marine sediments, coining term “cryptic cycle”. Here we present research on relationship between methanogenesis and AOM in a shallow hypersaline pool (∼130 PSU) southern California coastal wetland. Sediment (top 20 cm) was subjected geochemical analyses, in-vitro slurry experiments, radiotracer incubations using 35S-SO42−, 14C-mono-methylamine, 14C-CH4, study sulfate reduction, methylotrophic methanogenesis, AOM. An adapted radioisotope method used follow cryptic cycling 14C-mono-methylamine labeling with increasing incubation times (1 hour three weeks). Results showed peaks (max 13 nmol cm−3 d−1) reduction activity 728 top 6 cm. Below cm, continued 15 d−1), while absent despite 67 mM sulfate, suggesting coupled iron. Methane concentrations were low (<50 nM) throughout sediment. Batch sediment methylated substrates (mono-methylamine methanol) stimulated pointing presence methanogens. Incubations revealed simultaneous through stepwise transfer 14C from mono-methylamine CO2 via methane. Our results suggest that is crucial process wetland sediments prevent buildup zone. We propose largely overlooked wetlands resulting incomplete understanding carbon this environment.