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 of marine sediments, coining the term ``cryptic methane cycle". Here we present research on the relationship between methanogenesis and AOM in a shallow hypersaline pool (similar to 130 PSU) within a southern California coastal wetland. Sediment (top 20 cm) was subjected to geochemical analyses, in-vitro slurry experiments, and radiotracer incubations using S-35-SO42-, C-14-mono-methylamine, and C-14-CH4, to study sulfate reduction, methylotrophic methanogenesis, and AOM. An adapted radioisotope method was used to follow cryptic methane cycling in 14C-mono-methylamine labeling incubations with increasing incubation times (1 hour to three weeks). Results showed peaks in AOM (max 13 nmol cm(-3) d(-1)) and sulfate reduction activity (max 728 nmol cm(-3) d(-1)) within the top 6 cm. Below 6 cm, AOM activity continued (max 15 nmol cm(-3) d(-1)), while sulfate reduction was absent despite 67 mM sulfate, suggesting AOM was coupled to the reduction of iron. Methane concentrations were low (<50 nM) throughout the sediment. Batch sediment slurry incubations with methylated substrates (mono-methylamine and methanol) stimulated methanogenesis, pointing to the presence of methylotrophic methanogens. Incubations with C-14-mono-methylamine revealed the simultaneous activity of methanogenesis and coupled AOM through the stepwise transfer of C-14 from mono-methylamine to CO2 via methane. Our results suggest that AOM is a crucial process in coastal wetland sediments to prevent the buildup of methane in the sulfate-reducing zone. We propose that cryptic methane cycling has been largely overlooked in coastal wetlands resulting in incomplete understanding of carbon cycling in this environment. (C) 2021 The Authors. Published by Elsevier Ltd.