Furthermore, our data suggest that methylamine is utilized by both methanogenic archaea capable of methylotrophic methanogenesis and non-methanogenic microbial groups. The rapid cycling of carbon between methanogenesis and methanotropy likely prevents major build-up of methane in the sulfate-reducing zone. We conclude that the results presented in this work show strong evidence of cryptic methane cycling occurring within the top 20 cm of sediment in the Santa Barbara Basin. Discrepancies between the rate constants (K 1) of methanogenesis (from 14C- mono-methylamine) and AOM (from either 14C- mono-methylamine-derived 14C-CH 4 or from directly injected 14C-CH 4) suggest the activity of a separate, concurrent metabolic process directly metabolizing mono-methylamine to inorganic carbon. Estimated methanogenesis from mono-methylamine ranged from 0.2 nmol to 0.5 nmol cm -3 d -1. Metabolomic analysis showed that substrates for methanogenesis (i.e., acetate, methanol and methylamines) were mostly below the detection limit in the porewater, but some samples from the 1–2 cm depth section showed non-quantifiable evidence of these substrates, indicating their rapid turnover. Porewater sulfate concentrations remained high (~23 mM to ~29 mM), despite the detection of sulfate reduction activity from 35S-SO 4 2- incubations with rates up to 134 nmol cm -3 d -1. Methane concentrations were consistently low (~3 to ~16 µM) across the depth transect, despite AOM rates increasing with decreasing water depth (from max 0.05 nmol cm -3 d -1 at the deepest station to max 1.8 nmol cm -3 d -1 at the shallowest station). The top 0–20 cm of sediment from each station was subjected to geochemical analyses and radiotracer incubations using 35S-SO 4 2-, 14C-mono-methylamine, and 14C- CH 4 to find evidence of cryptic methane cycling. Here we present evidence of cryptic methane cycling activity within the upper regions of the sulfate-reducing zone, along a depth transect within the Santa Barbara Basin, off the coast of California, USA. The recently discovered cryptic methane cycle in the sulfate-reducing zone of marine and wetland sediments couples methylotrophic methanogenesis to anaerobic oxidation of methane (AOM).
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