Evidence of cryptic methane cycling and non-methanogenic methylamine consumption in the sulfate-reducing zone of sediment in the Santa Barbara Basin, California

Abstract. 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). 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 top 0–20 cm of sediment from each station was subjected to geochemical analyses and radiotracer incubations using ³⁵S-SO₄^2-, ¹⁴C-mono-methylamine, and ¹⁴C- CH₄ to find evidence of cryptic methane cycling. 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). Porewater sulfate concentrations remained high (~23 mM to ~29 mM), despite the detection of sulfate reduction activity from ³⁵S-SO₄^2- incubations with rates up to 134 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. Estimated methanogenesis from mono-methylamine ranged from 0.2 nmol to 0.5 nmol cm^-3 d^-1. Discrepancies between the rate constants (K₁) of methanogenesis (from ¹⁴C- mono-methylamine) and AOM (from either ¹⁴C- mono-methylamine-derived ¹⁴C-CH₄ or from directly injected ¹⁴C-CH₄) suggest the activity of a separate, concurrent metabolic process directly metabolizing mono-methylamine to inorganic carbon. 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. The rapid cycling of carbon between methanogenesis and methanotropy likely prevents major build-up of methane in the sulfate-reducing zone. Furthermore, our data suggest that methylamine is utilized by both methanogenic archaea capable of methylotrophic methanogenesis and non-methanogenic microbial groups. We hypothesize that sulfate reduction is responsible for the additional methylamine turnover but further investigation is needed to elucidate this metabolic activity.