08 May 2023
 | 08 May 2023
Status: this preprint is open for discussion.

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

Sebastian Jian Ernst Krause, Jiarui Liu, David J. Yousavich, DeMarcus Robinson, David Hoyt, Qianhui Qin, Frank Wenzhoefer, Felix Janssen, David Valentine, and Tina Treude

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-SO42-, 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-SO42- 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.

Sebastian Jian Ernst Krause et al.

Status: open (until 06 Jul 2023)

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  • RC1: 'Comment on egusphere-2023-909', Anonymous Referee #1, 14 May 2023 reply

Sebastian Jian Ernst Krause et al.

Sebastian Jian Ernst Krause et al.


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Short summary
Methane is a potent greenhouse gas and hence it is important to understand its sources and sinks in the environment. Here we present new data from organic-rich surface sediments below an oxygen minimum zone off the coast of California (Santa Barbara Basin) demonstrating the simultaneous microbial production and consumption of methane, which appears to be an important process preventing the build-up of methane in these sediments and the emission into the water column and atmosphere.