16 May 2024
 | 16 May 2024
Status: this preprint is open for discussion.

Unexpected scarcity of ANME Archaea in hydrocarbon seeps within Monterey Bay

Amanda Clare Semler and Anne Elizabeth Dekas

Abstract. Marine hydrocarbon seeps typically harbor a relatively predictable microbiome, including anaerobic methanotrophic (ANME) archaea. Here, we sampled two cold seeps in Monterey Bay, CA – Clam Field and Extrovert Cliff – which have been known for decades but never characterized microbiologically. Many aspects of these seeps were typical of seeps worldwide, including elevated methane and sulfide concentrations, 13C-depleted dissolved inorganic carbon, and the presence of characteristic macrofauna. However, we observed atypical microbial communities: extremely few ANME sequences were detected in either 16S rRNA or mcrA gene surveys at Clam Field (<0.1 % of total community reads), even after six months of incubation with methane in the laboratory, and only slightly more ANME sequences were recovered from Extrovert Cliff (<0.3 % of total community reads). At Clam Field, a lack of ANME mcrA transcription, a lack of methane-dependent sulfate reduction, and a linear porewater methane profile were consistent with low or absent methanotrophy. Although the reason for the scarcity of ANME is yet unclear, we postulate that non-methane hydrocarbon release excludes anaerobic methanotrophs directly or indirectly (e.g., through competitive interactions with hydrocarbon-degrading bacteria). Our findings highlight the potential for hydrocarbon seeps without this critical biofilter, with implications for their contribution to global methane emissions.

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Amanda Clare Semler and Anne Elizabeth Dekas

Status: open (until 18 Jul 2024)

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Amanda Clare Semler and Anne Elizabeth Dekas
Amanda Clare Semler and Anne Elizabeth Dekas


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Short summary
Marine hydrocarbon seeps typically host subsurface microorganisms capable of degrading methane before it is emitted to the water column. Here we describe a seep in Monterey Bay which virtually lacks known methanotrophs and where biological consumption of methane at depth is undetected. Our findings suggest that some seeps are missing this critical biofilter and that seeps may be a more significant source of methane to the water column than previously realized.