Paris, E. R. | Search bioRxiv

bioRxiv2026

Anaerobic methane oxidation by ANME-2a at two molar chloride in Orca Basin

Anaerobic methane oxidation, typically mediated by consortia of archaea and bacteria, is a key process in the global methane cycle, but little is known about its upper salinity limits. We characterized the microbial methane cycle in the anoxic, hypersaline Orca Basin using metagenomics, metatranscriptomics, fluorescence in situ hybridization, and geochemical measurements at sub-meter resolution. In the brine, we detected transcriptional activity of the halophilic methylotrophic methanogen Methanohalophilus, consistent with a biological source for Orca Basin methane. In the particle-rich halocline ([~]2 M Cl-; [~]2235 meters depth), high mcrA transcription by a novel ANME-2a species was co-located with a positive shift in {delta}13C-CH4 indicative of anaerobic oxidation of methane. ANME-2a also transcribed genes for biosynthesis of the osmolyte N({varepsilon})-acetyl-{beta}-L-lysine, supporting adaptation for hypersaline conditions. At the same depth, consortia of sarcina-like archaea, likely ANME-2a, were observed in association with vibrioid and filamentous bacteria, potentially members of a halotolerant genus in the order Desulfobulbales (family SURF-16, which includes the previously identified ANME partner Seep-DBB) that were active at the same depth. At and above the oxic-anoxic interface, aerobic methane oxidation appears to be mediated by three genera of uncultivated Methylococcales bacteria. Our results double the upper salinity range of ANME-2a to [~]2 M Cl- and reveal the key microbial players in the methane bio-filter between the Orca Basin brine and overlying seawater.

microbiology↗

bioRxiv2024

Abundance, identity, and potential diazotrophic activity of nifH-containing organisms at marine cold seeps

Diazotrophic microorganisms alleviate nitrogen limitation at marine cold seeps using nitrogenase, encoded in part by the gene nifH. Here, we investigated nifH-containing organisms (NCOs) inside and outside six biogeochemically heterogeneous seeps using amplicon sequencing and quantitative real-time PCR (qPCR) of nifH genes and transcripts. We detected nifH genes affiliated with anaerobic methane-oxidizing ANME-2 archaea and sulfate-reducing Desulfobacteraceae, consistent with previous studies, but also phylogenetically and likely metabolically diverse organisms, including Desulfoglaeba, Candidatus Methanoliparia, and Desulfuromonadales. In total, we recovered 10,734 bona fide nifH sequence variants affiliated with 18 bacterial and archaeal phyla (17 within seeps), a subset of which were transcribed at nearly all seeps investigated. We corrected our qPCR data based on our amplicon results, which found that 71% of recovered sequences were not bona fide nifH, and we recommend a similar correction in future qPCR studies that use broad nifH primers. NifH abundance was up to three orders of magnitude higher within seeps, was highly correlated with mcrA gene abundance, and, when corrected, was negatively correlated with porewater ammonium <25 uM, consistent with the inhibition of diazotrophy by ammonium. Our findings significantly expand the known diversity of NCOs at seeps and emphasize seeps as hotspots for deep-sea diazotrophy.

microbiology↗