Laso Perez, R. | Search bioRxiv

bioRxiv2026

Anaerobic oxidation of methane supports a minimal microbial community in a subsurface biofilm at Ginsburg Mud Volcano

Deep marine sediments generate large amounts of methane, but most of this gas is consumed by the anaerobic oxidation of methane (AOM) mediated by microscopic consortia of anaerobic methane-oxidizing archaea (ANME) and sulfate-reducing bacteria (SRB). In this study, we investigated the AOM within a sulfate-methane transition zone (SMTZ) at a depth of [~]9.6 m at the rim of the Ginsburg mud volcano in the Gulf of Cadiz. The SMTZ is supplied with sulfate from both, overlying seawater and an underlying evaporitic deposit, and it coincides with a fracture zone that hosts a visible biofilm. Here, carbon dioxide shows the strongest 13C-depletion, indicating intense methane consumption. Metagenomic and lipid biomarker analysis of the biofilm revealed an exceptionally simple microbial community dominated by ANME-1b archaea (63%), which predominantly produce strongly 13C-depleted glycerol dialkyl glycerol tetraethers and, to a lesser extent, the less common macrocyclic archaeols. The putative partner bacterium Seep-SRB1c (Desulfobacterota) is less abundant (9%). Additionally, the biofilm contained five low-abundance heterotrophs that likely rely on biomass or metabolites released from the ANME-SRB consortium. Our study highlights the presence of active methanotrophic biofilms in subsurface sediments and suggests that these communities may play an overlooked role in mitigating seafloor methane emissions.

microbiology↗

bioRxiv2024

Nitrogen cycling during an Arctic bloom: from chemolithotrophy to nitrogen assimilation

In the Arctic, phytoplankton blooms are recurring phenomena occurring during the spring-summer seasons and influenced by the strong polar seasonality. Besides, bloom dynamics is affected by nutrient availability, especially nitrogen, which is the main limiting nutrient in the Arctic. This study aimed to investigate the changes in an Arctic microbial community during a phytoplankton bloom with a special focus on the nitrogen cycle. Using metagenomic and metatranscriptomic samples from the Dease Strait (Canada) from March to July (2014), we reconstructed 176 metagenome-assembled genomes. Bacteria dominated the microbial community, although archaea reached up to 25% of genomic abundance in early spring, when Nitrososphaeria archaea actively expressed genes associated with ammonia oxidation to nitrite (amt, amoA, nirK). The resulting nitrite was presumably further oxidized to nitrate by a Nitrospinota bacterium that highly expressed a nitrite oxidoreductase gene (nxr). Since May, the constant increase in chlorophyll a indicated the occurrence of a phytoplankton bloom, promoting the successive proliferation of different groups of chemoorganotrophic bacteria (Bacteroidetes, Alphaproteobacteria and Gammaproteobacteria). These bacterial taxa showed different strategies to obtain nitrogen, whether it be from organic or inorganic sources, according to the expression patterns of genes encoding transporters for nitrogen compounds. In contrast, during summer, the chemolithotrophic organisms thriving during winter, reduced their relative abundance and the expression of their catabolic genes. Based on the functional analysis of our data, we see a transition from a community where nitrogen-based chemolitotrophy plays a relevant role, to a chemoorganotrophic community based on the carbohydrates released during the phytoplankton bloom, where different groups specialize in different nitrogen sources.

microbiology↗