Tully, B. | bioRxiv Science

bioRxiv2017

A dynamic microbial community with high functional redundancy inhabits the cold, oxic subseafloor aquifer

The rock-hosted subseafloor crustal aquifer harbors a reservoir of microbial life that may influence global marine biogeochemical cycles. Here we utilized genomic reconstruction of crustal fluid samples from North Pond, located on the flanks of the Mid-Atlantic Ridge, a site with cold, oxic subseafloor fluid circulation within the upper basement. Twenty-one samples were collected during a two-year period at three different depths and two locations with the basaltic aquifer to examine potential microbial metabolism and community dynamics. We observed minor changes in the geochemical signatures over the two years, yet a dynamic microbial community was present in the crustal fluids that underwent large shifts in the dominant taxonomic groups. An analysis of 195 metagenome-assembled genomes (MAGs) were generated from the dataset and revealed a connection between litho- and autotrophic processes, linking carbon fixation to the oxidation of sulfide, sulfur, thiosulfate, hydrogen, and ferrous iron in a diverse group of microorganisms. Despite oxic conditions, analysis of the MAGs indicated that members of the microbial community were poised to exploit hypoxic or anoxic conditions through the use of microaerobic cytochromes and alternative electron acceptors. Temporal and spatial trends from the MAGs revealed a high degree of functional redundancy that did not correlate with the shifting microbial community membership, suggesting functional stability in mediating subseafloor biogeochemical cycles.

microbiology↗

bioRxiv2017

Undocumented Potential For Primary Productivity In A Globally-Distributed Bacterial Photoautotroph

Aerobic anoxygenic phototrophs (AAnPs) are common in the global oceans and are associated with photoheterotrophic activity. To date, AAnPs have not been identified in the surface ocean that possess the potential for carbon fixation. Using the Tara Oceans metagenomic dataset, we have reconstructed draft genomes of four bacteria that possess the genomic potential for anoxygenic phototrophy, carbon fixation via the Calvin-Benson-Bassham cycle, and the oxidation of sulfite and thiosulfate. Forming a monophyletic clade within the Alphaproteobacteria and lacking cultured representatives, the organisms compose minor constituents of local microbial communities (0.1-1.0%), but are globally distributed, present in multiple samples from the North Pacific, Mediterranean Sea, the East Africa Coastal Province, and the South Atlantic. These organisms represent a shift in our understanding of microbially-mediated photoautotrophy in the global oceans and provide a previously undiscovered route of primary productivity.\n\nSignificance StatementIn examining the genomic content of organisms collected during the Tara Oceans expedition, we have identified a novel clade within the Alphaproteobacteria that has the potential for photoautotrophy. Based on genome observations, these organisms have the potential to couple inorganic sulfur compounds as electron donors to fix carbon into biomass. They are globally distributed, present in samples from the North Pacific, Mediterranean Sea, East Africa Coastal Current, and the South Atlantic. This discovery may require re-examination of the microbial communities in the global ocean to understand and constrain the impacts of this group of organisms on the global carbon cycle.

microbiology↗