At deep-sea hydrothermal vents, microbial communities thrive across geochemical gradients above, at, and below the seafloor. In this study, we determined the gene content and transcription patterns of microbial communities and specific populations to understand the taxonomy and metabolism both spatially and temporally across geochemically different diffuse fluid hydrothermal vents. Vent fluids were examined via metagenomic, metatranscriptomic, genomic binning, and geochemical analyses from Axial Seamount, an active submarine volcano on the Juan de Fuca Ridge in the NE Pacific Ocean, from 2013 to 2015 at three different vents: Anemone, Marker 33, and Marker 113. Results showed that individual vent sites maintained microbial communities and specific populations over time, but with spatially distinct taxonomic, metabolic potential, and gene transcription profiles. The geochemistry and physical structure of each vent both played important roles in shaping the dominant organisms and metabolisms present at each site. Genomic binning identified key populations of SUP05, Aquificales and methanogenic archaea carrying out important transformations of carbon, sulfur, hydrogen, and nitrogen, with groups that appear unique to individual sites. This work highlights the connection between microbial metabolic processes, fluid chemistry, and microbial population dynamics at and below the seafloor and increases understanding of the role of hydrothermal vent microbial communities in deep ocean biogeochemical cycles.
The chemolithoautotrophic microbial community of the rocky subseafloor potentially provides a large amount of organic carbon to the deep ocean, yet our understanding of the activity and metabolic complexity of subseafloor organisms remains poorly described. A combination of metagenomic, metatranscriptomic, and RNA stable isotope probing (RNA-SIP) analyses were used to identify the metabolic potential, expression patterns, and active autotrophic bacteria and archaea and their pathways present in low-temperature hydrothermal fluids from Axial Seamount, an active submarine volcano. Metagenomic and metatranscriptomic results showed the presence of genes and transcripts for sulfur, hydrogen, and ammonium oxidation, oxygen respiration, denitrification, and methanogenesis, as well as multiple carbon fixation pathways. In RNA-SIP experiments across a range of temperatures under reducing conditions, the enriched 13C fractions showed differences in taxonomic and functional diversity. At 30°C, and were the only autotrophs present at 80°C. Correspondingly, the predominant CO2 fixation pathways changed from the reductive tricarboxylic acid (rTCA) cycle to the reductive acetyl-CoA pathway with increasing temperature. By coupling RNA-SIP with meta-omics, this study demonstrates the presence and activity of distinct chemolithoautotrophic communities across a thermal gradient of a deep-sea hydrothermal vent.
In August 2014, I gave an oral presentation at the International Society for Microbial Ecology (ISME) 15th International Symposium in Seoul, South Korea highlighting interesting findings from work looking at the active autotrophic community of the subseafloor. My talk was part of the session entitled “Microbiomes of marine ecosystems: key functions from the cryosphere to the deep biosphere” which hosted an array of talks discussing various aspects of extreme life. Subseafloor communities have the potential to influence ocean biogeochemistry and, in particular, the chemolithoautotrophic populations could potentially provide a large amount of new production to the deep sea. Yet the contribution of and the mechanisms behind the different redox-driven autotrophic metabolisms and the extent of the carbon produced from these metabolic reactions have not been well described. Our findings, using both ‘omics and RNA-SIP techniques, show the presence of an active and metabolically diverse subseafloor chemolithoautotrophic community. Our RNA-SIP results also show there are specific taxonomic groups and autrophic metabolisms that dominate at the different temperatures of the subseafloor. Thanks to the support of C-DEBI, I was able to present this work at the high profile international ISME conference and as a result was also able to learn about other exciting new research scientists are doing to further explore subseafloor life.