The serpentinite-hosted Lost City hydrothermal field is a remarkable submarine ecosystem in which geological, chemical, and biological processes are intimately interlinked. Reactions between seawater and upper mantle peridotite produce methane- and hydrogen-rich fluids, with temperatures ranging from <40 degrees to 90 degrees C at pH 9 to 11, and carbonate chimneys 30 to 60 meters tall. A low diversity of microorganisms related to methane-cycling Archaea thrive in the warm porous interiors of the edifices. Macrofaunal communities show a degree of species diversity at least as high as that of black smoker vent sites along the Mid-Atlantic Ridge, but they lack the high biomasses of chemosynthetic organisms that are typical of volcanically driven systems.

SummaryAt 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.

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[1] Phase separation at mid-ocean ridge hydrothermal systems is a widely distributed frequently occurring process. It is highly sensitive to dynamic subsurface pressure and temperature conditions and has a significant impact on chloride concentrations of hydrothermal fluid. Although vapor phases produced by phase separation are observed in a variety of hydrothermal settings, observation of conjugate brines that must be produced concurrently are more rare and the fate of subsurface brines is still not well understood. Here we use an array of in situ chloride sensors deployed at the Main Endeavor Field on the Juan de Fuca Ridge in the wake of a magmatic event to monitor the behavior of hydrothermal vent effluent with high temporal resolution. Our results provide evidence of near-critical chloride depleted fluids within the crust up to 486 m beneath the seafloor. We estimate that a brine reservoir stored beneath the Main Endeavor Field is constrained to temperatures between 131°C and 326°C for chloride concentrations ranging from 6 to 25 Wt.% NaCl, respectively. Our data suggest that subsurface fluids and circulation pathways vary widely over spatial scales on the order of meters. Under these circumstances, simultaneous measurements of multiple fluid parameters at a high temporal resolution are essential to understanding subsurface hydrothermal processes associated with magmatic events.