Geochemically induced shifts in catabolic energy yields explain past ecological changes of diffuse vents in the East Pacific Rise 9°50'N area

For a different cluster of vents from the EPR 9°50'N area (Tube Worm Pillar), thermodynamic modeling is used to examine changes in subseafloor catabolic metabolism between 1992 and 2000. These reactions are deduced from deviations in diffuse fluid compositions from conservative behavior of redox-sensitive species. We show that hydrogen is significantly reduced relative to values expected from conservative mixing. While H2 concentrations of the hydrothermal endmember fluids were constant between 1992 and 1995, the affinities for hydrogenotrophic reactions in the diffuse fluids decreased by a factor of 15 and then remained constant between 1995 and 2000. Previously, these fluids have been shown to support subseafloor methanogenesis. Our calculation results corroborate these findings and indicate that the 1992-1995 period was one of active growth of hydrogenotrophic communities, while the system was more or less at steady state between 1995 and 2000.Microorganisms have the ability to gain energy for their metabolism by promoting a large range of redox reactions. Well-known energy sources are for example aerobic oxidation of methane or hydrogen sulfide, methanogenesis, fermentation, and sulfate reduction under anaerobic conditions [1]. In habitats like hydrothermal systems or mines, lacking sunlight and organic carbon sources, the primary production depends on electron donors that are released by water-rock reactions. High-temperature (> 400°C) processes of water-rock interaction determine the composition of seawater-derived hydrothermal fluids that are equilibrated with rocks at depths as much as several kilometers (Figure 1). Upon upwelling, these fluids cool (conductively and/or adiabatically) and mix with cold seawater to varying extents. High temperature fluids, venting focused via black smoker chimneys, often show little evidence for subseafloor mixing and are typically used as "hydrothermal endmember" compositions. Commonly, sites of diffuse venting are developed