Abstract
Hydrothermal vents are thermally and geochemically dynamic habitats, and the organisms therein are subject to steep gradients in temperature and chemistry. To date, the influence of these environmental dynamics on microbial sulfate reduction has not been well constrained. Here, via multivariate experiments, we evaluate the effects of key environmental variables (temperature, pH, H2S, , DOC) on sulfate reduction rates and metabolic energy yields in material recovered from a hydrothermal flange from the Grotto edifice in the Main Endeavor Field, Juan de Fuca Ridge. Sulfate reduction was measured in batch reactions across a range of physico-chemical conditions. Temperature and pH were the strongest stimuli, and maximum sulfate reduction rates were observed at 50°C and pH 6, suggesting that the in situ community of sulfate-reducing organisms in Grotto flanges may be most active in a slightly acidic and moderate thermal/chemical regime. At pH 4, sulfate reduction rates increased with sulfide concentrations most likely due to the mitigation of metal toxicity. While substrate concentrations also influenced sulfate reduction rates, energy-rich conditions muted the effect of metabolic energetics on sulfate reduction rates. We posit that variability in sulfate reduction rates reflect the response of the active microbial consortia to environmental constraints on in situ microbial physiology, toxicity, and the type and extent of energy limitation. These experiments help to constrain models of the spatial contribution of heterotrophic sulfate reduction within the complex gradients inherent to seafloor hydrothermal deposits.
Highlights
Sulfate-reducing bacteria and archaea gain energy by mediating the anaerobic oxidation of organic and inorganic substrates using sulfate as an electron acceptor
Diffuse hydrothermal flow was observed on the surface of the hydrothermal flange of Grotto ranging in temperature from 2 to 18.1◦C, while hot hydrothermal fluid (Tmax = 215.6◦C) was observed in pools accumulating on the underside of an overhanging flange
Multivariate experiments that couple empirically derived data and bioenergetic modeling can significantly advance our understanding of SR within complex hydrothermal systems by placing constraints on the factor(s) most likely governing SR activity at in situ conditions
Summary
Sulfate-reducing bacteria and archaea gain energy by mediating the anaerobic oxidation of organic and inorganic substrates using sulfate as an electron acceptor. Estimates of cell-specific SR rates range from 10−4 to 100 fmol cell−1 day−1 and are often lower than those of pure cultures by orders of magnitude (Jørgensen and Bak, 1991; Ravenschlag et al, 2000; Orcutt et al, 2005; Leloup et al, 2007, 2009; Holmkvist et al, 2011a,b). SR rates have been shown to be highly heterogeneous within marine sediments due to variations in substrate and organic carbon availability, as well as other geochemical and physical factors (Canfield, 1989; Isaksen et al, 1994; Kallmeyer et al, 2002; Orcutt et al, 2005; Fike et al, 2008; Treude et al, 2009; Holmkvist et al, 2011a)
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