Sulfide distribution in chemosynthetic communities at hydrocarbon seeps in the Gulf of Mexico

Abstract

Dense macrofaunal communities around hydrocarbon seeps in the Gulf of Mexico (GOM) are supported by the activity of chemoautotrophic microorganisms that couple the oxidation of sulfide (H₂S) or methane with the fixation of inorganic carbon into organic biomass. The connections and feedbacks between microbial activity and the geochemistry of these communities are not fully understood. The role of geochemistry in fueling microbially mediated processes was assessed by estimating sediment-water interface fluxes of H₂S. Gradient driven H₂S fluxes were calculated from fine-scale profiles and Fick's first law, for the top centimeter of sediments underlying Beggiatoa mats, tube worm bushes, and control sites. Profiles were obtained using solid-state mercury-gold amalgam microelectrodes capable of measuring O₂, H₂S, Mn²⁺, and Fe²⁺ simultaneously via square and cyclic wave voltammetry. Sediment cores were collected from GOM hydrocarbon seeps using the submersible, Johnson Sea Link. Results focused on H₂S as very low to undetectable concentrations of Mn²⁺ and Fe²⁺ were measured in porewaters. Lack of O₂ detection resulted from the high O₂ demand, which ranged from 0.006 to 0.584 mmol of O₂ hr⁻¹ according to estimates based on measured sulfate reduction rates, typical of these environments. Profiles of H₂S in cores from Beggiatoa mats showed a consistent sharp increase in concentration across the sediment-water interface with little inter-annual variation, reflecting the ability of Beggiatoa to exploit favorable chemical environments. Profiles in cores collected distally and proximally to tube worm bushes in 1997 and 1998 were generally depleted over the top three to four centimeters, respectively, reflecting the potential for H₂S uptake by underlying tube worm "roots". Fluxes in Beggiatoa mats ranged from -4.1 to -123.0 and -7.2 to -213.4 mmol m⁻² d⁻¹ in 1997 and 1998, respectively, into the overlying water. H₂S flux from cores collected from tube worm bushes ranged from -50.6 to 5.0 and 0 to -240.2 mmol m⁻² d⁻¹ in 1997 and 1998, respectively. Variations in H₂S profiles and fluxes in Beggiatoa mats, tube worms, and control sites may help delineate the distribution of different community types and their tolerance of what most consider a harsh environment.