THERMOPHILIC BACTERIAL SULFATE REDUCTION IN DEEP-SEA SEDIMENTS AT THE GUAYMAS BASIN HYDROTHERMAL VENT SITE (GULF OF CALIFORNIA)

Sulfate reduction was studied by radiotracer methods in geothermally heated mud retrieved from an area near black smokers in the southern trough of the Guaymas Basin at 2000 m water depth. Sediment cores were taken by the submersible Alvin from three closely spaced sites. Steep hydrothermal gradients from 2.7°C at the surface up to 126°C at 75 cm depth were measured in the sediments. Extensive conversion of sulfate to H2S, with 4 mM SO42- and 15 mM H2S at 5 cm depth, indicated thermogenic reduction at depth in the upwards percolating hot pore fluid. Pyrite concentrations were high, 200 μmol S cm-3, while FeS was low near detection limit. Bacterial sulfate reduction showed maximum rates of 30-140 nmol SO42- cm-3 d-1. While mesophilic sulfate reduction occurred near the cold (2-3°C) sediment surface, extremely thermophilic activity was observed in deeper, hot layers, the actual depth dependent on the temperature profile of the particular core. In the subsurface sediment at 10-45 cm depth, optimum temperatures for sulfate reduction increased from 63 to 83°C, with corresponding maximum temperatures of 66-90°C. © 1990.