Microbial sulfate reduction in deep-sea sediments at the Guaymas Basin hydrothermal vent area: Influence of temperature and substrates

Abstract Microbial sulfate reduction was studied by a 35 S tracer technique in sediments from the hydrothermal vent site in Guaymas Basin, Gulf of California, Mexico. In situ temperatures ranged from 2.7°C in the overlying seawater to > 120°C at 30 cm depth in the hydrothermal sediment. Sulfate reduction was measured in intact cores of hydrothermal sediment at 3°, 20°, 35°, 50°, 70°, and 90°C. The maximum rates of sulfate reduction were found in the upper 0–2 cm of the sediment and ranged from 32 nmol cm −3 d −1 at 90°C to 1563 nmol cm −3 d −1 at 70°C. The rates of sulfate reduction rapidly decreased with depth in the upper 0–10 cm of the sediment and the maximal depth-integrated rate (0–10 cm) was 70.3 mmol SO 4 2− m −2 d −1 at 70°C. In comparison, the sulfate reduction rate in nonhydrothermal sediment from the vent area was 0.85 mmol m −2 d −1 at the in situ temperature of about 3°C. The high subsurface rates of sulfate reduction in the hydrothermal vent area was attributed to an enhanced local substrate availability. In slurries of hydrothermal sediment, incubated at 10–120°C, microbial sulfate reduction extended to 102°C, and different temperature groups of microbial sulfate reducers had optimum temperatures at around 34°, 70°, and 80–88°C. The overall temperature response of thermophilic sulfate reduction was similar in hydrothermal sediment sampled at different sites. Addition of short-chain fatty acids and yeast extract to the sediment slurries stimulated sulfate reduction rates at all incubation temperatures. No sulfate reduction was detected in the temperature range from 102–120°C. Microbial rather than thermochemical sulfate reduction could be a possible source of H 2 S in sulfide deposits with formation temperatures at about 100°C.