MICROBIAL SULFATE REDUCTION IN DEEP-SEA SEDIMENTS AT THE GUAYMAS BASIN - HYDROTHERMAL VENT AREA - INFLUENCE OF TEMPERATURE AND SUBSTRATES

Microbial sulfate reduction was studied by a S-35 tracer technique in sediments from the hydrothermal vent site in Guaymas Basin, Gulf of California, Mexico. In situ temperatures ranged from 2.7-degrees-C in the overlying seawater to > 120-degrees-C at 30 cm depth in the hydrothermal sediment. Sulfate reduction was measured in intact cores of hydrothermal sediment at 3-degrees, 20-degrees, 35-degrees, 50-degrees, 70-degrees, and 90-degrees-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-degrees-C to 1563 nmol cm-3 d-1 at 70-degrees-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 SO42- m-2 d-1 at 70-degrees-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-degrees-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-degrees-C, microbial sulfate reduction extended to 102-degrees-C, and different temperature groups of microbial sulfate reducers had optimum temperatures at around 34-degrees, 70-degrees, and 80-88-degrees-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-degrees-C. Microbial rather than thermochemical sulfate reduction could be a possible source of H2S in sulfide deposits with formation temperatures at about 100-degrees-C.