Bacterial sulfate reduction in hydrothermal sediments of the Guaymas Basin, Gulf of California, Mexico

Depth distribution and temperature dependence of bacterial sulfate reduction were studied in hydrothermal surface sediments of the southern trough of the Guaymas Basin at 2000 m water depth. In situ temperatures ranged from 2.8°C at the sediment surface to >130°C at 30 cm depth in the proximity of active vent chimneys. Sediment cores recovered from geothermally heated mud were incubated in the laboratory at 12°C, 25°C, 35°C, 70°C, 80°C and 90°C. The peak rates of bacterial sulfate reduction, up to 2550 nmol cm−3 d−1, were found in surface sediments (0–5 cm) covered with Beggiatoa mats. In sediments with a higher diffuse flow of hydrothermal fluid, a substrate pool ascending with the fluid flow was apparently available in the subsurface sediment below 15 cm, and the thermophilic sulfate reduction rose to a subsurface maximum of 3350 nmol cm−3 d−1 at 70°C. In cold sediments, a few hundred meters outside the hydrothermal fields, sulfate reduction rates peaked at only 12 nmol cm−3 d−1, i.e. >200-fold lower. When incubated in a temperature gradient block at 31 increments over 0–120°C, the hydrothermal surface sediments revealed meso- to thermophilic optimum temperatures for sulfate reduction between 40°C and 60°C. In hydrothermal sediment from 15–20 cm depth with in situ temperatures of 71–93°C, thermo- to hyperthermophilic sulfate reduction was found in the temperature range 70–100°C. Sulfate reduction was not detected above 100°C.