Thermophilic sulfate‐reducing bacteria in cold marine sediment

Sulfate reduction was measured with the 35SO42− ‐tracer technique in slurries of sediment from Aarhus Bay, Denmark, where seasonal temperatures range from 0° to 15°C. The incubations were made at temperatures from 0°C to 80°C in temperature increments of 2°C to search for presence of psychrophilic, mesophilic and thermophilic sulfate‐reducing bacteria. Detectable activity was initially only in the mesophilic range, but after a lag phase sulfate reduction by thermophilic sulfate‐reducing bacteria were observed. No distinct activity of psychrophilic sulfate‐reducing bacteria was detected. Time course experiments showed constant sulfate reduction rates at 4°C and 30°C, whereas the activity at 60°C increased exponentially after a lag period of one day. Thermophilic, endospore‐forming sulfate‐reducing bacteria, designated strain P60, were isolated and characterized as D esulfotomaculum kuznetsovii. The temperature response of growth and respiration of strain P60 agreed well with the measured sulfate reduction at 50°–70°C. Bacteria similar to strain P60 could thus be responsible for the measured thermophilic activity. The viable population of thermophilic sulfate‐reducing bacteria and the density of their spores was determined in most probable number (MPN) dilutions. The density was 2.8·104 cells·.g−1 fresh sediment, and the enumerations suggested that they were all present as spores. This result agrees well with the observed lag period in sulfate reduction above 50°C. No environment with temperatures supporting the growth of these thermophiles is known in the region around Aarhus Bay.