Sulfate reduction in the deep biosphere.

Dissimilatory sulfate reduction is the quantitatively most important terminal electron acceptor process in marine sediments. Sulfate reducing microorganisms are known to be able to live under almost all pressure and temperature conditions that allow life. Sulfate reduction rates (SRR) in the deep sub-seafloor biosphere are several orders of magnitude lower than in shallow sediments. In most cases SRR in the deep biosphere were too low to be detected by standard radiotracer techniques. Therefore, a new radiotracer technique was developed that allows the detection of very low rates of sulfate reduction. This new technique was used to measure sulfate reduction rates in samples from the Peru continental margin to depths > 100 mbsf. Additionally to the measurements, rales were mathematically modeled. In many cases where no sulfate reduction could be measured, the model suggest rates that are lower than the minimum detection limit of the radiotracer technique.