Determination of dissimilatory sulfate reduction rates in marine sediment via radioactive 35S tracer

Rates of dissimilatory sulfate reduction in aquatic sediments have been measured over many years with S-35-radiotracer, and the method has been continuously modified and optimized. This article discusses the sequence of procedures that constitutes the method from sediment handling before incubation, via incubation and distillation, to statistical analysis of the results. We test modifications that have been added since previous method descriptions, and we recommend sound experimental procedures. We discuss the measurement of extremely low sulfate reduction rates whereby only one count per minute labeled sulfide may be produced. We show by numerical modeling that the measured rates are mostly representative for a small volume around the point where (SO42-)-S-35 is injected and that this can be used as an advantage to avoid edge effects. Finally, we show that oxidation will spoil samples during storage unless the samples are stored frozen. The main focus is on marine sediment, but the discussions are equally relevant for freshwater.