Influence of cell's growth phase on the sulfur isotopic fractionation during in vitro microbial sulfate reduction

• Sulfur isotope fractionation by microbial sulfate reduction was studied in different growth phase.
• The sulfur isotope fractionation is always larger in maintenance growth phase than exponential growth phase.
• Sulfate reducing bacterial in natural environment possibly grow not exponentially.

Culture experiments of sulfate reducing bacteria were conducted to produce large sulfur isotope fractionation in marine sediments. We determined the sulfur isotope fractionation factor for both the exponential growth phase and maintenance growth phase. The results show that the sulfur isotope fractionation during the maintenance phase is larger than it is in the exponential phase, irrespective of a temperature anywhere from 25 °C to 37 °C. In the natural environment, sulfate reducing bacteria may not dominantly grow exponentially. We suggest that sulfate reducers in the natural environment only metabolize at the minimum level to maintain their body. This may solve the apparent discrepancy between the large sulfur isotope fractionation observed in marine sediments and the smaller fractionation obtained from culture experiments.