Quantifying the expression of dissimilatory sulfite reductase as a metric for sulfate reduction rates

We tested whether the gene expression of dissimilatory sulfite reductase (dsr mRNA), a critical enzyme in the sulfate reduction pathway, can serve as an indicator of the rate of sulfate reduction in natural systems. We grew Desulfovibrio vulgaris in fed-batch reactors under electron-donor limiting conditions. To simulate conditions characteristic of oligotrophic environments such as anoxic aquifers, we constrained the rates of sulfate reduction from 0.1 ??M h???1 to 20 ??M h???1 (0.89 ??? 85.9 fmol cell???1d???1) by controlling the rate of formate addition into the system. We used quantitative-PCR to measure the number of dsr mRNA transcripts per cell from biomass sampled over the course of these experiments. We observed a well-defined relationship between the rate of sulfate reduction and the number of dsr mRNA transcripts per cell. Cells from reactors maintained with the highest rate of sulfate reduction contain 315 times more dsr mRNA per cell than those in reactors with the lowest reduction rate. These results suggest we might be able to infer rates of sulfate reduction in the field by measuring the amount of dsr mRNA per cell in biomass samples. Such estimates are difficult to make directly because the rate at bacteria consume reactants and generate products cannot be observed readily in many environments, such as aquifers open to groundwater flow