The marine sulfate reducer Desulfobacterium autotrophicum HRM2 can switch between low and high apparent half-saturation constants for dissimilatory sulfate reduction

Studies of the kinetics of dissimilatory sulfate reduction in marine sediment have shown that a mixture of marine sulfate-reducing bacteria (SRB) can reduce sulfate with both a high and low apparent sulfate half-saturation constant (K m). However, all marine pure cultures investigated have shown only low-sulfate affinity sulfate reduction kinetics. It remains unknown whether marine high sulfate-affinity sulfate reduction is catalyzed by unknown SRB or whether known SRB possess unrecognized high-affinity sulfate reduction systems. We used 35S-sulfate incubation experiments to show that cultures of Desulfobacterium autotrophicum HMR2 will switch from low-affinity to high-affinity sulfate reduction when sulfate concentrations fall below 500 μM. The mean K m was 150 μM at high sulfate concentrations and 8 μM at lowsulfate concentrations. The high-affinity K m value is comparable to values found in SRB inhabiting freshwater sediments and D. autotrophicum cultures could deplete sulfate to below our detection limit of 25 nM. The switch in K m value was accompanied by a change in the expression of genes encoding membrane-bound transport proteins putatively involved in sulfate uptake in D. autotrophicum. Our results demonstrate that a marine sulfate reducer can efficiently reduce sulfate at both high and low sulfate concentrations, possibly by activation of different sulfate transporters in the membrane.