Aerobic turnover of dimethyl sulfide by the anoxygenic phototrophic bacterium Thiocapsa roseopersicina

This is the first report describing the complete oxidation of dimethyl sulf ide (DMS) to sulfate by an anoxygenic, phototrophic purple sulfur bacterium . Complete DMS oxidation was observed in cultures of Thiocapsa roseopersici na M11 incubated under oxic/light conditions, resulting in a yield of 30.1 mg protein mmol(-1). No oxidation of DMS occurred under anoxic/light condit ions. Chloroform, methyl butyl ether, and 3-amino-1,2,4-triazole, which are specific inhibitors of aerobic DMS oxidation in thiobacilli and hyphomicro bia, did not affect DMS oxidation in strain M11. This could be due to limit ed transport of the inhibitors through the cell membrane. The growth yield on sulfide as sole electron donor was 22.2 mg protein mmol(-1) under anoxic /light conditions. Since aerobic respiration of sulfide would have resulted in yields lower than 22 mg protein mmol(-1), the higher yield on DMS under oxic/light conditions suggests that the methyl groups of DMS have served a s an additional car-bon source or as an electron donor in addition to the s ulfide moiety. The kinetic parameters V-max and K-m for DMS oxidation under oxic/light conditions were 12.4 +/- 1.3 nmol (mg protein)(-1) min(-1) and 2 mu M, respectively. T. roseopersicina M11 also produced DMS by cleavage o f dimethylsulfoniopropionate (DMSP). Specific DMSP cleavage rates increased with increasing initial substrate concentrations, suggesting that DMSP lya se was only partly induced at lower initial DMSP concentrations. A comparis on of T. roseopersicina strains revealed that only strain M11 was able to o xidize DMS and cleave DMSP. Both strain M11 and strain 5811 accumulated DMS P intracellularly during growth, while strain 1711 showed neither of these characteristics. Phylogenetic comparison based on 16S rRNA gene sequence re vealed a similarity of 99.0% between strain M11 and strain 5811, and 97.6% between strain M11 and strain 1711. DMS and DMSP utilization thus appear to be strain-specific.