Green sulfur bacteria oxidize sulfide and thiosulfate to sulfate with extracellular globules of elemental sulfur as intermediate. Here we investigated which genes are involved in the formation and consumption of these sulfur globules in the green sulfur bacterium Chlorobaculum tepidum. We show that sulfur globule oxidation is strictly dependent on the dissimilatory sulfite reductase (DSR) system. Deletion of dsrM/CT2244 or dsrT/CT2245 or the two dsrCABL clusters (CT0851-CT0854, CT2247-2250) abolished sulfur globule oxidation and prevented formation of sulfate from sulfide, whereas deletion of dsrU/CT2246 had no effect. The DSR system also seems to be involved in formation of thiosulfate, because thiosulfate is released from wild type cells during sulfide oxidation, but not from the dsr mutants. The dsr mutants incapable of complete substrate oxidation oxidized sulfide and thiosulfate about twice as fast as the wild type, while having only slightly lower growth rates (70-80% of wild type). The increased oxidation rates seem to compensate for the incomplete substrate oxidation to satisfy the requirement for reducing equivalents during growth. A mutant in which two sulfide:quinone oxidoreductases (sqrB/CT0117 and sqrD/CT1087) were deleted, exhibited a decreased sulfide oxidation rate (~50% of wild type), yet formation and consumption of sulfur globules were not affected. The observation that mutants lacking the DSR system maintain efficient growth, suggests that the DSR system is dispensable in environments with sufficient sulfide concentrations. Thus, the DSR system in GSB may have been acquired by horizontal gene transfer in a response to a need for improved substrate utilization in sulfide-limiting habitats.