Sulfur globule oxidation in green sulfur bacteria is dependent on the dissimilatory sulfite reductase system

Abstract

Green sulfur bacteria (GSB) oxidize sulfide and thiosulfate to sulfate, with extracellular globules of elemental sulfur as an 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 the formation of thiosulfate, because thiosulfate was 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 (sqrD/CT0117 and sqrF/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 sufficiently high sulfide concentrations. Thus, the DSR system in GSB may have been acquired by horizontal gene transfer as a response to a need for enhanced substrate utilization in sulfide-limiting habitats.

Mechanisms of extracellular S0 globule production and degradation in Chlorobaculum tepidum via dynamic cell–globule interactions

“…Polysulfides in the medium can then carry out several functions: (1) diffuse into other cells and reach inner membrane Dsr proteins that are required for S 0 utilization by Cba. tepidum (Holkenbrink et al, 2011), and (2) reductively open S 8 rings in S 0 globules, releasing more polysulfides. These functions are consistent with our observations that unattached cells grow and S 0 globules degrade in the absence of any cell contact.…”

Production and Consumption of Hydrogen in Hot Spring Microbial Mats Dominated by a Filamentous Anoxygenic Photosynthetic Bacterium

“…The genome sequence for C. aggregans (CP001337) shows that this bacterium has the gene encoding sulfide:quinone oxidoreductase (SQR) (4) but lacks the genes encoding dissimilatory sulfite reductase (DSR). Since DSR is essential for complete oxidation of sulfur compounds to sulfate (14), it appears that C. aggregans oxidizes sulfide to produce sulfur. Such production of sulfur by C. aggregans was supported by the microscopic observation of sulfur globules in C. aggregans cultures (data not shown).…”

Genomic insights into the uncultured genus ‘Candidatus Magnetobacterium’ in the phylum Nitrospirae

“…One gene (Peg.0221) encodes a protein with considerable similarity to sulfide:quinone oxidoreductase (sqr), which could catalyze the oxidation of sulfide (S 2 À ) to elemental sulfur (S 0 ) (Pott and Dahl, 1998;Reinartz et al, 1998). Elemental sulfur could be further oxidized to sulfite by the dissimilatory sulfite reductase encoded by dsrABDEFH genes that have been found in the sequenced genome (Dahl et al, 2005;Holkenbrink et al, 2011). In addition, genes for adenosine 5 0 -phosphosulfate reductase (Peg.0013 and Peg.0014, aprAB) and adenosine 5 0 -triphosphate (ATP) sulfurylase (Peg.0012, sat) mediating the oxidation of sulfite to sulfate have also been found in the genome.…”