Comparison of sulfide-oxidizing Sulfurimonas strains reveals a new mode of thiosulfate formation in subsurface...

Sulfur-oxidizing Sulfurimonas spp. are widespread in sediments, hydrothermal vent fields, aquifers and subsurface environments such as oil reservoirs where they play an important role in the sulfur cycle. We determined the genome sequence of the oil field isolate Sulfurimonas sp. strain CVO and compared its gene expression during nitrate-dependent sulfide oxidation to the coastal sediment isolate Sulfurimonas denitrificans. Formation of elemental sulfur (S⁰ ) and high expression of sulfide quinone oxidoreductase (SQR) genes indicates that sulfide oxidation in both strains is mediated by SQR. Subsequent oxidation of S⁰ was achieved by the sulfur oxidation enzyme complex (SOX). In the coastal S. denitrificans, the genes are arranged and expressed as two clusters: soxXY₁ Z₁ AB and soxCDY₂ Z₂ H, and sulfate was the sole metabolic end product. By contrast, the oil field strain CVO has only the soxCDY₂ Z₂ H cluster and not soxXY₁ Z₁ AB. Despite the absence of the soxXY₁ Z₁ AB cluster, strain CVO oxidized S⁰ to thiosulfate and sulfate, demonstrating that soxCDY₂ Z₂ H genes alone are sufficient for S⁰ oxidation in Sulfurimonas spp. and that thiosulfate is an additional metabolic end product. Screening of publicly available metagenomes revealed that Sulfurimonas spp. with only the soxCDY₂ Z₂ H cluster are widespread suggesting this mechanism of thiosulfate formation is environmentally significant.