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

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: soxXY₁Z₁AB and soxCDY₂Z₂H, and sulfate was the sole metabolic end product. By contrast, the oil field strain CVO has only the soxCDY₂Z₂H cluster and not soxXY₁Z₁AB. Despite the absence of the soxXY₁Z₁AB cluster, strain CVO oxidized S⁰ to thiosulfate and sulfate, demonstrating that soxCDY₂Z₂H 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 soxCDY₂Z₂H cluster are widespread suggesting this mechanism of thiosulfate formation is environmentally significant.