Sulfur Redox Cycling Dependent Abiotic Ferrihydrite Reduction by a Desulfitobacterium hafniense

Biogeochemical interactions of iron and sulfur drive their cycles in many environments, which makes understanding the mechanism of sulfur redox cycling dependent abiotic iron(III) reduction by sulfate-reducing bacteria (SRB) particularly important. Here, we present a newly isolated Desulfitobacterium hafniense strain DH with the potential of reducing sulfate, sulfite, thiosulfate, and elemental sulfur from a paddy soil. Strain DH cannot enzymatically reduce ferrihydrite, but it can abiotically reduce ferrihydrite through extracellular electron transfer with biogenic sulfide or other sulfur compounds acting as electron shuttles. Sulfite, elemental sulfur, and thiosulfate, the oxidation products of sulfide, complete the sulfur cycle between ferrihydrite and strain DH. The presence of strain DH at the ferrihydrite surface results in mineral dissolution and secondary mineralization. Proteomic analysis revealed that the expression level of sulfur reduction functional protein DsrC and a thiosulfate reductase in the sulfide/ferrihydrite amendment group was 1.79 and 1.99 times higher, respectively, than in the sulfide-only amendment setup. On the other hand, the protein expression level of DsrAB showed no significant variation. A number of carbon metabolism enzymes, including succinate dehydrogenase (2.46 fold), fumarate reductase (2.64 fold), acetyl-coenzyme A synthetase (2.64 fold) and phosphoenolpyruvate synthase (3.4 fold), were up-regulated significantly due to the stimulation of released ferrous ion that was generated during the sulfur redox cycling dependent abiotic ferrihydrite reduction process. This result confirms the important role of DsrC and thiosulfate reductase in sulfur redox cycling dependent abiotic ferrihydrite reduction. Sulfur redox cycling dependent abiotic iron oxide reduction mediated by SRB might be a widespread process in the environment.