Sulfite disproportionation by Desulfotomaculum from marine sediments

Sulfite disproportionation is a critical pathway of global sulfur cycle since primordial Earth, yet the microbial diversity underlying this process remains incompletely explored. Here, we report an autotrophic enrichment culture from marine sediments capable of producing both sulfate and sulfide by disproportionating sulfite. This culture was dominated (>95% relative abundance) by a novel bacterial strain closely related to Desulfotomaculum profundi. Genomic analysis revealed a complete pathway for dissimilatory sulfate reduction, which likely facilitates sulfite disproportionation by operating the enzymes for sulfate reduction to sulfite in reverse. Unlike the characterized species within the Desulfotomaculum genus, this highly enriched organism apparently lacked the physiological capacity to respire sulfate when tested with acetate and formate as electron donors, suggesting a specialized metabolic adaptation for sulfite disproportionation. Our findings represent the first experimental evidence linking sulfite disproportionation to the genus Desulfotomaculum. Given the widespread occurrence of functional genes across diverse ecosystems, we propose that Desulfotomaculum-mediated sulfite disproportionation is a biogeochemically significant yet underappreciated process in sulfur-enriched modern ecosystems.IMPORTANCEBacteria within the ubiquitous genus Desulfotomaculum (known as Desulfotomaculum subcluster Ia) are key actors in the sulfur cycle across diverse terrestrial and marine anoxic environments. Historically, cultivated strains have been characterized exclusively as sulfate reducers. Here, we expand the metabolic profile of this genus by characterizing a highly enriched Desulfotomaculum population capable of autotrophic sulfite disproportionation. In contrast to known Desulfotomaculum species, the enriched population showed no detectable sulfate reduction when tested with acetate or formate. These results indicate that Desulfotomaculum lineages can actively participate in sulfate formation and autotrophic carbon fixation, suggesting an expanded role in coupling the sulfur and carbon cycles across anoxic subsurface and sedimentary ecosystems.