The genetic repertoire underlying electrogenic sulphur oxidation in cable bacteria

Cable bacteria are filamentous, sulphur-oxidizing microorganisms of the Desulfobulbaceae family that conduct electrons over centimetre-scale distances, coupling sulphide oxidation in deeper sediments to oxygen reduction near the surface. Geochemical evidence demonstrates high rates of aerobic sulphide oxidation in sediments inhabited by cable bacteria. Still, the underlying physiological and molecular basis of this electrogenic sulphur metabolism remains unresolved. Previous genomic analysis proposed that cable bacteria oxidize sulphide by reversing the canonical dissimilatory sulphite reduction (Dsr) pathway. We evaluated the sulphur metabolism of cable bacteria and related Desulfobulbales through comparative genomics, using an expanded set of 31 quality-filtered cable bacteria genomes, including 7 closed assemblies. We showed that cable bacteria encode a complete Dsr pathway, including the previously missing dsrD and dsrT genes, as well as a novel gene cluster with DsrOP homologues. All Dsr genes were classified as reductive-type, and phylogenetic analyses indicated a close affiliation with those of other Desulfobulbaceae (sulphate-reducing and/or sulphur-disproportionating bacteria). In addition, several other previously unrecognized sulphur-metabolism genes were identified in both cable bacteria and closely related Desulfobulbales, including a novel subtype of sulphide:quinone oxidoreductase (SQR), a putative rhodanese–persulphide dioxygenase fusion (Rho–PDO), and a YTD gene cluster (consisting of five genes) previously proposed to be characteristic of sulphur-disproportionation lineages. Structural predictions indicate that three uncharacterized YTD-encoded proteins assemble into a DsrEFH-like double heterotrimer, albeit with highly divergent, non-orthologous sequences. Finally, we integrated publicly available transcriptomic and proteomic data to confirm the in vivo expression of these genes, with expression patterns mirroring those of Desulfolithobacter dissulfuricans and Desulfurivibrio alkaliphilus. Cable bacteria show minimal genetic divergence and little differential expression in their sulphur-metabolism genes compared to related organisms. Together, our findings challenge the idea that sulphide oxidation occurs via a reversed Dsr pathway. We propose a unique sulphur metabolism model for cable bacteria, in which a canonical reductive/disproportionating sulphur-metabolism repertoire (similar to Desulfolithobacter dissulfuricans and Desulfurivibrio alkaliphilus) is coupled to net sulphide oxidation through long-distance electron transport. Key steps include sulphide oxidation to polysulphide by SQR, putative conversion to sulphite via Rho–PDO and/or proteins encoded in the YTD cluster, and subsequent disproportionation through the Dsr pathway, where sulphide re-enters the cycle. Net sulphide oxidation and sulphate production arise because electrons are efficiently drained via long-distance electron transport, effectively coupling the metabolism to oxygen reduction.