Cave Thiovulum(CandidatusThiovulum stygium) differs metabolically and genomically from marine species

Thiovulumspp. (Campylobacterota) are large sulfur bacteria that form veil-like structures in aquatic environments. The sulfidic Movile Cave (Romania), sealed from the atmosphere for ~5 million years, has several aqueous chambers, some with low atmospheric O2(~7%). The cave’s surface-water microbial community is dominated by bacteria we identified as Thiovulum. We show that this strain, and others from subsurface environments, are phylogenetically distinct from marine Thiovulum. We assembled a closed genome of the Movile strain and confirmed its metabolism using RNAseq. We compared the genome of this strain and one we assembled from public data from the sulfidic Frasassi caves to four marine genomes, including CandidatusThiovulum karukerense and Ca. T. imperiosus, whose genomes we sequenced. Despite great spatial and temporal separation, the genomes of the Movile and Frasassi Thiovulum were highly similar, differing greatly from the very diverse marine strains. We concluded that cave Thiovulumrepresent a new species, named here CandidatusThiovulum stygium. Based on their genomes, cave Thiovulumcan switch between aerobic and anaerobic sulfide oxidation using O2and NO3-as electron acceptors, the latter likely via dissimilatory nitrate reduction to ammonia. Thus, Thiovulumis likely important to both S and N cycles in sulfidic caves. Electron microscopy analysis suggests that at least some of the short peritrichous structures typical of Thiovulumare type IV pili, for which genes were found in all strains. These pili may play a role in veil formation, by connecting adjacent cells, and in the motility of these exceptionally fast swimmers.