Disproportionation of inorganic sulfur compounds by a novel autotrophic bacterium belonging to Nitrospirota

The phylum Nitrospirota (previously known as Nitrospirae or Nitrospira) currently encompasses a limited number of bacterial species with validly published names, including sulfate-reducing bacteria (SRB) of the genus Thermodesulfovibrio. Some metagenome-assembled genomes (MAGs) of bacteria occur in this phylum, and genes involved in dissimilatory sulfur metabolism have been identified in them. Currently, however, there is no established way to discriminate SRB and sulfur-disproportionating bacteria (SDB), which obtain energy from the disproportionation of inorganic sulfur compounds. In this study, a thiosulfate-disproportionating enrichment culture was established from a hot spring microbial mat. The culture was dominated by a single species belonging to the phylum Nitrospirota, and growth of the novel bacterium was supported by disproportionation of thiosulfate and elemental sulfur. Its growth was not observed under sulfate-reducing conditions. Therefore, a comparative genomic analysis of SDB and SRB was performed using its draft genome sequence, in order to identify any genetic element that could be used as a marker for SDB. As a result, a characteristic gene cluster was identified as a putative genetic element that characterized the genomes of SDB. The gene cluster was found in some MAGs of the phylum Nitrospirota, and their corresponding bacteria may also be capable of the disproportionation of inorganic sulfur compounds.