A novel mesophilic, hydrogen-, and sulfur-oxidizing bacterium, designated strain ST-419T, was isolated from a deep-sea hydrothermal vent plume on the Carlsberg Ridge of the Northwestern Indian Ocean. The isolate was a Gram-staining-negative, non-motile and coccoid to oval-shaped bacterium. Growth was observed at 4–50 °C (optimum 37 °C), pH 5.0–8.6 (optimum pH 6.0) and 1.0–5.0 % (w/v) NaCl (optimum 3.0 %). ST-419T could grow chemlithoautotrophically with molecular hydrogen, sulfide, elemental sulfur and thiosulfate as energy sources. Molecular oxygen, nitrate and elemental sulfur could be used as electron acceptors. The predominant fatty acids were C16 : 1ω7c, C18 : 1ω7c and C16 : 0. The major polar lipids were phosphatidylethanolamine, diphosphatidylglycerol and phosphatidylglycerol. The respiratory quinone was menaquinone MK-6 and the G+C content of the genomic DNA was 42.4 mol%. Phylogenetic analysis based on 16S rRNA gene sequences revealed that ST-419T represented a member of genus Sulfurovum and was most closely related to Sulfurovum riftiae 1812ET, with 97.6 % sequence similarity. The average nucleotide identity (ANI) and digital DNA–DNA hybridization (dDDH) values between ST-419T and S. riftiae 1812ET were 74.6 and 19.6 %, respectively. The combined genotypic and phenotypic data indicate that ST-419T represents a novel species within the genus Sulfurovum , for which the name Sulfurovum indicum sp. nov. is proposed. The type strain is ST-419T (=MCCC 1A17954T=KCTC 25164T).

The genus Alcanivorax is common in various marine environments, including in hydrothermal fields. They were previously recognized as obligate hydrocarbonoclastic bacteria, but their potential for autotrophic carbon fixation and Fe(II)-oxidation remains largely elusive. In this study, an in situ enrichment experiment was performed using a hydrothermal massive sulfide slab deployed 300 m away from the Wocan hydrothermal vent. Furthermore, the biofilms on the surface of the slab were used as an inoculum, with hydrothermal massive sulfide powder from the same vent as an energy source, to enrich the potential iron oxidizer in the laboratory. Three dominant bacterial families, Alcanivoraceae, Pseudomonadaceae, and Rhizobiaceae, were enriched in the medium with hydrothermal massive sulfides. Subsequently, strain Alcanivorax sp. MM125-6 was isolated from the enrichment culture. It belongs to the genus Alcanivorax and is closely related to Alcanivorax profundimaris ST75FaO-1T (98.9% sequence similarity) indicated by a phylogenetic analysis based on 16S rRNA gene sequences. Autotrophic growth experiments on strain MM125-6 revealed that the cell concentrations were increased from an initial 7.5 × 105 cells/ml to 3.13 × 108 cells/ml after 10 days, and that the δ13CVPDB in the cell biomass was also increased from 234.25‰ on day 2 to gradually 345.66 ‰ on day 10. The gradient tube incubation showed that bands of iron oxides and cells formed approximately 1 and 1.5 cm, respectively, below the air-agarose medium interface. In addition, the SEM-EDS data demonstrated that it can also secrete acidic exopolysaccharides and adhere to the surface of sulfide minerals to oxidize Fe(II) with NaHCO3 as the sole carbon source, which accelerates hydrothermal massive sulfide dissolution. These results support the conclusion that strain MM125-6 is capable of autotrophic carbon fixation and Fe(II) oxidization chemoautotrophically. This study expands our understanding of the metabolic versatility of the Alcanivorax genus as well as their important role(s) in coupling hydrothermal massive sulfide weathering and iron and carbon cycles in hydrothermal fields.