Two new Vibrio diazotrophicus strains isolated from deep-sea hydrothermal iron-rich microbial mats display unexpected metabolic capacities

Iron-rich microbial mats at low-temperature sites in deep-sea hydrothermal environments make a significant contribution to element cycling, especially the iron cycle. From these mats collected at the MIR zone (TAG hydrothermal field, Mid-Atlantic Ridge), enrichment cultures were performed to isolate strains involved in iron metabolism. Two strains, affiliated to Vibrio diazotrophicus (HER2-O and HER2-R), were isolated from this environment and might be able to oxidize iron under autotrophic and anaerobic or microaerophilic conditions, in addition to grow under organoheterotrophic conditions. In particular, genomic analyses revealed the presence of a c4-type cytochrome (cyc1) that could be involved in electron transfer in the Fe(II)-oxidation pathway and a complete inorganic carbon fixation pathway (reductive glycine pathway), arguing in favor of growth based on autotrophic metabolism, using iron as an energy source. Prophages were identified in both genomes. Caudoviricetes-type virus morphotypes were evidenced by direct observations during growth under organoheterotrophic conditions. Genes providing metabolic adaptations to deep-sea hydrothermal vent conditions were also identified and could confer an ecological advantage to these two novel strains to thrive and compete effectively in metal-rich deep-sea environments. Their unexpected abilities regarding their potential to oxidize Fe(II), in addition to their physiological capacities, may help to explain their presence in iron-rich microbial mats.