Adaptation strategies of iron-oxidizing bacteria Gallionella and Zetaproteobacteria crossing the marine–freshwater barrier

Iron-oxidizing Betaproteobacteria and Zetaproteobacteria are generally associated with freshwater and marine environments, respectively. Despite repeated cross-environment observations of these taxa, there has been no focused exploration of genomes of marine Gallionella (Betaproteobacteria) to understand transitions between freshwater and marine habitats. Consequently, their roles in these environments remain uncertain. Here, we present strong evidence for co-occurrence of Gallionella and Zetaproteobacteria at deep-sea hydrothermal vents at the Arctic Mid-Ocean Ridges through metagenomic analyses. Phylogenomics analysis of Gallionella metagenome-assembled genomes (MAGs) suggests that seawater adaptation is an evolutionary event which occurred multiple times in distinct lineages. Similarly, several distinct evolutionary events for freshwater and terrestrial Mariprofundus and other Zetaproteobacteria are predicted. The presence of cyc2 iron oxidation genes in co-occurring marine Betaproteobacteria and Zetaproteobacteria implies an overlap in niches of these iron-oxidizers. Functional enrichment analyses reveal genetic differences between marine MAGs of both iron-oxidizing groups and their terrestrial aquatic counterparts linked to salinity adaptation. Though scanning electron microscopy confirms the presence of Fe(III) oxyhydroxide stalks where Gallionella and Mariprofundus co-occur, Gallionella MAGs from hydrothermal vents lack evidence of putative stalk formation genes. Mariprofundus is therefore the likely sole stalk-producing iron-oxidizer in this environment. Conversely, discovery of putative stalk formation genes in Mariprofundus MAGs across the marine-freshwater barrier suggests that Fe(III) oxyhydroxide stalks might not be an exclusive signature for single iron-oxidizing taxa in marine and freshwater environments. Our research provides novel insights into the iron-oxidizing capacities, stalk production, environmental adaptation, and evolutionary transitions between marine and freshwater habitats for Gallionella and Zetaproteobacteria.