Marine Fe-oxidizing Zetaproteobacteria: Historical, ecological, and genomic perspectives
The Zetaproteobacteria are a class of bacteria typically associated with marine Fe oxidizing environments. First discovered in the hydrothermal vents at Loihi Seamount, Hawaii, they have become model organisms for marine microbial Fe oxidation. In addition to deep sea and shallow hydrothermal vents, Zetaproteobacteria are found in coastal sediments, other marine subsurface environments, steel corrosion biofilms, as well as saline terrestrial aquifers and springs. Isolates from a range of environments all grow by Fe oxidation. Their success lies partly in their microaerophily, which enables them to compete with abiotic Fe oxidation at the low O2 concentrations common to Fe(II)-rich oxic/anoxic transition zones. Also, Zetaproteobacteria make a variety of biomineral morphologies as a repository for Fe(III) waste, and as attachment structures. To determine the known diversity of the Zetaproteobacteria, we have used 16S rRNA gene sequences to define 59 operational taxonomic units (OTUs), at 97% similarity. While some Zetaproteobacteria taxa appear to be cosmopolitan, various habitats enrich for different sets of Zetaproteobacteria. OTU networks show that certain Zetaproteobacteria co-exist, sharing compatible niches. These niches may correspond with adaptations to O2, H2, and nitrate availability, based on genomic analyses. Also, a putative Fe oxidation gene has been found in diverse Zetaproteobacteria taxa, suggesting that the Zetaproteobacteria evolved as specialists in Fe oxidation. In all, culture, genomic, and environmental studies suggest that Zetaproteobacteria are widespread, and therefore have a broad influence on marine and saline terrestrial Fe cycling.