Discovery of free-living Fe-oxidizing Zetaproteobacteria at the Mid-Atlantic Ridge

Chemolithoautotrophic Fe-oxidizing bacteria play an essential role in the iron cycle on the Earth. Thus far, the majority of marine Fe-oxidizers have been associated with a novel class, Zetaproteobacteria, within the phylum Proteobacteria. Fe-oxidizing communities have been found at volcanically-active seamounts, crustal spreading centers, and in coastal waters. One conspicuous absence is at hydrothermal systems associated with the slow-crustal spreading center along the Mid-Atlantic Ridge (MAR). We used the DSV Jason to visit three well-known hydrothermal vents at the MAR: Rainbow, TAG, and Snakepit, to determine if Fe-oxidizing Zetaproteobacteria were present. Small, diffuse flow venting areas with high Fe(II) concentrations, and rust-colored microbial mats were observed at all three sites proximal to black smoker chimneys. A novel, syringe-based precision sampler was used to collect multiple, individual microbial iron mats at all three sites. Morphologically, sheath-forming Fe-oxidizers were observed at all sites, although the degree of mineralization varied substantially between the samples. Tagged pyrosequencing of the 16S rRNA gene from these samples revealed that Zetaproteobacteria were the most abundant class of any bacterial group in all the samples (5.1-92.0%; m=55.2%). Beta diversity estimates showed that there was as much variability between microbial mats at a specific vent site as there was between different vent fields. Principal component analysis revealed two distinct clusters of samples dominated by five different Zeta operational taxonomic units (OTUs). Single cell genome analysis of two representative samples was also dominated by Zetas, and confirmed the pyrosequencing results. We are in the process of associating community diversity data with in situ geochemical analysis to determine what geochemical factor(s) might be driving these patterns of diversity and community assembly. Together these data confirm that Fe-oxidizing Zetaproteobacteria are present at the MAR, and likely control Fe-oxidation at localized diffuse vent sites.