Award: OCE-1155754

Intellectual Merit. A grand challenge in microbial ecology is understanding what drives the structure of microbial communities. The work from this project focused on a specific novel class of iron-oxidizing bacteria, the Zetaproteobacteria that live at deep-sea hydrothermal vent communities. Because the Zetaproteobacteria are ancient, have unique metabolic and morphological attributes, and appear to be restricted to a well-defined redox habitat, they offer an interesting model for understanding fundamental ecological concepts that drive microbial diversity and evolution. We combined novel, high resolution sampling techniques using deep-sea submersibles, with state of the art molecular analysis methods to gain a better understanding of both the diversity of these communities and their functional role. This work involved three research expeditions to three separate vent sites along the Mid-Atlantic Ridge; the active undersea volcano, Loihi Seamount, near Hawaii, and active seamounts and spreading centers along the Mariana forearc and backarc in the western Pacific. A total of nine distinct vent sites were used as sampling sites, each would be characterized as a low-temperature (<100°C) diffuse-flow system, a type of vent less well-studied than high temperature (>200°C) focused flow vents that are epitomized by black smokers. Each of these sites is unique, in terms of seafloor characteristics, geological setting, and geochemistry, and in turn, within each site there are different vent fluid flow patterns, seafloor topologies, as well as different iron and oxygen concentrations. In total, we collected over 300 discrete samples of microbial mats from these three regions. All of the samples where iron was the prevalent energy source were dominated by Zetaproteobacteria, and a pattern emerged where specific groups of bacteria that utilized methane, or organic matter were found; in addition, several groups of bacteria of unknown function and provenance were found almost exclusively in iron-dominated mats. Together this data shows there is a unique microbiome associated with chemosynthetic microbial communities that derive energy from iron. This iron mat microbiome is very different, from the microbiome of the deep ocean water column surrounding it, or from vents that derive energy from sulfur-rich hydrothermal vent fluids, sometimes these latter vents are only a few meters away from iron mats. A new method, oligotyping, was able to discern biogeographic patterns among cosmopolitan groups of Zetaproteobacteria between the Atlantic and Pacific oceans that was not discernable using conventional methods. The use of centimeter-scale sampling techniques showed that the primary driver of differences in community composition in iron mats was due to subtle changes in temperature, fluid flow pattern, oxygen concentration, or iron concentration within a vent site. These differences due to niche selection were more significant in terms of driving community diversity than larger scale biogeographic differences between vent sites. We were also able to collect intact microbial mats from the seafloor for the first time and show there is a remarkable coordination in the way filament-producing Zetaproteobacteria grow and structure the mats they produce. These bacteria act as ecological engineers, using their growth as a way to influence the fluid flow and chemistry of their habitat and provide substrate for secondary colonizers. Other significant discoveries were the finding of a diffuse venting ?tower? nearly 10m tall in the Mariana backarc that was composed primary of biogenic iron oxides. An analysis of in situ growth rates of iron mats found iron mat accretion rates were about 2 – 3 cm per year. We also isolated new members of the Zetaproteobacteria that are able to grow on hydrogen as a substrate, in addition to iron, and sequenced the genomes of two isolates, one from the Atlantic and one from the Pacific. They represent a new genus, Ghiorsea bivora, in the Zetaproteobacteia. These organisms are now playing an instrumental role in our quest to understanding the mechanism of how bacteria gain energy from iron-oxidation. Broader Impacts. A better understanding of iron-oxidizing bacteria that include Zetaproteobacteria is of fundamental interest to scientists interested in areas of earth science and oceanography because they illustrate how microbes can fundamentally influence geochemical cycling and mineral deposition. This work has contributed substantially to that understanding, and how these unique iron-oxidizing communities contribute to the production of biogenic iron oxides and how this may influence the iron budget of the global ocean. This work has provided research training to two postdoctoral research scientists, as well as either directly or indirectly, to half a dozen undergraduates. It has resulted in over a dozen peer-reviewed scientific papers, and presentations at national or international scientific meetings. Last Modified: 10/03/2016 Submitted by: David Emerson