While typically investigated as a microorganism capable of extracellular electron transfer to minerals or anodes, Shewanella oneidensis MR-1 can also facilitate electron flow from a cathode to terminal electron acceptors, such as fumarate or oxygen, thereby providing a model system for a process that has significant environmental and technological implications. This work demonstrates that cathodic electrons enter the electron transport chain of S. oneidensis when oxygen is used as the terminal electron acceptor. The effect of electron transport chain inhibitors suggested that a proton gradient is generated during cathode oxidation, consistent with the higher cellular ATP levels measured in cathode-respiring cells than in controls. Cathode oxidation also correlated with an increase in the cellular redox (NADH/FMNH2) pool determined with a bioluminescence assay, a proton uncoupler, and a mutant of proton-pumping NADH oxidase complex I. This work suggested that the generation of NADH/FMNH2 under cathodic conditions was linked to reverse electron flow mediated by complex I. A decrease in cathodic electron uptake was observed in various mutant strains, including those lacking the extracellular electron transfer components necessary for anodic-current generation. While no cell growth was observed under these conditions, here we show that cathode oxidation is linked to cellular energy acquisition, resulting in a quantifiable reduction in the cellular decay rate. This work highlights a potential mechanism for cell survival and/or persistence on cathodes, which might extend to environments where growth and division are severely limited.
Chemolithoautotrophic iron oxidizers play an important role in biogeochemical cycling in deep ocean biosphere. Zetaproteobacteria are widely distributed in deep ocean biosphere and are known to be obligately dependent on Fe(II) oxidation to fix carbon dioxide to grow. Thus Zetaproteobacteria play a potentially significant role in biogeochemical iron cycling in the deep ocean biosphere. To elucidate and quantify their contribution to iron cycling and microbial communities, it is important to understand the genes involved and mechanisms of iron oxidation. This objective will require a genetic system to perform genetic manipulation in Zetaproteobacteria. Currently there are no genetic systems available owing to the low growth yield, accumulation of mineral oxides during growth and their inability to make colonies on solidified medium, a prerequisite for classical genetic techniques. I propose to develop a genetic system in Zetaproteobacteria using Mariprofundus ferrooxydans PV-1 as a model organism. Using this genetic system, a new metabolism will be provided to domesticate this strain to make colonies on solidified medium to further study iron oxidation mechanism. I propose to participate as an instructor for microbial diversity to incoming students. Furthermore, I will mentor undergraduate students to follow a research career in microbiology.