(Invited) Using the Model Bacterium Geobacter Sulfurreducens to Understand Microbial Electrochemical Conversion of Nitrogen Gas into Ammonium

There is a growing need to decrease reliance on the Haber-Bosch process. To convert nitrogen gas (N 2 ) into ammonia (NH 3 ) for fertilizers, this temperature- and pressure-intensive process consumes roughly 2–3% of global energy and releases over 740 million tons of CO 2 each year. Many microorganisms naturally convert or “fix” N 2 into ammonium (NH 4 + ) at room temperature and pressure using an enzyme called the nitrogenase. Attempts to increase NH 4 + yields from microorganisms through genetic engineering are seeing signs of success, but we lack methods to increase NH 4 + generation rates and overcome one of the largest challenges: O 2 gas. Increasing O 2 gas concentrations typically increases metabolic rates of aerobic microorganisms; however, in the case of microorganisms that fix N 2 (called diazotrophs), O 2 can shut down the nitrogenase and stop cell growth. To overcome these challenges, we are using electricity to drive N 2 fixation in exoelectrogenic bacteria. These bacteria have the unique ability to naturally transfer electrons to anode electrodes in microbial electrochemical technologies (METs). Increasing the voltage applied to METs increases the metabolic rates of exoelectrogens. The anode chamber is kept anaerobic, which eliminates O 2 -driven inhibition of the nitrogenase. We previously showed that the N 2 fixation rates of a mixed microbial community in a MET increased more than three times when the applied whole-cell potential increased from 0.7 V to 1.0 V. By adding a chemical inhibitor that prevented the incorporation of NH 4 + into larger biomolecules, NH 4 + was excreted by the cells and into the medium. Based on the acetylene reduction assay (a proxy for N 2 fixation), we estimated that we recovered about 10% of the theoretical NH 4 + generated by the cells. If close to 100% of the theoretical NH 4 + could be excreted from the cells and recovered, we predicted an energy demand of around 3 MJ/mol-NH 4 + , which is close to the range of value reported for the Haber-Bosch process. To develop METs that generate NH 4 + at competitive production rates and energy demands, we are now focusing on the model exoelectrogenic diazotroph Geobacter sulfurreducens . Using a single organism with a fully sequenced genome allows us to understand how the N 2 fixation process responds to electrochemical variables. Towards these efforts, we conducted a transcriptomic analysis of G. sulfurreducens at two fixed anode potentials: +0.15 V and −0.15 V. This