Identifying Redox Potentials used by Microbes within Crustal Fluid of the Juan de Fuca Ridge Flank

More than half of the bacteria on Earth are found in the oceanic and terrestrial subsurface. Recent technology advances have provided access to these subsurface environments, allowing the microbes living in these previously inaccessible environments to be collected and studied. The research presented here relied on wells drilled into the oceanic crust along the eastern flank of the Juan de Fuca Ridge in the Northwest Pacific Ocean for sample collection. In May of 2019, oceanic crustal fluid was collected to investigate the microbial metabolisms used to survive in the extreme environment of the oceanic crust. Subsequently, microbial fuel cells were used to identify energy generating oxidation/reduction reactions favored by cells collected from the oceanic crustal fluid, with a focus on iron oxidation/reduction reactions coupled to lactate oxidation. Shewanella oneidensis MR-1 was used as a positive control for iron reduction. Voltage parameters that mimic iron reduction were successfully identified when 0.4 volts vs. Ag/AgCl were applied to the working electrode. At this voltage, S. oneidensis MR-1 cells reduced iron and caused an increase in current over time, indirectly showing an increase in biomass. Unfortunately, significant current increases were not observed in the crustal fluid sample at 0.4 volts possibly due to a naturally lower biomass in the crustal fluid or microbial members in crustal fluid are unable to oxidize lactate and reduce iron at this specific voltage. Sequential data from voltage sweeps identified crustal fluid reduction events occurred at voltages that mimic iron hydroxide reduction (-0.6 volts), which will guide the next long-term experiment.