Mapping Redox Potentials in River Bed Sediment using Open Circuit Microbial Fuel Cells: An Evaluation.

Bedform driven hyporheic exchange transports solutes and heat from surface water into shallow river sediments. The advection of solutes and heat into the hyporheic zone (HZ) controls the microbial processing of environmentally important nutrients. Recent studies focused on understanding the dynamic coupling and spatial variability of physical, chemical and biological processes in the HZ. Latest advances include the tracking of oxygen dynamics and nitrogen cycling, the mapping and identification of microbial communities in both flume and natural settings, and the development of predictive models for the 'hot spots' of biochemical reactions. Despite this progress, experimental and field data sufficient for testing and development of these models are in short supply, leaving many basic questions open. Here we aim to advance the observational methods that can be used to predict the spatial distribution of specific redox reactions within streambed HZ. Specifically, we focus on the technique of using open circuit microbial fuel cells (O-MFC) for measuring redox potentials within a bedform. O-MFCs are composed of anode sensors and cathode reference cells with no external resistance. O-MFCs take advantage of the extracellular electron transfer, occurring during microbial metabolic processes, to produce voltage. In absence of resistance, the electron flow is eliminated and the electrons are stored on the O-MFC sensor. The established capacitance is indicative of microbial potential to carry specific redox reactions. Thus, it can be used to predict the redox state at specific locations within the HZ. Our study evaluates the efficacy of using O-MFCs in identifying redox conditions in the HZ starting with a controlled flume environment. The expected results should reflect spatial variability in potentials across a single bedform. Preliminary flume work with closed circuit microbial fuel cells (C-MFC) showed that the established microbial communities are responsive to induced changes such as substrate addition and introduction of oxygenated water. However, unlike O-MFC, the C-MFC are prone to voltage losses. Therefore, O-MFC may be more representative of actual microbial redox potentials. Future field deployments of O-MFCs will compare the outcomes across natural settings to those predicted by numerical models.