Probing Carboxylate Anolytes for Photo-Biofuel Cells through Combination of Bioinformatics and Electrochemistry

Abstract

Photoheterotrophic purple bacterium Rhodobacter capsulatus has recently gained attention for its high halotolerance and its photo-enhanced extracellular electron transfer via exogenous quinone redox mediators, opening opportunity for self-powered decontamination of saline wastewater. Biodegradation of malate and succinate in R. capsulatus electrochemical systems has undergone prior examination, although a consistent bioelectrochemical system to comparatively study multiple carbon sources has not previously been developed. In this study, electrochemical techniques, biological assays, and computational tools were combined to evaluate malate, succinate, propionate, and lactate as oxidizable fuels in photo-enhanced microbial electrochemical systems. Specifically, cyclic voltammetry and amperometry data demonstrated that R. capsulatus generates distinctive photo-enhanced current densities dependent on both fuel identity and concentration. Bacterial growth curve studies were in agreement with the electrochemical data, indicating that lactate, which yielded the greatest bio-anodic current density (9.5 ± 1.9 µA cm-2), allowed the bacteria to grow more rapidly than the other substrates, suggesting its effectiveness as a fuel. Moreover, propionate appeared to be the fuel least efficiently utilized by R. capsulatus, having the slowest growth curve and the lowest current density generation (1.3 ± 0.2 µA cm-2). Finally, high-throughput differential gene expression analysis allowed for illuminating the physiological underpinnings of the observed differences between substrates. Most remarkably, cells grown in lactate were found to overexpress light harvesting complex II proteins and to underexpress flagellar motility proteins, which both correspond to the high photo-enhanced current density in lactate bioanodes compared to malate and propionate.