Electrochemistry of R. palustris Azul during phototrophic growth

Abstract

Facultative phototrophs are considered within the most versatile bacteria with respect to energy metabolism, since their genomes code for the machinery for living under any of the four modes of metabolism. In photoautotrophic conditions, Rhodopseudomonas palustris' growth requires electrons provided by inorganic donors to produce enough reducing equivalents to complement energy generation by light. When growing photo heterotrophically, these electrons are provided by organic donors, and the excess of reducing power is balanced through carbon dioxide (CO2) fixation or through the reduction of other electron acceptors. Here we report the electrochemical characterization of an autochthonous R. palustris strain identified as AZUL, which is capable of not only accepting electrons from an electrode (cathodic conditions) during photoautotrophic growth but also using the electrode as an electron acceptor (anodic conditions) under a photoheterotrophic conditions. In the first condition, cells were able to grow and accept electrons from an electrode polarized at negative potentials (i.e. replacing inorganic electron donors). We propose that R. palustris AZUL presents mechanisms for both direct and indirect electron uptake, as evidenced by growth curves and cyclic voltammetry experiments. When grown in anodic conditions, cells formed biofilms with a particular architecture on poised graphite electrodes. We detected a specific voltammetric profile characterized by two reduction processes at 0.2 and 0.4 V and an oxidation process at 0.57 V (vs Ag/AgCl). These redox signals progressively changed with the time of growth towards an oxidation catalysis signal with a gate potential of 0.5 V. Biofilm charge accumulation experiments indicated that this bacteria accumulates electrons that can be subsequently discharged upon polarization. The results presented herein suggest that R. palustris AZUL is capable of exchanging electrons with an electrode in a bidirectional fashion. In addition, our experiments indicate that this strain has a mechanism of redox balance that includes charge storage in redox molecules that can transfer charge to the extracellular space when a high potential acceptor is available.