CONICET | Buscador de Institutos y Recursos Humanos

Facultative phototrophs are considered within the most versatile bacteria with respectto energy metabolism, since their genomes code for the machinery for living under anyof the four modes of metabolism. In photoautotrophic conditions, Rhodopseudomonaspalustris´ growth requires electrons provided by inorganic donors to produce enoughreducing equivalents to complement energy generation by light. When growing photoheterotrophically, these electrons are provided by organic donors, and the excess ofreducing power is balanced through carbon dioxide (CO2) fixation or through thereduction of other electron acceptors. Here we report the electrochemicalcharacterization of an autochthonous R. palustris strain identified as AZUL, which iscapable of not only accepting electrons from an electrode (cathodic conditions) duringphotoautotrophic growth but also using the electrode as an electron acceptor (anodicconditions) under a photoheterotrophic metabolism. In the first condition, cells wereable to grow and accept electrons from an electrode polarized at negative potentials(i.e. replacing inorganic electron donors). We propose that R. palustris AZUL presentsmechanisms for both direct and indirect electron uptake, as evidenced by growthcurves and cyclic voltammetry experiments. When grown in anodic conditions, cellsformed biofilms with a particular architecture on poised graphite electrodes. Wedetected a specific voltammetric profile characterized by two reduction processes at0.2 and 0.4 V and an oxidation process at 0.57 V (vs Ag/AgCl). These redox signalsprogressively changed with the time of growth towards an oxidation catalysis signalwith a gate potential of 0.5 V. Biofilm charge accumulation experiments indicated thatthis bacteria accumulates electrons that can be subsequently discharged uponpolarization. The results presented herein suggest that R. palustris AZUL is capable ofexchanging electrons with an electrode in a bidirectional fashion. In addition, ourexperiments indicate that, during photoheterotrophic growth, this strain has amechanism of redox balance that includes charge storage in redox molecules that cantransfer charge to the extracellular space when a high potential acceptor is available.