Abstract
Exoelectrogenic bacteria are defined by their ability to respire on extracellular and insoluble electron acceptors and have applications in bioremediation and microbial electrochemical systems (MESs), while playing important roles in biogeochemical cycling. Shewanella oneidensis MR-1, which has become a model organism for the study of extracellular respiration, is known to display taxis toward insoluble electron acceptors, including electrodes. Multiple mechanisms have been proposed for MR-1's tactic behavior, and, here, we report on the role of electrochemical potential by video microscopy cell tracking experiments in three-electrode electrochemical cells. MR-1 trajectories were determined using a particle tracking algorithm and validated with Shannon's entropy method. Tactic response by MR-1 in the electrochemical cell was observed to depend on the applied potential, as indicated by the average velocity and density of motile (>4 µm/s) MR-1 close to the electrode (<50 µm). Tactic behavior was observed at oxidative potentials, with a strong switch between the potentials -0.15 to -0.25 V versus the standard hydrogen electrode (SHE), which coincides with the reduction potential of flavins. The average velocity and density of motile MR-1 close to the electrode increased when riboflavin was added (2 µM), but were completely absent in a ΔmtrC/ΔomcA mutant of MR-1. Besides flavin's function as an electron mediator to support anaerobic respiration on insoluble electron acceptors, we propose that riboflavin is excreted by MR-1 to sense redox gradients in its environment, aiding taxis toward insoluble electron acceptors, including electrodes in MESs.IMPORTANCE Previous hypotheses of tactic behavior of exoelectrogenic bacteria are based on techniques that do not accurately control the electrochemical potential, such as chemical-in-plug assays or microscopy tracking experiments in two-electrode cells. Here, we have revisited previous experiments and, for the first time, performed microscopy cell-tracking experiments in three-electrode electrochemical cells, with defined electrode potentials. Based on these experiments, taxis toward electrodes is observed to switch at about -0.2 V versus standard hydrogen electrode (SHE), coinciding with the reduction potential of flavins.
Highlights
Exoelectrogenic bacteria are defined by their ability to respire on extracellular and insoluble electron acceptors and have applications in bioremediation and microbial electrochemical systems (MESs), while playing important roles in biogeochemical cycling
KEYWORDS Shewanella oneidensis, extracellular electron transfer, flavin, insoluble electron acceptors, microbe-mineral interactions, motility, respiration, taxis, tracking microscopy. Exoelectrogens, such as Shewanella oneidensis MR-1, are a group of microbes able to transfer respiratory electrons to extracellular and insoluble electron acceptors. They have been employed in the development of microbial electrochemical systems (MESs) and bioremediation and are implicated in biogeochemical cycling [1,2,3,4,5,6]
The role of exoelectrogens in biogeochemistry, MESs, and bioremediation is underpinned by their ability for extracellular electron transfer (EET)
Summary
Exoelectrogenic bacteria are defined by their ability to respire on extracellular and insoluble electron acceptors and have applications in bioremediation and microbial electrochemical systems (MESs), while playing important roles in biogeochemical cycling. KEYWORDS Shewanella oneidensis, extracellular electron transfer, flavin, insoluble electron acceptors, microbe-mineral interactions, motility, respiration, taxis, tracking microscopy We decided to revisit the function of electrochemical potentials in the tactic behavior of MR-1 toward electrode surfaces.
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