Investigations of the periplasmic physiology of phenazines in Pseudomonas for improved mediated electron transfer in bioelectrochemical systems
Phenazines are natural products of some microorganisms, especially bacteria. They are very peculiar to Pseudomonas aeruginosa where they are a pathogenic factor performing several roles. These include acting as an antibiotic against non-producing bacteria, facilitating iron acquisition, and enhancing the survival of P. aeruginosa in oxygen-limited environment by shuttling electrons to distant oxygen or other extracellular electron acceptors- a process known as extracellular electron transfer (EET). The latter function is employed in a bioelectrochemical system (BES) using phenazines redox mediators to shuttle electrons from microorganisms to the anode (electrode) thereby generating electricity. This phenazine EET in the BES is being explored to develop bioprocesses, which can function even under limited oxygen conditions. Hence, biotechnologically important strains such as P. putida KT2440 and E. coli have been engineered to produce phenazines. A challenge, however, in using phenazines as electron mediators in a BES is that only a small proportion or percentage of electrons generated from the metabolism of a given substrate can be delivered to the anode, i.e., the coulombic efficiency of phenazine EET is low. We wanted to unravel the reasons behind the low coulombic efficiency, and we hypothesized that most of the electrons that are available for phenazine reduction are obtained in the periplasm of the cell. This is because no putative re-entry path into the cytoplasm has so far been described for the phenazines. Thus, it was not certain if the phenazines, once released from the cytoplasm where they are produced, can re-access this part of the cell. Consequently, only electrons emanating from the various metabolic processes in the periplasm would be available for phenazine reduction, possibly excluding electrons from the cytoplasm where the bulk of the metabolic activities occur. ...
