Unveiling Purple Bacteria Salt Tolerance Mechanisms for Environmental Monitoring in Photo-Bioelectrochemical Systems
Authors: Gaffney Erin , Grattieri Matteo , Minteer Shelley D. 
Published: February 2020

Photo-bioelectrochemical systems allow for the integration of photosynthetic bacteria at an electrode surface for the conversion of solar energy into electrical current.1 Among various applications, these systems open for the continuous monitoring of toxic compounds in the environment based on their cytotoxic effects on bacteria activity. However, a challenge for the on-field application is the exposure of bacterial cells to a diverse range of condition, requiring robust, versatile microorganisms capable of tolerating dynamic environments.

Rhodobacter capsulatus (R. capsulatus) is a purple, photosynthetic bacterium with an outstanding versatile metabolism. Specifically, its versatility is thought to be due to a bacteria phage-like element, the R. capsulatus gene transfer agent (rcGTA), enabling horizontal gene transfer across microorganisms, which results in an expedited evolution to new environmental stresses. The rcGTA has been seen to facilitate resistance to antibiotics2 and provides a mechanism for adaptation to various environmental conditions. Integrating this bacterium with an electrode for photo-bioelectrochemical system development proves to be challenging due to the active redox center’s location inside of the thick cellular membrane. Previous work in our group has succeeded in mediating this redox active center employing monomeric quinones for mediating the extracellular electron transfer to the electrode.3 Current research is focused on engineering redox hydrogels to enhance the extracellular electron transfer in high saline, resulting in equal or higher currents compared to non-saline. To further improve photo-bioelectrocatalysis performance through adaptation of the cells to high saline conditions, we investigated the adaptation mechanism using techniques to study the rcGTA, and bioinformatics to evaluate differential expression of genes in both saline and non-saline conditions. Further research will be focused on harnessing these findings to decrease adaptation time to high salinities and evaluating the bioelectrochemical performance of these adapted strains through chronoamperometry and cyclic voltammetry experiments.
The successful completion of this study will contribute towards the design of a photo-bioelectrochemical system for toxic compound detection in high saline conditions, and further increase our knowledge of salt adaptation mechanisms and the gene transfer agent of Rhodobacter capsulatus.
References:
(1) Grattieri, M.; Minteer, S. D. Decoupling Energy and Power. Nat. Energy
2018, 3 (1), 8–9. https://doi.org/10.1038/s41560-017-0076-x.
(2) Lang, A. S.; Zhaxybayeva, O.; Beatty, J. T. Gene Transfer Agents: Phage-like Elements of Genetic Exchange. Nat. Rev. Microbiol. https://doi.org/10.1038/nrmicro2802.
(3) Grattieri, M.; Rhodes, Z.; Hickey, D. P.; Beaver, K.; Minteer, S. D. Understanding Biophotocurrent Generation in Photosynthetic Purple Bacteria. ACS Catal.
2019, 9 (2), 867–873. https://doi.org/10.1021/acscatal.8b04464.