Extracellular Electron Transfer Mechanisms in a Moderately Halophilic Bacterium from the Great Salt Lake for High Salinity Heavy Metal Biosensing
Authors: Gaffney Erin , Grattieri Matteo , Minteer Shelley D. 
Published: February 2020

High saline contamination events are a growing problem due to the increase in frequency and severity of hurricanes and the accompanying torrential rains.1 Storms affecting coastal areas can flood Superfund sites, which are designated hazardous waste sites managed by the Environmental Protection Agency (EPA), and other dangerous industrial waste sites. These events can result in the release of harmful pollutants into the environment, including heavy metals.2 The dimension of the contaminated area, as well as the cost and labor intensive approaches utilized, make the monitoring through standard laboratory methods complicated.3 Accordingly, a comprehensive screening of a community after a contamination event is not feasible. In this contest, biosensors offer a cost-effective alternative. Microbial biosensors are of particular interest for environmental monitoring due to their stability, wide range of analytes, and the ability to report on physiological toxicity.4 However, high salinities pose stress on the cellular membrane of bacteria cells, and few microbes can survive in salinities higher than the ocean (~3.5% NaCl). Such salinities can frequently occur after a natural disaster, due to stagnant water and subsequent evaporative water loss, demonstrating a need for a halophilic microorganism that can report on toxic contaminants.

A bacterium that has these characteristics was previously isolated for electroactivity and further evaluated for use in the microbial analysis of toxic pollutants applicable for a widescale screening process in high saline contamination events.5 The bacterium, Salinivibrio EAGSL, was found to tolerate from 0.1 M to 3 M NaCl and showed anode-respiring capability. The bacterium was isolated as a new strain based on 16s rRNA gene sequencing.6 Therefore bioinformatics was employed for sequencing and assembling the genome to discover traits for NaCl, heavy metal, and general environmental stress factors. Interestingly, bioinformatics analysis contributed to unveiling the mechanism of extracellular electron transfer in this bacterium. The strategies for arsenic analysis included a high throughput 96-well plate cytotoxicity assay and an electrochemical assay for steps towards an online hazardous contaminant detection system. The 96-well plate assay provided a screening method for ~50 samples at once, taking around 6 hours to detect concentrations of 75 μM arsenic. To overcome the need for sampling and analysis, an electrochemical method was also developed for continuous monitoring of toxic contaminants in high saline. The net result is a microbial cytotoxicity assay to evaluate the toxicity of contaminants in a high saline environment. Additionally, this study elucidated the endogenous electron mediation mechanism in this bacterium through bioinformatics and electrochemical characterization, in addition to performance with an exogenous monomeric mediation system for increased current output.

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