Abstract
While most biological materials are insulating in nature, efficient extracellular electron transfer is a critical property of biofilms associated with microbial electrochemical systems and several microorganisms are capable of establishing conductive aggregates and biofilms. Though construction of these conductive microbial networks is an intriguing and important phenomenon in both natural and engineered systems, few studies have been published related to conductive biofilms/aggregates and their conduction mechanisms, especially in mixed-species environments. In the present study, current-producing mixed species biofilms exhibited high conductivity across non-conductive gaps. Biofilm growth observed on the inactive electrode contributed to overall power outputs, suggesting that an electrical connection was established throughout the biofilm assembly. Electrochemical gating analysis of the biofilms over a range of potentials (-600–200 mV, vs. Ag/AgCl) resulted in a peak-manner response with maximum conductance of 3437 ± 271 μS at a gate potential of -360 mV. Following removal of the electron donor (acetate), a 96.6% decrease in peak conductivity was observed. Differential responses observed in the absence of an electron donor and over varying potentials suggest a redox driven conductivity mechanism in mixed-species biofilms. These results demonstrated significant differences in biofilm development and conductivity compared to previous studies using pure cultures.
Highlights
The efficient extracellular transference of electrons is critical to the functioning of many biological processes in both natural and engineered environmental systems [1,2,3]
While most biological materials are insulating in nature, efficient extracellular electron transfer is a critical property of biofilms associated with microbial electrochemical systems and several microorganisms are capable of establishing conductive aggregates and biofilms
Much of our current understanding of extracellular electron transfer in these environments is largely based on the indirect transfer of small molecules such as hydrogen and formate, but recent evidence suggests that extracellular electron transfer through electrical current is prevalent [1,4,5,6]
Summary
The efficient extracellular transference of electrons is critical to the functioning of many biological processes in both natural and engineered environmental systems [1,2,3]. Much of our current understanding of extracellular electron transfer in these environments is largely based on the indirect transfer of small molecules such as hydrogen and formate, but recent evidence suggests that extracellular electron transfer through electrical current is prevalent [1,4,5,6]. In diffusion-limited environments, such as biofilms and sediments, direct extracellular electron transfer via electrical currents could offer significant advantages over small molecule exchange. It is likely that physical connections in the form of aggregates and biofilms are often established in order to support electrical interactions between microorganisms and extracellular electron acceptors including other microorganisms and electrodes [1,4,6,7]. Biofilms and PLOS ONE | DOI:10.1371/journal.pone.0155247 May 9, 2016
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