Understanding Electron Transport in a Durable Mixed Community Microbial Biocathode Enriched from Seawater
Authors: Yates Matthew D , Eddie Brian J , Kotloski Nicholas J , Lebedev Nikolai , Malonoski Anthony P , Lin Baochuan , Strycharz-Glaven Sarah , Tender Leonard M 
Published: February 2020

Biocathode MCL (Marinobacter, Chromatiaceae, Labrenzia) is a cathode associated microbial consortium enriched from seawater.(1-4) It is extremely durable, having been maintained in the laboratory for at least 6 years with minimal care, and acquires energy by coupling electron uptake from graphite or gold electrodes with reduction of O2, directing a portion of the acquired electrons and energy to reduce CO2 for cell growth.
Here we report electrochemical gating measurements performed on Biocathode MCL grown on interdigitated microelectrode arrays (IDAs) to characterize its extracellular electron transport (EET) properties in the same manner recently utilized for Geobacter sulfurreducens.(5-9) These measurements revealed a peak-shaped dependency of electrical conductivity on gate potential, consistent with redox conduction (i.e., electron hopping). The maximum biofilm conductivity at 30°C is 60 µS/cm, more than 10-fold greater than that of G. sulfurreducens biofilms.(5) In addition, conductivity of Biocathode MCL decreases with decreasing temperature in an Arrhenius manner also consistent with redox conduction. The apparent reorganization energy (λ = 0.58 ± 0.015 eV) is similar to that of G. sulfurreducens, suggesting hemes act as the EET cofactors. Voltammetric peaks attributable to EET cofactors are not observed under non-turnover condition (oxygen removed) however, indicating relatively low cofactor abundance. The apparent cofactor formal potential, indicated by the conductivity vs. gate potential dependency, is 0.14 V positive of the O2 reduction catalytic midpoint potential, indicating that the consortium utilizes electrons for oxygen reduction at potentials for which the majority of EET cofactors are reduced and the biofilm possesses only a fraction of its maximum conductivity. Confocal microscopy and fluorescent in situ hybridization indicate that Biocathode MCL biofilms are spatially organized such that the putative electroautotrophic organism is localized at the electrode surface. 16S rRNA gene sequencing analysis indicates that the community composition is stable on different electrode materials (gold or graphite electrodes). Finally, an unidentified iron-sulfur protein was detected by confocal resonance Raman microscopy (CRRM). We study biocathode MCL with the long-term goal of gaining mechanistic insights for engineering rugged and efficient electrode catalysts for electrosynthesis of liquid fuel precursors from CO2 accumulating in seawater. The results reported her expand upon our fundamental understanding of its EET properties.
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