Electrode Potential Dependency of Single-Cell Activity Identifies the Energetics of Slow Microbial Electron Uptake Process.

Xiao Deng,Akihiro Okamoto

doi:10.3389/fmicb.2018.02744

Xiao Deng, Akihiro Okamoto

Open Access

https://doi.org/10.3389/fmicb.2018.02744

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Abstract

Electrochemical measurements have been widely applied to study microbial extracellular electron transport processes. However, because electrochemistry detects not only microbial electron transport but also other reactions, background signals comparable to or larger than microbial ones hamper the identification of microbial electrochemical properties. This problem is crucial especially for the detection of electron uptake processes by slow-growing microbes in low-energy subsurface sediments, as the environmental samples contain electrochemically active humus and mineral particles. In this study, we report a cell-specific stable isotope analysis to quantify the electrode potential dependency of anabolic activity in individual cells for identifying the electron uptake energetics of slow-growing bacteria. Followed by the incubation of Desulfovibrio ferrophilus IS5 cells with isotopic 15N-ammonium as the sole N source on electrodes poised at potentials of -0.2, -0.3, -0.4, and -0.5 V [vs. standard hydrogen electrode (SHE)], we conducted nanoscale secondary ion mass spectroscopy (NanoSIMS) to quantify 15N assimilation in more than 100 individual cells on the electrodes. We observed significant 15N assimilation at potentials of -0.4 and more 15N assimilation at -0.5 V, which is consistent with the onset potential for electron uptake via outer-membrane cytochromes (OMCs). The activation of cell energy metabolism was further examined by transcriptome analysis. Our results showed a novel methodology to study microbial electron uptake energetics. The results also serve as the first direct evidence that energy acquisition is coupled to the electron uptake process in sulfate-reducing bacteria that are ubiquitous in the subsurface environments, with implications on the electron-fueled subsurface biosphere hypothesis and other microbial processes, such as anaerobic iron corrosion and anaerobic methane oxidation.

Highlights

Vast and active microbial biospheres have been discovered in subsurface environments with scarce energy sources (Parkes et al, 1994; Fredrickson and Onstott, 1996; D’Hondt et al, 2004, 2015)
Anoxic electrochemical reactors equipped with Indium tin-doped oxide (ITO) electrodes poised at four different potentials, −0.2, −0.3, −0.4, and −0.5 V, were used for the electrical incubation of D. ferrophilus IS5 cells
Followed by the background current stabilization to less than −0.05 μA cm−2, we introduced IS5 cells to the electrode surface poised at −0.2 V, and the increase in cathodic current, which is the negative current, was negligible during the following electrical incubation of 48 h (Figure 1 and Supplementary Figure S1)

Summary

Introduction

Vast and active microbial biospheres have been discovered in subsurface environments with scarce energy sources (Parkes et al, 1994; Fredrickson and Onstott, 1996; D’Hondt et al, 2004, 2015). These environments lack sufficient soluble electron donors, they preserve abundant minerals, such as iron sulfides (FeS), and different redox states to generate a flow of natural

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