Electrochemical Detection of Circadian Redox Rhythm in Cyanobacterial Cells via Extracellular Electron Transfer.

Recent research on cellular circadian rhythms suggests that the coupling of transcription-translation feedback loops and intracellular redox oscillations is essential for robust circadian timekeeping. For clarification of the molecular mechanism underlying the circadian rhythm, methods that allow for the dynamic and simultaneous detection of transcription/translation and redox oscillations in living cells are needed. Herein, we report that the cyanobacterial circadian redox rhythm can be electrochemically detected based on extracellular electron transfer (EET), a process in which intracellular electrons are exchanged with an extracellular electrode. As the EET-based method is non-destructive, concurrent detection with transcription/translation rhythm using bioluminescent reporter strains becomes possible. An EET pathway that electrochemically connected the intracellular region of cyanobacterial cells with an extracellular electrode was constructed via a newly synthesized electron mediator with cell membrane permeability. In the presence of the mediator, the open circuit potential of the culture medium exhibited temperature-compensated rhythm with approximately 24 h periodicity. Importantly, such circadian rhythm of the open circuit potential was not observed in the absence of the electron mediator, indicating that the EET process conveys the dynamic information regarding the intracellular redox state to the extracellular electrode. These findings represent the first direct demonstration of the intracellular circadian redox rhythm of cyanobacterial cells.

Regulation of the cyanobacterial circadian clock by electrochemically controlled extracellular electron transfer.

There is growing awareness that circadian clocks are closely related to the intracellular redox state across a range of species. As the redox state is determined by the exchange of the redox species, electrochemically controlled extracellular electron transfer (EC-EET), a process in which intracellular electrons are exchanged with extracellular electrodes, is a promising approach for the external regulation of circadian clocks. Herein, we discuss whether the circadian clock can be regulated by EC-EET using the cyanobacterium Synechococcus elongatus PCC7942 as a model system. In vivo monitoring of chlorophyll fluorescence revealed that the redox state of the plastoquionone pool could be controlled with EC-EET by simply changing the electrode potential. As a result, the endogenous circadian clock of S. elongatus cells was successfully entrained through periodically modulated EC-EET by emulating the natural light/dark cycle, even under constant illumination conditions. This is the first example of regulating the biological clock by electrochemistry.

Extracellular electron transfer across bacterial cell membranes via a cytocompatible redox-active polymer.

A redox-active phospholipid polymer with a phospholipid-mimicking structure (2-methacryloyloxyethyl phosphorylcholine; MPC) was synthesized to construct a biocompatible electron mediator between bacteria and an electrode. In this study, a copolymer of MPC and vinylferrocene [VF; poly(MPC-co-VF)] (PMF) is synthesized. When PMF is added to cultures of the bacterial species Escherichia coli (Gram negative) and Lactobacillus plantarum (Gram positive), which have different cell wall structures, a catalytic current mediated by PMF is observed. In addition, growth curves and live/dead assays indicate that PMF does not decrease metabolic activity or cell viability. These results indicate that PMF mediates extracellular electron transfer across bacterial cell membranes without associated cytotoxicity.

Digestion of algal biomass for electricity generation in microbial fuel cells.

Algal biomass serves as a fuel for electricity generation in microbial fuel cells. This study constructed a model consortium comprised of an alga-digesting Lactobacillus and an iron-reducing Geobacter for electricity generation from photo-grown Clamydomonas cells. Total power-conversion efficiency (from Light to electricity) was estimated to be 0.47%.

Light/electricity conversion by a self-organized photosynthetic biofilm in a single-chamber reactor.

Biological energy-conversion systems are attractive in terms of their self-sustaining and self-organizing nature and are expected to be applied to low-cost and environment-friendly processes. Here we show a biofilm-based light/electricity-conversion system that was self-organized from a natural microbial community. A bioreactor equipped with an air cathode and graphite-felt anode was inoculated with a green hot-spring microbial mat. When the reactor was irradiated with light, electric current was generated between the anode and cathode in accordance with the formation of green biofilm on the anode. Fluorescence microscopy of the green biofilm revealed the presence of chlorophyll-containing microbes of approximately 10 microm in size, and these cells were abundant close to the surface of the biofilm. The biofilm community was also analyzed by sequencing of polymerase chain reaction-amplified small-subunit rRNA gene fragments, showing that sequence types affiliated with Chlorophyta, Betaproteobacteria, and Bacteroidetes were abundantly detected. These results suggest that green algae and heterotrophic bacteria cooperatively converted light energy into electricity.

Factors affecting electric output from rice-paddy microbial fuel cells.

Rice-paddy microbial fuel cells generate electricity from organic matter that is photosynthesized by rice plants and exudated from the roots. We examined factors that might affect cell performance, and found that cathode modification with platinum catalysts, anode position, and external load largely affected the power output.

Light/electricity conversion by defined cocultures of Chlamydomonas and Geobacter.

Biological energy-conversion systems are attractive in terms of their self-organizing and self-sustaining properties and are expected to be applied towards environmentally friendly bioenergy processes. Recent studies have demonstrated that sustainable light/electricity-conversion systems, termed microbial solar cells (MSCs), can be constructed using naturally occurring microbial communities. To better understand the energy-conversion mechanisms in microbial communities, the present study attempted to construct model MSCs comprised of defined cocultures of a green alga, Chlamydomonas reinhardtii, and an iron-reducing bacterium, Geobacter sulfurreducens, and examined their metabolism and interactions in MSCs. When MSC bioreactors were inoculated with these microbes and irradiated on a 12-h light/dark cycle, periodic current was generated in the dark with energy-conversion efficiencies of 0.1%. Metabolite analyses revealed that G. sulfurreducens generated current by oxidizing formate that was produced by C. reinhardtii in the dark. These results demonstrate that the light/electricity conversion occurs via syntrophic interactions between phototrophs and electricity-generating bacteria. Based on the results and data in literatures, it is estimated that the excretion of organics by the phototroph was the bottleneck step in the syntrophic light/electricity conversion. We also discuss differences between natural-community and defined-coculture MSCs.

The effect of the encapsulation of bacteria in redox phospholipid polymer hydrogels on electron transfer efficiency in living cell-based devices.

Development of living cell-based devices holds great promise in many biomedical and industrial applications. To increase our understanding of the process, we investigated the biological and electrochemical properties of a redox phospholipid polymer hydrogel containing an electron-generating bacteria (Shewanella oneidensis MR-1). A water-soluble and amphiphilic phospholipid polymer, poly(2-methacryloyloxyethyl phosphorylcholine-co-n-butyl methacrylate-co-p-vinylphenylboronic acid-co-vinylferrocene) (PMBVF), was our choice for incorporation into a hydrogel matrix that promotes encapsulation of bacteria and acts as an electron transfer mediator. This hydrogel formed spontaneously and encapsulated Shewanella in three-dimensional structures. Visual analysis showed that the encapsulated Shewanella maintained viability and metabolic activity even after long-term storage. Cyclic voltammetry measurement indicated that the PMBVF/poly(vinyl alcohol) (PMBVF/PVA) hydrogel had stable and high electron transfer efficiency. Amperometric measurement showed that the hydrogel could maintain the electron transfer efficiency even when Shewanella was encapsulated. Thus, the PMBVF/PVA hydrogel not only provides a mild environment for long-term bacterial survival but also maintains electron transfer efficiency from the bacteria to the electrode. We conclude that hydrogel/bacteria hybrid biomaterials, such as PMBVF/PVA/Shewanella, may find application in the fabrication of living cell-based devices.