Ferrihydrite Reduction by Photosynthetic Synechocystis sp. PCC 6803 and Its Correlation With Electricity Generation.

Kenya Tanaka,Ginga Shimakawa,Souichiro Kato,Shoko Kusama,Takashi Harada,Shuji Nakanishi

doi:10.3389/fmicb.2021.650832

Abstract

Microbial extracellular electron transfer (EET) to solid-state electron acceptors such as anodes and metal oxides, which was originally identified in dissimilatory metal-reducing bacteria, is a key process in microbial electricity generation and the biogeochemical cycling of metals. Although it is now known that photosynthetic microorganisms can also generate (photo)currents via EET, which has attracted much interest in the field of biophotovoltaics, little is known about the reduction of metal (hydr)oxides via photosynthetic microbial EET. The present work quantitatively assessed the reduction of ferrihydrite in conjunction with the EET of the photosynthetic microbe Synechocystis sp. PCC 6803. Microbial reduction of ferrihydrite was found to be initiated in response to light but proceeded at higher rates when exogenous glucose was added, even under dark conditions. These results indicate that current generation from Synechocystis cells does not always need light irradiation. The qualitative trends exhibited by the ferrihydrite reduction rates under various conditions showed significant correlation with those of the microbial currents. Notably, the maximum concentration of Fe(II) generated by the cyanobacterial cells under dark conditions in the presence of glucose was comparable to the levels observed in the photic layers of Fe-rich microbial mats.

Highlights

Photosynthetic microorganisms are able to transport intracellular electrons to an anode and this process, known as extracellular electron transfer (EET), has received attention within the field of biophotovoltaics (BPVs), as summarized in recent reviews (Rosenbaum et al, 2010; McCormick et al, 2015; Tschörtner et al, 2019)
The anodic current was not observed in the presence of cells or in the presence of dead cells even when the glucose was added into the system (Supplementary Figure 2)
In contrast to the light-dependent current increases reported elsewhere, the current in the present work was found to increase under dark conditions upon the addition of glucose

Summary

Introduction

Photosynthetic microorganisms are able to transport intracellular electrons to an anode and this process, known as extracellular electron transfer (EET), has received attention within the field of biophotovoltaics (BPVs), as summarized in recent reviews (Rosenbaum et al, 2010; McCormick et al, 2015; Tschörtner et al, 2019). In a typical BPV system, oxygenic photosynthetic microorganisms such as cyanobacteria and green algae serve both as light absorbers and living anodes/catalysts for water oxidation. Photosynthetic and respiratory electron transfer chains share the interchain components plastoquinone, the cytochrome b6/f complex, plastocyanin, and cytochrome c (Binder, 1982; Peschek et al, 2004). The electrons injected into the anode via EET are transferred to the cathode where oxygen reduction occurs, generating electricity. Light to electrical energy conversion in BPVs is achieved on the basis of the functions of living photosynthetic microorganisms

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