Extracellular Electron Transfer via Outer Membrane Cytochromes in a Methanotrophic Bacterium Methylococcus capsulatus (Bath)

Abstract

Electron exchange reactions between microbial cells and solid materials, referred to as extracellular electron transfer (EET), have attracted attention in the fields of microbial physiology, microbial ecology, and biotechnology. Studies of model species of iron-reducing, or equivalently, current-generating bacteria such as Geobacter spp. and Shewanella spp. have revealed that redox-active proteins, especially outer membrane c-type cytochromes (OMCs), play a pivotal role in the EET process. Recent (meta)genomic analyses have revealed that diverse microorganisms that have not been demonstrated to have EET ability also harbor OMC-like proteins, indicating that EET via OMCs could be more widely preserved in microorganisms than originally thought. A methanotrophic bacterium Methylococcus capsulatus (Bath) was reported to harbor multiple OMC genes whose expression is elevated by Cu starvation. However, the physiological role of these genes is unknown. Therefore, in this study, we explored whether M. capsulatus (Bath) displays EET abilities via OMCs. In electrochemical analysis, M. capsulatus (Bath) generated anodic current only when electron donors such as formate were available, and could reduce insoluble iron oxides in the presence of electron donor compounds. Furthermore, the current-generating and iron-reducing activities of M. capsulatus (Bath) cells that were cultured in a Cu-deficient medium, which promotes high levels of OMC expression, were higher than those cultured in a Cu-supplemented medium. Anodic current production by the Cu-deficient cells was significantly suppressed by disruption of MCA0421, a highly expressed OMC gene, and by treatment with carbon monoxide (CO) gas (an inhibitor of c-type cytochromes). Our results provide evidence of EET in M. capsulatus (Bath) and demonstrate the pivotal role of OMCs in this process. This study raises the possibility that EET to solid compounds is a novel survival strategy of methanotrophic bacteria.

Methanotrophs: Discoveries, Environmental Relevance, and a Perspective on Current and Future Applications

“…has been detected in anaerobic cultures of nitrate-dAOM microorganisms, where the expression of the pmoA gene increased upon exposure to 5% oxygen v/v , highlighting the survival and latency of aerobic methanotrophs under anoxic methane/nitrate-rich conditions (Guerrero-Cruz et al, 2018). Besides nitrate, methane oxidation coupled to iron (in the form of ferrihydrite) and manganese reduction through direct electron transfer via cytochromes, or via an artificial organic electron acceptor (e.g., anthraquinone-2,6-disulfonate) have been implicated for both gammaproteobacterial (Methylococcus, Methylomonas) and alphaproteobacterial (Methylosinus) methanotrophs under hypoxia (Oswald et al, 2016;Tanaka et al, 2018;Zheng et al, 2020). Proteobacterial methanotrophs (e.g., Methylomicrobium alcaliphilum and Methylocystis-affiliated) excrete methanederived organic compounds (e.g., acetate, lactate, succinate) during oxygen-limited growth (Costa et al, 2000;Kalyuzhnaya et al, 2013).…”

Facultative methanotrophs – diversity, genetics, molecular ecology and biotechnological potential: a mini-review

“…Where methane is abundant and oxygen may be limiting, some methanotrophs thrive at low-oxygen tensions. At the extreme, Candidatus Methylomirabilis oxyfera uses nitric oxide dismutation to generate its own molecular oxygen for use by methane monooxygenase, while other methanotrophs economize on oxygen consumption by denitrification [118–121], iron reduction [122, 123], or by fermentation [124, 125], maintaining a supply of molecular oxygen for methane activation. While the denitrification enzymes are also important for detoxification [126], the use of nitrogen compounds as electron acceptor may be significant in methane oxidation [127].…”

Gene Sets and Mechanisms of Sulfate-Reducing Bacteria Biofilm Formation and Quorum Sensing With Impact on Corrosion

“…Extracellular electron transfer (EET) is a mode of energy conservation by which some microorganisms exchange intracellular electrons with extracellular electron donor/acceptor (Rathinam et al, 2018;Tanaka et al, 2018;Shrestha et al, 2020). Microbial EET potentially endangers corrosion of iron structures.…”