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Biotechnology Advances
Volume 48, 2021, 107728

Anodic electro-fermentation: Empowering anaerobic production processes via anodic respiration

Igor Vassileva, Nils J.H.Avereschb, Pablo Ledezmac, Marika Kokkoa

Faculty of Engineering and Natural Sciences, Tampere University, Tampere, Finland.

Abstract

In nature as well as in industrial microbiology, all microorganisms need to achieve redox balance. Their redox state and energy conservation highly depend on the availability of a terminal electron acceptor, for example oxygen in aerobic production processes. Under anaerobic conditions in the absence of an electron acceptor, redox balance is achieved via the production of reduced carbon-compounds (fermentation). An alternative strategy to artificially stabilize microbial redox and energy state is the use of anodic electro-fermentation (AEF). This emerging biotechnology empowers respiration under anaerobic conditions using the anode of a bioelectrochemical system as an undepletable terminal electron acceptor. Electrochemical control of redox metabolism and energy conservation via AEF can steer the carbon metabolism towards a product of interest and avoid the need for continuous and cost-inefficient supply of oxygen as well as the production of mixed reduced by-products, as is the case in aerobic production and fermentation processes, respectively. The great challenge for AEF is to establish efficient extracellular electron transfer (EET) from the microbe to the anode and link it to central carbon metabolism to enhance the synthesis of a target product. This article reviews the advantages and challenges of AEF, EET mechanisms, microbial energy gain, and discusses the rational choice of substrate-product couple as well as the choice of microbial catalyst. Besides, it discusses the potential of the industrial model-organism Bacillus subtilis as a promising candidate for AEF, which has not been yet considered for such an application.

Keywords: Anodic electro-fermentation, Microbial electrochemical technology, Aerobic vs. anaerobic production, Unbalanced fermentation, Terminal electron acceptor, Redox balance, Energy conservation, Bacillus subtilis, Acetoin.

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