Guo et al., 2010; Moscoviz et al., 2016). The process of fermentation is carried out by a large diversity of microorganisms (e.g., pure or mixed cultures), which can utilize different types of substrates and organic wastes (Ghimire et al., 2015; Guo et al., 2010). There are many different environmental parameters, such as the inoculum type, medium composition, pH, temperature, hydraulic retention time, and accumulation of bi- and end-products that can affect alter the fermentative pathways (Moscoviz et al., 2016). Despite the extensive studies conducted for controlling of aforementioned environmental parameters for the fermentation processes, targeting a specific end product is still challenging, especially concerning mixed cultures (Moscoviz et al., 2016). Hence, instead, engineering the oxidation-reduction potential (ORP) of the fermentation medium (e.g., also known as the extracellular ORP) can be an alternative to control the microbial metabolism to generate a target product (Wong et al., 2014; Zhu et al., 2014). The extracellular ORP corresponds to the activity of electrons in the medium (e.g., in this case, substrates or organic wastes during fermentation), where it is mainly influenced by the chemical composition of the medium, the degree of reduction of the metabolites produced by fermentation, and the temperature (Moscoviz et al., 2016). Particularly, the extracellular ORP is critical as it substantially impacts the intracellular ORP via NADH and NAD+ (e.g., reduced/oxidized form of NAD) balance (Liu et al., 2013). The intracellular ORP represents the redox state inside a cell, which can control enzyme synthesis and gene expression; hence, it ultimately affects the entire metabolic process and it can modify the metabolic pathways during fermentation (Liu et al., 2013). Previously, studies have successfully demonstrated the chemical control of using extracellular ORP to enhance the production of succinate (Chen et al., 2012; Li et al., 2010) and 1,3-propanediol (Du et al., 2006) during fermentation, in which, the MXCs can also potentially be implemented to modify the extracellular ORP via supplying or collecting electrical energy (e.g., electrons, current) using electrodes. This process of combining the fermentation process with an MXC was named “electro-fermentation”.
Electro-fermentation (EF) is a unique process, which introduces electrical energy to microbial fermentative metabolism (Moscoviz et al., 2016). The electrical energy in the EF system can control and stabilize the fermentation process, overcoming the metabolic limitations of balanced reactions (Moscoviz et al., 2016). Nonetheless, the EF system requires lower electrical energy compared with other aforementioned applications of MXCs, where high electrical energy (e.g., high current densities) can result in reduced performance or system failure due to the inhibition of fermentative bacteria (Lai and Lan, 2020). More importantly, even small applied current densities can affect both extracellular and intracellular ORP (e.g., overall biological regulation) through the changes in NADH/NAD+ ratio, which can substantially affect the final fermentation products (Moscoviz et al., 2016; Speers et al., 2014; Sturm-Richter et al., 2015; Zhou et al., 2015). In this context, the electrical energy (e.g., current) in the EF system is not the product of interest nor the main driving energy source, but it is a trigger that allows the fermentation to occur under unbalanced conditions (Moscoviz et al., 2016). Furthermore, the features of EF can significantly overcome the problems of the conventional fermentation process. For instance, using electroactive bacteria (e.g., capable of converting volatile fatty acids to electrons, protons, and carbon dioxide) and electrodes in the EF system can alleviate the challenges, such as accumulation of short-chain volatile fatty acids (SCVFAs) (Lai et al., 2016b) and toxicants (e.g., nitrite, if nitrate is used as an electron acceptor) (Takeno et al., 2007), which are experienced by the conventional fermentation. With such advantages, EF systems present an emerging platform for future biorefinery for the synthesis of various value-added products from organic feedstocks. To date, the EF systems have been implemented for enhancing various biofuels and chemical productions, such as carboxylates, alcohols, solvents, lipids, acetoin, biopolymer, and many more (see Figure 1.1) (Bursac et al., 2017; Choi et al., 2014; Jiang et al., 2020; Lai and Lan, 2020; Liu et al., 2019; Mostafazadeh et al., 2016; Vassilev et al., 2018; Villano et al., 2017). This book chapter presents fundamental mechanisms, applied and scientific aspects of EF to produce different value-added products, and finally, perspectives for future development.
Figure 1.1 Overview of various value-added products produced via electro-fermentation.
1.2 Fundamental Mechanisms
A system for EF consists of an anode and a cathode, and the chambers can be separated by an ion-exchange membrane (see Figure 1.2). The use of a membrane is optional; used when preventing product crossover is critical. Briefly describing the entire process, the EF comprises the fermentation of an energy-rich substrate, where the solid electrodes present in the EF system serves as inexhaustible electron donors or acceptors that does not limit the entire fermentation process (Jiang et al., 2019; Moscoviz et al., 2016). The EF system is generally connected with power sources (e.g., power supply, potentiostat, etc.), where the externally poised potential/voltage is utilized to regulate the fermentation pathways for pure and mixed cultures (Jiang et al., 2019; Moscoviz et al., 2016; Schievano et al., 2016). Briefly speaking, the electrons are transferred between the fermentation medium and the bacteria (e.g., fermentative and/or electroactive), and between the bacteria and the electrodes (e.g., anode or cathode). The electron transfer process between bacteria and electrodes are known as extracellular electron transfer (EET). Depending on the type of fermentation, the EET can be outward (during anodic EF) and inward (during cathodic EF) (Gong et al., 2020; Kracke et al., 2018). Although EET mechanisms have not been studied exclusively for various EF processes, literature suggests that bi-directional EET can occur via multiple mechanisms, including direct electron transport via extracellular redox co-factors (e.g., cytochromes, and other redox proteins), nanowires, and mediators (Gong et al., 2020; Kracke et al., 2018).
Depending on the target product (e.g., final product more oxidized vs. reduced form than the initial substrate), the EF can be classified into two main types: (i) anodic electro-fermentation (AEF); and (ii) cathodic electro-fermentation (CEF). When the final product of EF is more oxidized form than the substrate (e.g., ethanol from glycerol), the working electrode acts as an anode and is used to dissipate the excess electrons, known as the AEF. On the other hand, if the final product is in a more reduced form than the substrate (e.g., butanol from glucose), then the working electrode acts as an electron donor, known as the CEF. The electron sinks during AEF are known to synthesize more adenisine triphospates (ATPs) through creating a proton gradient, while the electron sources during CEF have impacts on the generation of more reduced redox cofactors, such as NADH (Kracke and Krömer, 2014). Hence, both AEF and CEF can significantly enhance the entire fermentation performance (e.g., product selectivity, production rate and yield) (Xafenias et al., 2017). Detail descriptions of underlying mechanisms or EF mechanisms can be found elsewhere (Gong et al., 2020; Jiang et al., 2019; Moscoviz et al., 2016).
Figure 1.2 Mechanisms of electro-fermentation: (a) anodic electro-fermentation; (b) cathodic electro-fermentation.
As discussed earlier, in most cases, the reactions and electron transfers associated with EF are usually performed via syntrophic interactions between the fermentative bacteria and electroactive bacteria (Jiang et al., 2019; Moscoviz et al., 2016). However, sometimes, none of the fermentative bacteria are electroactive (e.g., Clostridium species), in which, the redox mediators,
