mutant JG806 (Dfrd DadhE DldhA Dpta-ack) to achieve high pyruvate production, for the optimal production of acetoin. First, Förster et al. (2017) initiated their using nitrate as an electron acceptor (e.g., anoxic fermentation) for the production of acetoin and achieved a yield of 90% of maximum theoretical. However, the anoxic fermentation led to the accumulation of pyruvate and acetate. On the other hand, when the EF system was introduced, an acetoin yield of 79% of maximum theoretical was observed, and more importantly, there were no other end products besides acetoin with such high columbic efficiency (93.4%). Despite lower acetoin yield demonstrated by the EF system e.g., (79% vs. 90% - anoxic fermentation), Förster et al. (2017) have clearly demonstrated that the EF system can still provide an efficient acetoin generation along with the elimination of aeration and volatile fatty acids accumulations.
1.3.6 Biopolymer
Polymers have been recognized as one of the versatile class of materials due to their enormous contributions to our standard of living, quality of life, and human well-being across the globe (Rajan et al., 2018). However, the extensive usage of polymers has challenged solid waste management, concerning the adverse impact of plastic wastes on the ecosystem (Rajan et al., 2018; Urbanek et al., 2018). Specifically, for instance, the petrochemical-derived plastics have been extensively generated over the last decades, where their recalcitrance (e.g., strong resistance to biodegradation) posed significant threats to both environment and human health (Mostafa et al., 2020; Rajan et al., 2018). Hence, the production of environmental-benign polymers is highly necessary in order to address the challenges associated with solid waste management (Mostafa et al., 2020). Recently, renewable bio-based plastics, such as polyhydroxyalkanoate (PHAs) have received significant attention due to their potential application as a green alternative over other types of polymers (Liu et al., 2020a; Mostafa et al., 2020; Myung et al., 2017; Rajan et al., 2018). PHAs are non-toxic, biocompatible, and biodegradable, which have similar properties to conventional plastics (Kourmentza et al., 2017; Myung et al., 2014). The accumulation of PHA inside the bacterial cells provides a defense mechanism, when the bacterial growth is limited due to the stress conditions, such as nutrient imbalance (Chien et al., 2007; Myung et al., 2017). To date, many different variations in functional groups (e.g., methyl to tridecyl, unsaturated bonds, chain length, etc.) make PHA suitable biopolymers for various applications (Verlinden et al., 2007). The most common form of PHA is known as poly-3-hydroxybutyrate (PHB), which accumulates in various types of microbes by binding the -hydroxybutyrate monomers with ester bonds (Juengert et al., 2018).
Different fermentation methods can be utilized for the production of bio-based plastics like PHA. First, a sugar-based fermentation, which was the conventional method, has been implemented for the production of bioplastics (Jabeen et al., 2015; Rossell et al., 2002), but this conventional approach was not favorable due to reasons such as accumulation of shortchain volatile fatty acids (discussed in the previous sections) and the redox imbalance (e.g., restraining the product selectivity) (Lai and Lan, 2020). A newer fermentation approach using methanotrophs has been developed recently (Liu et al., 2020a; Myung et al., 2017). However, the major challenge of using methanotrophic fermentation for the production of bioplastics was the high production costs (e.g., feedstock, aeration), low yields, and mass transfer limitations (e.g., oxygen transfer, low methane solubility in water) (Liu et al., 2020a). To address such challenges, Myung et al. (2016) grew methanotrophs in water-in-oil emulsions (e.g., using the fact that methane was more soluble in oil than water) and increased the mass transfer efficiency, yielding higher production of bioplastics. However, utilizing oil may not be environmental-friendly and the high costs triggered by the aeration requirement and feedstock must still be considered (Liu et al., 2020a).
On the other hand, the EF system showed a high potential to greatly assist the production of bioplastics. To the best of the author’s knowledge, Lai and Lan (2020) were the only studies that investigated the application of the EF system on bioplastic production. In their study, Ralstonia eutropha H16 species went under growth-suppressing conditions (e.g., nitrogen starvation) for the production of PHB in two different bioreactor setups, a control (e.g., without any electrical energy input) and a test reactor (e.g., EF system). When the electrical current (10 mA) was applied along with a redox mediator (methyl blue), the PHB productivity remarkably increased by 30% (e.g., after 60 h of operation) along with higher final biomass residual (13.6 g/L vs. 11.8 g/L), higher maximum specific growth rate (mmax = 0.133/h vs. 0.127/h), and higher specific substrate utilization rate (qs = 0.187 g/g vs. 0.169 g/g) compared with the control. The enhancement of the PHB accumulation was due to the additionally supplied electrons and redox mediators accelerated the glycolytic pathway and redox cycling of NADH/NAD+, which boosted the adenosine triphosphate (ATP) generation, facilitating the biosynthesis of PHB (Lai & Lan, 2020).
1.3.7 L-lysine
The market of amino acid, which is known to be a key sector of industrial biotechnology, has been rapidly growing recently (Vassilev et al., 2018; Xafenias et al., 2017). Specifically, such a high production rate for L-lysine, an amino acid (e.g., 2.2 million tons per year with 7% increase per year) has been reported due to the growing demand for meat because the L-lysine was extensively used as an additive in animal nutrition (Ajinomoto Co., 2013; Eggeling and Bott, 2015; Xafenias et al., 2017). Over the decades, various industrial biotechnologies, based on Gram-positive soil bacteria (e.g., Corynebacterium glutamicum), have been utilized for the production of lysine due to their advantages (e.g., a safe production host) (Eggeling and Bott, 2005; Tatsumi and Inui, 2012; Vassilev et al., 2018; Wittmann and Becker, 2007). To begin with, using C. glutamicum in aerobic bioreactors (e.g., using oxygen as a terminal electron acceptor) was the initial attempt to produce L-lysine, but low product yields via substrate loss and oxygen mass transfer were the limitations for further development and scaling-up (Gill et al., 2008; Hannon et al., 2007; Takeno et al., 2007). In fact, aerobic bioreactors will result in higher capital costs compared to anaerobic systems (Garcia-Ochoa and Gomez, 2009). Alternatively, a study in 2004 revealed that C. glutamicum was capable of performing fermentation of glucose to organic acids, such as lactate and acetate under oxygen deprivation conditions (Inui et al., 2004), which demonstrated another pathway for L-lysine production. Despite the advantages of the anaerobic process (e.g., low cost), due to the low yield, efforts (e.g., the introduction of nitrate, anoxic condition) have been made to promote the growth of C. glutamicum, but the growth was inhibited due to the production of toxicants (e.g., nitrite) in the bioreactor (Takeno et al., 2007).
To enhance the growth of C. glutamicum and to increase the production of L-lysine, studies have utilized the EF system (Vassilev et al., 2018; Xafenias et al., 2017). The production of lysine using the EF system was first investigated by Xafenias et al. (2017). In their study, glucose, C. glutamicum, and nitrate were selected as substrate, bacterial type, and additional electron acceptor, respectively, and the EF system was performed in reductive mode. This study had three highlighted features, which were: (1) the performance of the EF system, in terms of lysine production; (2) effects and comparison of different environmental conditions (e.g., CO2-vs. N2-gas environment); and (3) impact of exogenous redox mediators. The lysine generation was comparable between the open circuit control (e.g., ~75 g/L) and the EF system under reductive conditions of -1.25 V (e.g., ~154 g/L). The maximum lysine production significantly increased by ~10-folds when the EF system was switched from the N2-gas environment (e.g., ~12 g/L) to the CO2-gas environment (e.g., ~112 g/L). Especially, under the reductive condition (EF system), the maximum lysine production can further be increased by >3-folds (e.g., from ~112 g/L to ~357 g/L) by adding redox mediators, such as anthraquinone-2-sulfonate. Furthermore, the values for lysine yields relative to glucose consumed were significantly varying (e.g., 5 – 40 mol-lysine/mol-glucose) with each environmental condition, highlighting the importance of electrochemical parameters, N2 vs. CO2-gas, and presence of additional redox mediators
