Sustainable Solutions for Environmental Pollution. Группа авторов

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 (m_max = 0.133/h vs. 0.127/h), and higher specific substrate utilization rate (q_s = 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 (Xafenias et al., 2017). Next, Vassilev et al. (2018) also supported the application of the EF system on lysine production using similar conditions (e.g., glucose and C. glutamicum) as Xafenias et al. (2017). Their study identified that the microbial interaction (C. glutamicum) with the anode was very limited as illustrated by a low current output (e.g., 0.022 mA/cm²). Hence, their study first focused on enhancing the electron transfer by introducing an additional redox mediator, ferricyanide to support the oxidation of the metabolic substrate (fermentation process), which can potentially increase the lysine production. When the ferricyanide was introduced, it was reduced to ferrocyanide by C. glutamicum, then, reoxidized by the anode, where this resulted in transfer of electrons to the anode and development in current output (e.g., 0.022 mA/cm² to ~0.090 mA/cm²). Similar to a previous study (Xafenias et al., 2017), their EF system showed a higher volumetric production rate of lysine (e.g., 35 μmol/L/hr vs. 24 μmol/L/hr) compared with the open circuit control. Additionally, when higher biomass inoculum of C. glutamicum (e.g., 6.6 times higher, 0.42 g/L vs. 2.8 g/L) was used, the volumetric production rate of lysine significantly increased from 35 μmol/L/hr to 202 μmol/L/hr due to higher glucose consumption (e.g., > 97% glucose was consumed in 13.4 h compared to 78 h in lower biomass inoculum), achieving the total lysine concentration of 2.94 mM.

      1.3.8 1,3-propanediol

      1,3-propanediol is an important industrial chemical widely used as a monomer to synthesize various commercial products, including cosmetics, plastics, foods, and medicines (Yang et al., 2018). The global market size for1,3-propanediol is expected to reach ~690 million USD by the year 2025 (www.marketsandmarkets.com). Although 1,3-propanediol can be produced chemically through two methods (hydroformylation of ethylene and the hydration of acrolein), these traditional chemical synthesis methods are not considered sustainable due to high energy consumption, the requirement of expensive catalysts, and the generation of hazardous intermediates (Yang et al., 2018). Therefore, the biological production of 1,3-propanediol from waste biomass (e.g., glycerol waste from the biodiesel production process) is considered as a greener and safer approach (Vivek et al., 2017; Yang et al., 2018). Particularly, microbial conversion of glycerol with various fermentative bacteria (e.g., Citrobacter, Klebsiella, Lactobacillus, Enterobacter, and Clostridium) has been extensively investigated (Drozdzynska et al., 2011; Vivek et al., 2017; Yang et al., 2018). Crude glycerol, a major by-product from biodiesel production process can serve as a feedstock for 1,3-propanediol synthesis via fermentation. Typically, 1 L of crude glycerol is generated per 10 L of biodiesel production via transesterification of triglycerides (vegetable oil or animal fats), in the presence of primary alcohol (e.g., methanol) and a catalyst (Sarma et al., 2012). However, low yield, product inhibition, etc., have been identified as the major bottlenecks in the fermentation of glycerol to 1,3-propanediol (Vivek et al., 2017; Yang et al., 2018).

Various approaches proposed by researchers for enhancing biosynthesis of 1,3-propanediol from glycerol include the customization of metabolic pathways of fermentative bacteria by genetic engineering (Yang et al., 2018). As demonstrated by a few studies, cathodic EF appeared to be a promising method for enhancing the biosynthesis of 1,3-propanediol from glycerol (Choi et al., 2014; Moscoviz et al., 2018; Xafenias et al., 2015; Zhou et al., 2013; Zhou et al., 2015). Notably, for 10% increase in 1,3-propanediol yield, the required energy for EF operation was <1% of the total electron equivalents of