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 a substrate (e.g., glycerol) (Moscoviz et al., 2018). Choi et al. (2014) studied 1,3-propanediol production via cathodic EF of glycerol (~300 mM) with pure culture Clostridium pasteurianum DSM 525 in a dual-chamber bioreactor. Their study showed that Clostridium pasteurianum could directly accept electrons from the cathode electrode. Electrochemically induced reducing power could considerably influence intracellular redox condition and accelerate the synthesis of NADH-consuming compounds like 1,3-propanediol. Notably, as compared to the control (i.e., conventional fermentation), EF could increase NADH consumption towards 1,3-propanediol synthesis by 3.5 times; other by-products were butanol and butyrate. Nonetheless, NADH generation from electricity was very minimal as compared to the NADH generated from substrate. Thus, EF could provide a shift in metabolic pathways in Clostridium pasteurianum to provide a higher 1,3-propanediol yield from glycerol.
A few studies also investigated mixed-culture EF of glycerol for improving 1,3-propanediol yield (Xafenias et al., 2015; Zhou et al., 2013). Xafenias et al. (2015) studied mixed-culture cathodic EF of glycerol in a dual-chamber bioreactor operated under different cathode potential ranged from −0.80 V to −1.10 V vs. SHE (Standard hydrogen electrode). Compared to the conventional fermentation, EF could increase 1,3-propanediol production rate up to 6 times. Moreover, 1,3-propanediol concentration was considerably higher in the EF fermentation (EF: 42 g/L; Control: 18 g/L), suggesting microbial consortia in EF had a higher tolerance to product inhibition. As mentioned earlier, product inhibition is a commonly identified phenomenon in fermentative 1,3-propanediol production. The authors hypothesized that the higher local pH near the cathode could provide a favorable metabolic condition for microbial consortia in the cathode biofilms by decreasing concentrations of undissociated acids. This study also demonstrated the importance of cathode potential optimization for improving EF performance. Notably, known 1,3- propanediol-producing microbes (Clostridiaceae) were enriched at an applied cathode potential of −1.1 V vs. SHE, while unfavorable applied potential led to the enrichment of propionate-producing microbes (e.g., Veillonellaceae), which also corroborated with propionate accumulation. Moreover, after replacing the biocathode with an electrode lacking biofilms, glycerol consumption considerably decreased with a lower yield of 1,3- propanediol. The absence of EF stimulated the growth of Lactobacillaceae followed by the production of lactate.
A recent study by Moscoviz et al. (2018) demonstrated biocathode pre-enriched with G. sulfurreducens, a known electroactive bacterial species, could improve mixed-culture glycerol EF with considerably shortened lag time and enhanced 1,3- propanediol production by up to 10%. The authors suggested that G. sulfurreducens, pre-enriched on the cathode, could serve as a living mediator between the electrode and fermentative bacteria and promote the selection of kinetically efficient 1,3- propanediol producers. Based on these studies, EF opens up promising opportunities for increasing glycerol fermentation rates and 1,3- propanediol yields.
1.4 Challenges and Future Outlook
Still, several challenges must be addressed before further development or scaling-up of the EF system for biofuel and value-added chemical synthesis (e.g., lipids, acetoin, PHB, L-lysine). To start with, more explorations of different working conditions, such as temperature, pH, and working electrode potentials are required for the commercialization of the EF system. General mechanism and fundamental knowledge on bacterial strains and their impact on the EF performance are still lacking. For instance, Bursac et al. (2017) were not able to identify the reasons behind the positive impact on anode cell densities in their EF system due to -prophage deletion during acetoin production. Furthermore, Lai and Lan (2020) stated that an optimal energy input should be evaluated in EF, where their EF performance was reduced due to higher current input inhibiting their bacteria (e.g., Ralstonia eutropha H16). High electrical current supply can cause direct cell damage (sometimes resulting in cell death) via electroporation, production of reactive oxygen species, or generation of other toxic substances (Lai & Lan, 2020). Hence, more experimental evidence is needed for the selection of optimal energy input for the EF system. Particularly, minimal information is available on the complex interactions among electrode polarization, substrate composition, and microbial consortia.
Also, fundamental understandings, such as (1) interactions between fermentative bacteria; (2) dissolved redox couples of the medium; and (3) interactions between fermentative bacteria and the surface of the electrodes via different electron
