Dynamics of Cathode-Associated Microbial Communities and Metabolite Profiles in a Glycerol-Fed Bioelectrochemical System

Abstract

e Electrical current can be used to supply reducing power to microbial metabolism. This phenomenon is typically studied in pure cultures with added redox mediators to transfer charge. Here, we investigate the development of a current-fed mixed microbial community fermenting glycerol at the cathode of a bioelectrochemical system in the absence of added mediators and identify correlations between microbial diversity and the respective product outcomes. Within 1 week of inoculation, a Citrobacter population represented 95 to 99% of the community and the metabolite profiles were dominated by 1,3-propanediol and ethanol. Over time, the Citrobacter population decreased in abundance while that of a Pectinatus population and the formation of propionate increased. After 6 weeks, several Clostridium populations and the production of valerate increased, which suggests that chain elongation was being performed. Current supply was stopped after 9 weeks and was associated with a decrease in glycerol degradation and alcohol formation. This decrease was reversed by resuming current supply; however, when hydrogen gas was bubbled through the reactor during open-circuit operation (open-circuit potential) as an alternative source of reducing power, glycerol degradation and metabolite production were unaffected. Cyclic voltammetry revealed that the community appeared to catalyze the hydrogen evolution reaction, leading to a ؉400-mV shift in its onset potential. Our results clearly demonstrate that current supply can alter fermentation profiles; however, further work is needed to determine the mechanisms behind this effect. In addition, operational conditions must be refined to gain greater control over community composition and metabolic outcomes.

Anode potential influences the structure and function of anodic electrode and electrolyte-associated microbiomes

“…As previously observed, the diversity of the inoculum microbial community changed rapidly upon introduction to the reactors, reflecting the highly selective nature of MFC environments51318. With the onset of current generation, the composition of anodic electrode and electrolyte associated communities significantly diverged (Fig.…”

Microbial Conversion of Waste Glycerol from Biodiesel Production into Value-Added Products

“…An MFC with graphite fiber brush anodes can produce electrical current at power densities of 4.6 W/m 3 using pure glycerol and 2.3 W/m 3 using waste glycerol [77]. A recent study suggests that by externally applying current to a glycerol fermentation reactor inoculated with a mixed culture, metabolite formation can be significantly influenced with associated increases of highly reduced chemicals such as propanol and valerate [146]. Although the mechanism behind this metabolic shift was not clear, it is possibly related to the high hydrogen partial pressure caused by the current supplied in the vicinity of cathode.…”

Metabolic mechanisms and regulation of mixed culture fermentation

“…Previous studies using electro-fermentation did report increased production of electron sink products, but either did not control the terminal redox state via mediators 138 , or studied the effects using a pure culture 123,124 . However, comparison between the measured electron uptake and the shifted product spectrum indicates that the additional electron uptake alone does not always account for the additional reduced product formation.…”

“…Similarly, there is evidence that use of a cathode to supply reducing power in substrate limited conditions will cause both mixed 138 and pure 123,124 cultures to increase production of electron sink products. Emde and Schink 123 demonstrated an increased yield of reduced products with mediator compounds of increasingly negative redox potential, indicating that at least some bacteria can sense an increased electron load, and will manipulate their product spectrum accordingly.…”

“…As shown by Emde and Schink 123 , the use of mediators with more negative reduction potentials results in increased yield of reduced products in pure culture fermentation. Cathodes without mediators have also been used in mixed-culture fermentation 138 . However, no reports of mediated mixedculture electro-fermentation could be found at this time.…”