Understanding extracellular electron transport of industrial microorganisms and optimization for production application

Abstract

Microbial electrosynthesis (MES) and electro-fermentation are novel approaches to increase microbial production by stimulating the metabolism of the cells electrically. The technique is still in its infancy and while already hyped as promising method to convert CO 2 or cheap carbon sources and electrical energy into valuable chemicals and fuels, little is known about its true potential. This project uses a combination of in silico and in vivo approaches to gather novel information about the benefits and limitations of microbial electrosynthesis as well as its fundamental mechanisms.Understanding microbial electron transport mechanisms is the key to optimization of any bioelectrochemical technology. Therefore, this work analyses the different electron transport chains that nature offers for organisms such as metal respiring bacteria and acetogens, but also standard biotechnological organisms currently used in bioproduction.Special focus lies on the essential connection of redox and energy metabolism, which is often ignored when studying bioelectrochemical systems. The possibility of extracellular electron exchange at different points in each organism is discussed regarding required redox potentials and effect on cellular redox and energy levels. Key compounds such as electron carriers (e.g. cytochromes, ferredoxins, quinones, flavins) are identified and analysed regarding their possible role in electrode-microbe-interactions.Based on these findings a stoichiometric network analysis is performed analysing the effect of electrical stimulation on the microbial metabolism theoretically. Escherichia coli is used as model organism for production with the aim to identify target processes for MES.For the first time, 20 different valuable products were screened for their potential to show increased yields during anaerobic electrically enhanced fermentation. Surprisingly it was found that an increase in product formation by electrical enhancement is not necessarily dependent on the degree of reduction of the product but rather the metabolic pathway it is derived from. Contrary to the usual assumption a reduced product would always require electron supply by a cathode this study shows that a complex metabolic analysis is needed to identify the overall redox state of each process. A variety of beneficial processes is presented with product yield increases of maximal +36% in reductive and +84% in oxidative fermentations and final theoretical product yields up to 100%. This includes compounds that are already produced at industrial scale such as succinic acid, ii lysine and diaminopentane as well as potential novel bio-commodities such as isoprene, para-hydroxybenzoic acid and para-aminobenzoic acid. Furthermore, it is shown that the way of electron transport has major impact on achievable biomass and product yields. The coupling of electron transport to energy conservation could be identified as crucial for most processes.The in vivo approach of this project introduces the development of a standardised reactor platfor...

Metabolic mechanisms and regulation of mixed culture fermentation

“…For redox stress, it has been shown that degree of reduction of the product spectrum can be much greater than the stoichiometric changes that would be anticipated by simple electron uptake 137 , indicating that electron uptake cannot be the only factor influencing the change in product spectrum. Metaproteomic analysis will be used to analyze the switching mechanisms in both scenarios.…”

“…This generally refers only to increasing the electron load relative to the substrate rather than providing an electron sink, and has been studied in pure culture fermentations as a means to increase the production of reduced products 123,124,137 . Electron transfer can be accomplished through direct electron transfer via bionanowires, or through indirect electron transfer via electrode-generated hydrogen or using a mediating compound of known redox potential (Section 1.3.3).…”

“…It has been assumed in previous electro-fermentation studies that interaction with the electron mediation system, via surface flavoproteins such as cytochromes, is responsible for the observed changes in product spectrum 123,124,137 .…”

“…While successful electro-fermentation has been demonstrated across a range of mediators using a variety of pure cultures 124 , it has also been shown that biological interaction with thereduced mediator is dependent on the available cellular machinery and specific thermodynamics of each species 137 . Each cell relies on a variety of proteins and mechanisms within the electron transport chain to maintain balanced redox potential in several interacting pools of EMCs, which range in SRP from ubiquinones in the +50 to +100 mV range to ferredoxins in the -400 to -500 mV range.…”

“…Additionally, mediators having high toxicities should be avoided in favor of synthetic cobalt compounds, such as those used by Emde and Schink 123 and Kracke 137 . Finally, the range of mediators should extend beyond the reduction potentials of the intracellular mechanisms while avoiding H2 production in cathodic cells and O2 production in anodic cells.…”