Microbial electron transport and energy conservation – the foundation for optimizing bioelectrochemical systems

Abstract

Microbial electrochemical techniques describe a variety of emerging technologies that use electrode–bacteria interactions for biotechnology applications including the production of electricity, waste and wastewater treatment, bioremediation and the production of valuable products. Central in each application is the ability of the microbial catalyst to interact with external electron acceptors and/or donors and its metabolic properties that enable the combination of electron transport and carbon metabolism. And here also lies the key challenge. A wide range of microbes has been discovered to be able to exchange electrons with solid surfaces or mediators but only a few have been studied in depth. Especially electron transfer mechanisms from cathodes towards the microbial organism are poorly understood but are essential for many applications such as microbial electrosynthesis. We analyze 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 bio-production. 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, ferredoxin, quinones, flavins) are identified and analyzed regarding their possible role in electrode–microbe interactions. This work summarizes our current knowledge on electron transport processes and uses a theoretical approach to predict the impact of different modes of transfer on the energy metabolism. As such it adds an important piece of fundamental understanding of microbial electron transport possibilities to the research community and will help to optimize and advance bioelectrochemical techniques.

Performance and recent improvement in microbial fuel cells for simultaneous carbon and nitrogen removal: A review

“…The electron transfer mechanisms at the anode have been widely investigated, however, the transfer mechanisms from the cathode to microorganisms are poorly understood (Kracke et al, 2015;Rosenbaum et al, 2011).…”

“…Electrons produced from the oxidation of carbon are transferred to the anode by means of several mechanisms: direct as well as mediated electron transfer mechanisms (Kracke et al, 2015;Mook et al, 2013b;Rahimnejad et al, 2015;Schroder, 2007). Direct electron transfer (DET) refers to electrons transferred by c-type cytochromes (CTCs), flavin bound to c-type cytochromes and conductive pili, which may serve as biological nanowires (Fig.…”

Anodic respiration of Pseudomonas putida KT2440 in a stirred-tank bioreactor

“…Furthermore mass transfer limitations were significantly reduced. It could therefore be assumed that the metabolism is ATP limited during the electro-fermentation as suggested by theoretical considerations [45]. This would explain lower biomass concentrations as well as slower specific rates in the bio-electrochemical process.…”

Biologically activated graphite fiber electrode for autotrophic acetate production from CO2in a bioelectrochemical system

“…On the other hand, the interaction between microorganism and carbon materials is still unknown, which is the main factor limiting further improvement of the performance of the MES process. For example, insufficient information about microbecarbon interactions is delaying the advance of the process significantly when the input potential is <−410 mV vs standard hydrogen electrode (SHE), which is the theoretical minimum potential for hydrogen production [9].Carbon has been recognized as the best material for fuel cell applications owing to its good electrical conductivity, its potential for the impregnation of catalysts, and its cost effectiveness [10,11]. For the same reasons, a range of morphologies of carbon materials have been utilized as anode and cathode electrodes in BES in the form of carbon paper, cloth, graphite rod, and felt in BES systems [12].…”