Engineering electron transfer pathways of exoelectrogens

Extracellular electron transfer (EET) of exoelectrogens and electrophilic cells plays fundamental rules in natural biogeochemical processes, and enabled many applications in energy and environments, such as microbial electrochemical systems (MES) including microbial fuel cells, microbial electrolysis cells, microbial reverse-electrodialysis cells, and microbial electrosynthesis, etc. MES technologies are appealing in capturing energy from organic wastes and biomass and converting these organics into value-added chemicals via inward transferred electrons as reducing equivalent. Bacterial EET that dictates the exchange of electrons between bacteria and conductive surfaces of electrodes is a major bottleneck in determining the efficiency of MES. In this talk, I will present our recent researches in engineering microbes and the interactions between microbes and electrodes to facilitate EET. Based on the molecular mechanisms of EET, i.e., directcontact via c-type cytochromes, and shuttle-mediated EET, we engineered a few well-established exoelectrogenic bacteria (in particular Shewanella oneidensis) by synthetic biology approaches: (1) exogenous synthesis of riboflavin (vitamin B2) as electron shuttle in S. oneidensis; (2) engineering de novo NADH biosynthesis and cofactor engineering in regulation of NADH/NAD+ ratio, thus increasing intracellular releasable electron pool to enhance EET; and (3) to increase the spectrum of carbon sources that can be used by exoelectrogens, we rationally engineered a number of microbial consortia that included fermenter and exoelectrogens for simultaneously enhanced metabolism and EET. In all, such synthetic biology efforts in engineering EET efficiency of exoelectrogens facilitated their practical adoption into many energy and environmental applications.