Extracellular Electron Transfer in Mixed Species Biofilms: The Role of Rikenella Microfusus

Abstract

The capability of microorganisms to transfer electrons to/from an electrode surface is critical for the development of bioelectrochemical systems. However, the extracellular electron transfer (EET) mechanisms have been elucidated only for few microorganisms, in particular Geobacter sulfurreducens (Sh.) and Shewanella oneidensis (G.S.) respectively. A Microbial fuel cell (MFC) is a bioelectrochemical system that couples the possibility to directly convert chemical energy from civil and industrial wastewater into electrical energy thank to the bioelectrocatalytic activity of bacteria colonizing the electrodes. For on-field application of the technology, a mixed bacterial community will be most likely obtained, since wastewater solutions and biomasses are commonly utilized as inoculum. The presence of mixed-species biofilm can also facilitate the oxidation of complex substrates, since many microorganisms capable of EET can only use low-molecular organic acids and alcohols provided by fermenting bacteria. Accordingly, it is important to understand how microorganisms rather than Sh. and G.S. can contribute to the EET process. Different studies have reported a particularly high presence of Bacteroidetes in the microbial community at the anode and the cathode of MFCs. The biological characterizations have indicated an high percentages of Rikenella Microfusus (R.M.), which belong to the order of Bacteroidales, but only few information about its metabolism are available. R.M. was isolated by fecal materials, it is an obligated anaerobic, nonmotile and gram-negative bacterium, which is capable to oxidize a number of substrates as glucose, mannose, lactose producing acids. Herein, a pure culture of R.M. (ATCC® 29728™) was anaerobically grown and the EET was investigated on carbon cloth electrodes (non-wet proofed, E-TEK) potentiostatically polarized at a positive potential (+0.2 V vs. Ag|AgCl Sat.) and maintained in anaerobic conditions by constant purging of nitrogen gas. For the study, glucose was used to provide the substrate for the bacterium. Electrochemical experimental evidences revealed that flavins might be involved in the EET, facilitating the communication of the bacterium with the electrode surface. Moreover, oxygen influence on the current response was detected, critically affecting the capability of the bacterium to perform EET. The impact of the new finding on future researches devoted to clarify the EET for real application of bioelectrochemical systems will be discussed.