Morphological characteristics of Shewanella biofilms formed on graphite electrodes in bioelectrochemical systems
1Tokyo Univ. Pharm. Life Sci., 2, 3, 4, 5, 6, 7, 8, 9, 10
Shewanella oneidensis MR-1 has been widely studied as a model to uncover molecular mechanisms underlying bacterial electricity generation in bioelectrochemical systems (BESs). Previous studies have shown that MR-1 forms biofilms on electrodes in BESs, and it has been believed that biofilm formation is an important step for electricity generation. However, there has been no report describing morphological characteristics and growth processes of intact electrogenic biofilms formed on graphite electrodes. In the present study, we developed experimental setups for observing intact biofilms on graphite electrodes, and biofilms were observed using a confocal laser scanning microscope (CLSM).
An electrochemical flow cell developed in the present study is a three-electrode single-chambered cell, in which a graphite working electrode (WE) was placed at the bottom, and biofilms formed on WE were observed by soaking an objective lens of CLSM from the top. We used a platinum wire as a counter electrode and an Ag/AgCl electrode as a reference. For biofilm formation, the flow cell was continuously supplied with a deaerated lactate minimal medium (LMM) and inoculated with MR-1 constitutively expressing anaerobic GFP. Currents were measured at a WE potential of +0.2V. We found that biofilms formed under an electrogenic condition were flat and much thin compared to those formed under an aerobic condition (using aerated LMM, open circuit). Mushroom-like structures typical for bacterial biofilms were observed only in aerobic biofilms. It is likely that the observed biofilm structures reflect catabolic styles of MR-1 under respective conditions, namely respiration with oxygen or electrode. Studies are underway to examine effects of the medium composition, electrode potential and flow rate on biofilm formation under electrogenic conditions.
keywords:Biofilm,Microbial fuel cells (MFCs),Electrogenesis,,,