Attachment under current – biofilm formation by electroactive bacteria
Bioelectrochemical systems (BES) are hybrid systems using electroactive bacteria and electrochemical techniques. Solid electrodes serve as electron donor for or acceptor from microorganisms for the production of current and/or the generation of valuable substances. Research conducted on BES in this thesis ranged from fundamental investigations on microbial attachment to electrodes to the development of electrode materials for advanced reactor concepts. The first part of this theses was the biochemical analysis of the extracellular polymeric substances (EPS) secreted by G. sulfurreducens under electroactive conditions. G. sulfurreducens was cultivated in MFC-mode on graphite based electrodes polarized to +400 mV vs. Ag/AgCl for 8 d. A maximum current density of 172 ± 29 μA cm-2 was reached after 7 d. Routine methods for the biofilm harvest and the EPS processing were established. Electroactive cultures secreted significantly more EPS compared to cells grown under standard heterotrophic conditions (fumarate respiration). With 116 pg cell-1, the highest amount of EPS was measured for the soluble EPS fraction of G. sulfurreducens using anode respiration, followed by the tightly bound (18 pg cell-1) and loosely bound (11 pg cell-1) fractions of the EPS. Proteins were found to dominate all EPS fractions of the biofilms grown under electrochemical conditions. The second part was the development of a membrane separated flow cell for the simultaneous electrochemical impedance spectroscopy (EIS) and confocal laser scanning microscopy (CLSM) [Stöckl et al. 2016]. A flow cell made from PEEK was constructed, using a transparent indium tin oxide electrode as working electrode. A fluorescent S. oneidensis was cultivated under MFC conditions. A decrease of the charge transfer (RCT) from 292 kΩ to 120 kΩ was observed with an increased current of 0.52 μA cm-2 after 17 h of operation. The CLSM images revealed an increasing cell number of S. oneidensis on the WE electrode to a monolayer with 26 cells 100 μm-2 after 17 h under MFC conditions. As final part a straight forward approach to synthesize magnetic electrode particles allowing the artificial fixation of electroactive bacteria was developed [Stöckl, et al. 2016, DE102014112685A, Frankfurt, Germany]. The microwave assisted synthesis of magnetite was applied for the production of the magnetic electrode particles with activated carbon (PMAG/AC). The surface area is around 300 m2 g-1 and the particle size ranges between 20 and 200 μm. Resting cells of S. oneidensis attached to a maximum concentration of 8 · 10^10 ± 3 · 10^9 resting cells g-1 PMAG/AC. Electrochemical examination revealed that magnetically immobilized PMAG/AC showed a capacitive current response during cyclic voltammetry. Linear sweep voltammetry indicated that particles were stable down to a potential of –680 mV vs. Ag/AgCl.