Microbial fuel cells represent a promising technology for simultaneous wastewater treatment and renewable electricity production. However, the electricity recovery is still poor, typically <10% of what is theoretically possible and the extracellular electron transfer mechanisms are poorly understood.
The use of co-cultures to improve substrate (glucose) turnover rate and hence electricity recovered was investigated initially. A co-culture of Shewanella oneidensis and Clostridium beijerinckii gave a maximum power density (Pmax) of 87 mWm-2 (67% COD reduction) compared to 60 mWm-2 for C.beijerinckii alone and 48 mWm-2 for S.oneidensis alone. Co-culturing Geobacter sulphurreducens, C. beijerinckii and Saccharomyces cerevisiae gave the highest Pmax value of 80 mWm-2 (41% COD reduction) compared to other strain combinations.
Another study investigated the contribution of direct electron transfer mechanism on electricity production by physically retaining Shewanella oneidensis cells close to or away from the anode electrode using a dialysis membrane (as well as immobilisation of the cells in alginate). Pyruvate was used as the substrate. The outcome of this study indicated a Pmax value of 114±6 mWm-2 when cells were retained close to the anode, 3.5 times more than when the cells were separated from the anode. Without the membrane Pmax was 129±6 mWm-2 (57% COD reduction).
To understand the role played by c-type cytochromes MtrA, MtrB and MtrC in extracellular electron transfer in S.oneidensis, the genes mtrA, mtrB, mtrC and their combinations were heterologously expressed in non-electrogenic bacteria (Escherichia coli; glucose as substrate). The mtrCAB transformant gave the highest Pmax of 24 mWm-2 compared to 1 mWm-2 for the wild type although cell growth was slower.
The results demonstrate the importance of co-cultures and of the MtrCAB pathway (direct electron transfer mechanism) in improving bacterial electricity production.