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.
Synthetic biology was employed in this study to overexpress the genes: mtrA (periplasmic membrane cytochrome), mtrB (outer membrane β-barrel protein) and mtrC (outer membrane decaheme cytochrome C) involved in the Mtr pathway for extracellular electron transfer in Shewanella oneidensis. The genes were expressed individually or as operons and the effect on electricity production and substrate utilisation determined using H-type microbial fuel cells.
The power generated by the mtrAB construct (144 mWm-2) was 3 fold higher than that produced by the wild type (48mWm-2). Constructs mtrCAB, mtrA, mtrBC produced similar power densities amounting to 78 mWm-2, 77 mWm-2, 74 mWm-2 respectively. Substrate (glucose) utilisation was in the order mtrCAB construct (94%) > mtrC construct (88%) > mtrA construct (76%) > mtrAB construct (36%) > wild type (30%).
The results demonstrate the importance of overexpression of mtrA and mtrB as an operon in S. oneidensis with regard to power production. However, to enhance substrate utilisation, avenues to control the levels of expression of the genes are needed.