Cyanobacteria are used as anode catalyst in photo-bioelectrochemical cells to generate electricity in a sustainable, economic and environmental friendly manner. Though cyanobacteria (CB) have huge advantage for solar energy conversion through its robust photosynthesis, they could not perform extracellular electron transfer effectively as other electricigens. This is because, unlike electricigens such as Geobacter sp., CB do not possess any special key players on their outer membrane to carry out direct electron transfer. In this work, we genetically engineered a cyanobacterium named Synechococcus elongatus PCC7942 to express an outer membrane c-type cytochrome called OmcS¹. The genetically modified CB (Eng-CB) enhanced the photocurrent generation significantly in photo-bioelectrochemical cells compared to that of wild type without the addition of any redox mediator. A specific redox peak corresponding to OmcS (~ 0.1 V vs Ag/AgCl) was observed in Eng-CB in a low-scan rate cyclic voltammetry (1 mV/s) and the same was found absent in the wild type. Both these evidences confirmed the establishment of direct electron transfer in Eng-CB. Also, extracellular electron transfer of the Eng-CB was investigated biochemically based on its ability to reduce the exogenously added soluble redox compound like potassium ferricyanide during its growth in BG11 medium. The enhanced ferricyanide reduction as well as photocurrent generation by the Eng-CB under both light and dark conditions elucidates the fact that the heterologously expressed OmcS could interact with both photosynthetic and respiratory electron transport chains. This work demonstrates the importance of interdisciplinary and integrated approach to enhance the photocurrent generation using CB and aid in advancing the understanding of biological interfaces with the electrode leading to for rapid development of photosynthetic microbial fuel cell technology. 

References:

1. N. Sekar, R. Jain, Y. Yan, R. P. Ramasamy (2015) Biotechnol Bioeng.  doi: 10.1002/bit.25829