Applications and perspectives of phototrophic microorganisms for electricity generation from organic compounds in microbial fuel cells

There is an increasing interest to integrate phototrophic microorganisms into microbial fuel cells (MFCs) to assist electricity generation. In general, this integration can be accomplished in three ways: (1) phototrophic microorganisms function as or provide a substrate for supplying electrons; (2) photoheterotrophic microorganisms catalyze the anode reaction; and (3) photoautotrophic microorganisms provide oxygen as an electron acceptor to the cathode reaction. Direct use of phototrophic microorganisms for electricity production in MFCs faces significant challenges, because of the complex composition of microbial cells and their resistance to hydrolysis, and low conversion efficiency to electric energy by MFCs. Proper pretreatment using chemical or biological methods may improve degradability of microbial cells. Some purple nonsulfur bacteria exhibit strong electrochemical catalysis of organic compounds in the anode of an MFC, and the effect of illumination on the catalytic performance needs further investigation. Electricity generation via syntrophic relationship between photosynthetic microorganisms (providing organic compounds) and heterotrophic bacteria (oxidizing organics) in the anode is generally low due to low concentration of the electron donors and adverse effect of oxygen as a result photosynthesis on anode activities. It is promising to apply photosynthetic microorganisms in the cathode with multiple functions of oxygen supply, nutrient removal and biomass production. To address some of the challenges, two paradigms are proposed to encourage further investigation and development of effective processes with strong synergy between phototrophic microorganisms and MFCs.