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Abstract
The ability of microorganisms to transfer electrons to electron acceptors present outside their cellular envelope is called exoelectrogenicity. This study focuses on investigating exoelectrogenicity of cyanobacteria (CB) to generate electricity in a photo-bioelectrochemical cell. Investigation using site-specific photosynthesis inhibitors revealed that the electrons were redirected predominantly from photosystem II to the electrode via plastoquinone pool. Despite having huge advantages for solar energy conversion, CB could not efficiently perform extracellular electron transfer (EET). The reasons being, (1) unlike natural exoelectrogens, CB did not possess any special features on outer membrane to exhibit EET and (2) electrons generated in photosynthetic electron transport chain were channeled into respiratory pathways rather than to the electrode. To overcome the first caveat, CB named Synechococcus elongatus PCC7942 was genetically engineered to express outer membrane c-type cytochrome (OmcS). The genetically engineered CB (omcs) was found to exhibit two-fold greater rate of ferricyanide reduction and generated ~ nine-fold higher photocurrent compared to the wild-type (wt). To address the second issue, each of three respiratory terminal oxidases present in S. elongatus was knocked-out and their role towards EET was investigated. The mutant cyd- (strain lacking bd quinol oxidase) was observed to exhibit higher EET than wt as evident from its higher ferricyanide reduction rate. This result clearly corroborated the fact that bd-quinol oxidase distracted more electrons from the plastoquinone pool. Further, the strain cyd-omcs (i.e., CB that contained OmcS and lacked bd-quinol oxidase) was observed to generate the most photocurrents compared to wt, cyd- and omcs. In addition to enhancing the exoelectrogenicity of CB, EET of a hyperthermophilic archaeon named Pyrococcus furiosus was explored by studying its ability to (1) reduce exogenously added ferric oxide and ferric citrate, and (2) generate electricity in a two chamber microbial fuel cell operated at 90 oC. As a concluding note, a mathematical model has been developed to study the effect of incident light intensity on performance of a biofilm-based anode. Results and findings presented in this dissertation could make significant contributions to the existing knowledge of microbial exoelectrogenicity and benefit advancing the technology of sustainable electricity generation from microorganisms.