Photo-Bioelectrochemistry of Cyanobacteria Lacking Respiratory Terminal Oxidases

Abstract

In cyanobacteria, the excess electrons produced through photosynthetic electron transport chain at light intensities, are re-oxidized by respiratory terminal oxidases (RTOs) present in the respiratory electron transport chain providing an overflow pathway for excess electrons to minimize photo damage [1]. We have previously demonstrated enhanced photo-bioelectrochemical activity by Synechococcus elongatus if the organism was genetically modified to express an outer membrane cytochrome namely OmcS [2]. We hypothesize that by knocking out the genes corresponding to the RTOs in the genetically engineered Synechococcus elongatus PCC7942 with outer membrane cytochrome (OmcS), the organism’s extracellular electron transfer ability towards the electrode could be further enhanced through the blockage of the overflow pathways. In this study, the genes responsible for the various RTOs in PCC7942 include bd-type quinol oxidase encoded by cydBA (cyd), aa3-type cytochrome oxidase encoded by coxBAC (cox) and cbb3-type cytochrome oxidase encoded by ccoNQ (cco) were knocked out and the resulting mutants were evaluated for their extracellular electron transfer ability through both chemical and electrochemical tests. PCR was carried out to confirm the successful construction of single mutants. Further, double and triple mutants of Synechococcus elongatus PCC7942 in different combinations of the RTO knockouts were also performed and the resulting electrochemical performance was measured using conventional electrochemical techniques.