Terminal oxidase mutants of the cyanobacterium Synechocystis sp. PCC 6803 show increased electrogenic activity in biological photo-voltaic systems

Abstract

Biological photo-voltaic systems are a type of microbial fuel cell employing photosynthetic microbes at the anode, enabling the direct transduction of light energy to electrical power. Unlike the anaerobic bacteria found in conventional microbial fuel cells that use metals in the environment as terminal electron acceptors, oxygenic photosynthetic organisms are poorly adapted for electron transfer out of the cell. Mutant strains of the cyanobacterium Synechocystis sp. PCC 6803 were created in which all combinations of the three respiratory terminal oxidase complexes had been inactivated. These strains were screened for the ability to reduce the membrane-impermeable soluble electron acceptor ferricyanide, and the results were compared to the performance of the mutants in a biological photo-voltaic system. Deletion of the two thylakoid-localised terminal oxidases, the bd-quinol oxidase and cytochrome c oxidase resulted in a 16-fold increase in ferricyanide reduction rate in the dark compared to the wild-type. A further improvement to a 24-fold increase was seen upon deletion of the remaining "alternative respiratory terminal oxidase". These increases were reflected in the peak power generated in the biological photo-voltaic systems. Inactivation of all three terminal oxidase complexes resulted in a substantial redirection of reducing power; in the dark the equivalent of 10% of the respiratory electron flux was channelled to ferricyanide, compared to less than 0.2% in the wild-type. Only minor improvements in ferricyanide reduction rates over the wild-type were seen in illuminated conditions, where carbon dioxide is preferentially used as an electron sink. This study demonstrates the potential for optimising photosynthetic microbes for direct electrical current production.

Phycobilisome-Deficient Strains of Synechocystis sp. PCC 6803 Have Reduced Size and Require Carbon-Limiting Conditions to Exhibit Enhanced Productivity

“…Therefore, drawing any reliable comparison between our results and those of Kwon et al (2013) is not possible. However, the olive mutant may be useful for applications that require higher photosynthetic capacity, such as the generation of hydrogen (McCormick et al, 2013) or electricity in a biophotovoltaic device (Bombelli et al, 2011;Bradley et al, 2013).…”

Tapping into cyanobacteria electron transfer for higher exoelectrogenic activity by imposing iron limited growth

“…1). 12,25 Nevertheless, biolms of photosynthetic microorganisms are capable of electricity generation with no addition of mediators (mediatorless), revealing direct electron transfer, 7 and potentially representing a more sustainable platform.…”

“…generated in the photolysis of water are directed to CO 2 xation, it is only a very small fraction reaching out the cell surface. 12 The mechanisms and metabolic pathways involved in the redirection of electrons to the extracellular membrane are a matter of study and yet to be fully understood. 13,14 Good exoelectrogenic bacteria are those coupling respiration to an extracellular nal electron acceptor, and examples of exoelectrogens such as Shewanella oneidensis and Geobacter sulfurreducens have been reported extensively in the literature.…”

“…PCC6803 (wild-type). 12 We estimate a redirection of reducing power from photosynthesis in the order of 1% by iron sufficient PCC7942 at neutral pH and 28% by iron limited PCC7942 at neutral pH (ESI eqn (1) †), considering photosynthetic oxygen production of 350 pmol nmol chl À1 min À1 , determined for PCC7942 in the studied conditions (ESI Fig. 1 †).…”

Photosynthetic Energy Conversion: Recent Advances and Future Perspective

“…Some or all of the competing oxidases in R-ETC were knocked out of cyanobacteria through genetic manipulations that resulted in more electrons being available for photocurrent generation. 46 Further, the superior electron transfer ability of DMRB such as Geobacter and Shewanella can be predominantly attributed to the cytochromes present in their outer membrane collectively called as outer membrane cytochromes (OMC). Imparting these efficient extracellular electron transfer ability of DMRB to cyanobacteria through genetic manipulations can be a ground breaking and remarkable milestone in cyanobacterial photosynthetic energy conversion.…”