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.
Highlights
The Earth has a rapidly growing and developing population which is projected to result in a 53% increase in global power consumption for the period 2008–2035 to an annual average of B26 TW.[1]
Oxygenic photosynthetic organisms such as cyanobacteria have evolved to couple the capture of light energy to carbon dioxide fixation as efficiently as possible
During respiration or in times of excess electron production, reducing power can be dissipated to oxygen using terminal oxidases
Summary
The Earth has a rapidly growing and developing population which is projected to result in a 53% increase in global power consumption for the period 2008–2035 to an annual average of B26 TW.[1] A heavy reliance on fossil fuels is likely to result in climate change with potentially devastating consequences for ecosystems around the world.[2] Biological photo-voltaics is an emerging energy production technology which has the potential to use the planet’s most abundant source of energy – sunlight – to produce electrical power directly with lower carbon emissions than with fossil fuels. Cyanobacteria are oxygenic photosynthetic microbes that are well adapted for light energy capture and charge separation, but their metabolism
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