Cyanobacteria: A metabolic power house for harvesting solar energy to produce bio-electricity and biofuels

Abstract Cyanobacteria are a group of light harvesting prokaryotic microorganisms displaying a vast diversity in terms of their morphology, physiology, and metabolic capabilities, which appear to be important factors for their survival in diverse ecological niches. The metabolism of cyanobacteria does not fit well into a linear understanding of generalized photosynthetic microorganisms. In addition to the water oxidizing photosynthesis accomplished by coupling photosystem I and photosystem II activities, they also possess intersecting photosynthetic and respiratory electron transport chains in thylakoid membranes which help them to adjust electron flow in the membranes and linked energy metabolism as per the need or demand of the situation. The cyanobacteria have an incomplete tricarboxylic acid (TCA) cycle as they lack 2-oxoglutarate dehydrogenase. However, the enzymes, 2-oxoglutarate decarboxylase and succinic semialdehyde dehydrogenase encoded by their genes convert 2-oxoglutarate to succinate, and thereby use this shunt pathway not only to support the cells to maintain production of reducing equivalents (NADPH), but also to provide unique flexibility to its metabolic system that manifested in their various functions some of which are being progressively understood. The existence of unusual TCA cycle shunt in cyanobacteria opens up a new research avenue for engineering cyanobacteria for biotechnological applications including production of various biofuels of high commercial interest. The unique respiratory metabolisms could also be exploited to generate electrogenic cyanobacterial cells for production of bioelectricity in a fuelcell setup.