Nitrogenase and Hydrogenase: Enzymes for Nitrogen Fixation and Hydrogen Production in Cyanobacteria

Cyanobacteria, one of the most ancient groups of oxygenic photoautotrophs are ubiquitously found in almost all habitats on earth. Under nitrogen limitation, some of the cyanobacteria have the ability to differentiate a specialized cell called heterocyst which provides the microoxic environment favorable for proper functioning of oxygen-sensitive enzyme nitrogenase. In heterocystous cyanobacteria, nitrogenase complex is involved in N2 fixation which is also accompanied by hydrogen production. Today, the limited sources of fossil fuel prompt us to exploit various unconventional methods including hydrogen gas as an alternate source of energy. H2 gas is an environmentally clean and efficient alternative for energy. Several microorganisms including cyanobacteria can produce hydrogen. Therefore, this chapter first of all, highlights the basics of nitrogen assimilation and its regulation in cyanobacteria. It also deals and discusses in detail the nitrogenase and its involvement in H2 production. Further, the cyanobacterial hydrogenases and their role in hydrogen production and regulation have also been discussed. Various approaches used to enhance hydrogen production, like genetic and metabolic engineering, have been emphasized along with information on the different types of photobioreactor which are being used for a large-scale industrial production of hydrogen gas. Overall, this chapter summarizes the information about the nitrogenases and hydrogenases involved in H2 production in cyanobacteria and the technologies being utilized for increasing hydrogen production at industrial level. Understanding the underlying mechanisms of hydrogen production in cyanobacteria and improving production methods enable commercialization of molecular hydrogen as an efficient and environment-friendly energy source.