Cyanobacteria is a remarkable group of prokaryotic photosynthetic microorganisms, with several genera capable of fixing atmospheric nitrogen (N 2 ) and presenting a wide range of morphologies. Although the nitrogenase complex is not present in all cyanobacterial taxa, it is spread across several cyanobacterial strains. The nitrogenase complex has also a high theoretical potential for biofuel production, since H 2 is a by-product produced during N 2 fixation. In this review we discuss the significance of a relatively wide variety of cell morphologies and metabolic strategies that allow spatial and temporal separation of N 2 fixation from photosynthesis in cyanobacteria. Phylogenetic reconstructions based on 16S rRNA and nif D gene sequences shed light on the evolutionary history of the two genes. Our results demonstrated that ( i ) sequences of genes involved in nitrogen fixation ( nif D) from several morphologically distinct strains of cyanobacteria are grouped in similarity with their morphology classification and phylogeny, and ( ii ) nif D genes from heterocytous strains share a common ancestor. By using this data we also discuss the evolutionary importance of processes such as horizontal gene transfer and genetic duplication for nitrogenase evolution and diversification. Finally, we discuss the importance of H 2 synthesis in cyanobacteria, as well as strategies and challenges to improve cyanobacterial H 2 production.
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