ABSTRACT
Cyanobacteria are probably the most diverse group of prokaryotes in the number of species, type of habitats, and morphological and physiological properties. Nitrogen fixation occurs only in prokaryotes. Several cyanobacterial species are able to reduce atmospheric dinitrogen, a process detrimental to the oxygen evolved by photosynthetic activity and present in the environment. Some filamentous strains, such as Plectonema, fix nitrogen only microaerobically, during which all cells participate in this process, or by differentiating vegetative cells to heterocysts, such as in certain Anabaena-Nostoc species under external aerobic conditions. In heterocysts, ATP formation is not completely understood. Oxygen generated by photosynthesis is detrimental to nitrogen fixation; hence, the oxygenic photosystem II is inactive in heterocysts. Biological nitrogen fixation has been studied extensively in different proteobacteria such as Klebsiella pneumoniae, Azotobacter vinelandii, and Rhodobacter capsulatus. Measurements of the Anabaena genome size yielded approximately 6.42 Mb. The fraction of differentially expressed genes in heterocysts was estimated by DNA-RNA hybridization to be 600–1000 genes.
