Heterocyst and akinete differentiation in cyanobacteria

Cyanobacteria are an ancient and morphologically diverse group of photosynt hetic prokaryotes. They were the first organisms to evolve oxygenic photosy nthesis, and so changed the Earth's atmosphere from anoxic to oxic. As a co nsequence, many nitrogen-fixing bacteria became confined to suitable anoxic environmental niches, because the enzyme nitrogenase is highly sensitive t o oxygen. However, in the cyanobacteria a number of strategies evolved that protected nitrogenase from oxygen, including a temporal separation of oxyg enic photosynthesis and nitrogen fixation and, in some filamentous strains, the differentiation of a specialized cell, the heterocyst, which provided a suitable microaerobic environment for the functioning of nitrogenase. The evolution of a spore-like cell, the akinete, almost certainly preceded tha t of the heterocyst and, indeed, the akinete may have been the ancestor of the heterocyst. Cyanobacteria have the capacity to differentiate several ad ditional cell and filament types, but this review will concentrate on the h eterocyst and the akinete, emphasizing the differentiation and spacing of t hese specialized cells.