A–F: Nostoc commune G–H: Nostoc calcicola
I–M: Tolypothrix distorta N–R: Scytonema hyalinum
Scale bar = 10 μm. Abbreviations: hc - heterocyst, ak - akinete, hm - hormogonium, nd - necridia
Heterocysts or heterocytes are specialized nitrogen-fixing cells formed during nitrogen starvation by some filamentous cyanobacteria, such as Nostoc, Cylindrospermum, and Anabaena.[1] They fix nitrogen from dinitrogen (N2) in the air using the enzyme nitrogenase, in order to provide the cells in the filament with nitrogen for biosynthesis.[2]
Nitrogenase is inactivated by oxygen, so the heterocyst must create a microanaerobic environment. The heterocysts' unique structure and physiology require a global change in gene expression. For example, heterocysts:
- produce three additional cell walls, including one of glycolipid that forms a hydrophobic barrier to oxygen
- produce nitrogenase and other proteins involved in nitrogen fixation
- degrade photosystem II, which produces oxygen
- up-regulate glycolytic enzymes
- produce proteins that scavenge any remaining oxygen
- contain polar plugs composed of cyanophycin which slows down cell-to-cell diffusion
Cyanobacteria usually obtain a fixed carbon (carbohydrate) by photosynthesis. The lack of water-splitting in photosystem II prevents heterocysts from performing photosynthesis, so the vegetative cells provide them with carbohydrates, which is thought to be sucrose. The fixed carbon and nitrogen sources are exchanged through channels between the cells in the filament. Heterocysts maintain photosystem I, allowing them to generate ATP by cyclic photophosphorylation.
Single heterocysts develop about every 9-15 cells, producing a one-dimensional pattern along the filament. The interval between heterocysts remains approximately constant even though the cells in the filament are dividing. The bacterial filament can be seen as a multicellular organism with two distinct yet interdependent cell types. Such behavior is highly unusual in prokaryotes and may have been the first example of multicellular patterning in evolution. Once a heterocyst has formed it cannot revert to a vegetative cell. Certain heterocyst-forming bacteria can differentiate into spore-like cells called akinetes or motile cells called hormogonia, making them the most phenotyptically versatile of all prokaryotes.
- ^ Basic Biology (18 March 2016). "Bacteria".
- ^ Wolk, C.P.; Ernst, A.; Elhai, J. (1994). "Heterocyst Metabolism and Development". The Molecular Biology of Cyanobacteria. pp. 769–823. doi:10.1007/978-94-011-0227-8_27. ISBN 978-0-7923-3273-2.