Heterocysts (also called heterocytes) are specialized nitrogen-fixing cells produced by certain filamentous cyanobacteria—such as Nostoc, Cylindrospermum, and Anabaena—when environmental nitrogen is limited. They house the enzyme nitrogenase, which converts atmospheric N2 into a usable nitrogen form for the filament. Since nitrogenase is deactivated by oxygen, heterocysts create a microanaerobic interior. Their formation involves extensive changes in gene expression and cell structure. Heterocysts:
- add three extra cell-wall layers, including a glycolipid layer that restricts oxygen and CO2 entry
- synthesize nitrogenase and other nitrogen-fixation proteins
- dismantle photosystem II (the oxygen-evolving complex)
- upregulate glycolytic enzymes
- produce proteins that scavenge residual oxygen
- form polar cyanophycin plugs that slow diffusion between cells
Because heterocysts lack the water-splitting photosystem II, they cannot carry out typical photosynthesis; surrounding vegetative cells provide them with fixed carbon, probably as sucrose. Carbon and fixed nitrogen are exchanged between cells through intercellular channels. Heterocysts retain photosystem I and generate ATP via cyclic photophosphorylation.
Heterocysts typically appear every 9–15 cells, creating a stable one-dimensional pattern along the filament that persists as cells divide. The filament functions like a simple multicellular organism with two complementary cell types—an unusual feature for prokaryotes and possibly an early example of multicellular patterning. Once differentiated, heterocysts do not revert to vegetative cells. Some species also form akinetes (spore-like cells) or hormogonia (motile fragments), demonstrating considerable phenotypic flexibility.
Genetic control of heterocyst development Low nitrogen triggers heterocyst differentiation through the transcriptional regulator NtcA. NtcA controls multiple genes, including hetR, a central regulator that activates genes such as patS and hepA by binding their promoters. ntcA and hetR regulate each other and can induce heterocyst formation even in the presence of nitrogen. Other important regulators include PatA and hetP: PatA helps determine heterocyst spacing along the filament and supports cell division, while PatS inhibits neighboring cells from differentiating into heterocysts during early stages. Maintenance of heterocysts relies on hetN. The presence of fixed nitrogen (ammonium or nitrate) blocks heterocyst formation.
Stages of heterocyst differentiation from a vegetative cell:
- The cell enlarges.
- Granular inclusions decline.
- Photosynthetic lamellae are reorganized.
- The cell wall becomes triple-layered, with three layers added outside the existing outer layer (a uniform middle layer and a laminated inner layer).
A senescent heterocyst becomes vacuolated and may detach from the filament, giving rise to hormogonia that reproduce asexually. Heterocyst-forming cyanobacteria are found in the orders Nostocales (simple filaments) and Stigonematales (branching filaments), which form a monophyletic group with relatively low genetic diversity.
Symbioses and ecological significance Heterocyst-forming cyanobacteria can establish symbiotic relationships with certain plants. In these partnerships, plant-derived signals—rather than environmental nitrogen levels—induce heterocyst formation; up to 60% of bacterial cells can differentiate into heterocysts. The plant signal and its exact target remain unknown, but hetR is required for symbiotic heterocyst formation, and ntcA is necessary for infection.
A notable example is Anabaena azollae in association with Azolla ferns. Anabaena colonize Azolla stems and leaves; the plant supplies fixed carbon through photosynthesis, while Anabaena heterocysts fix atmospheric nitrogen into ammonia, benefiting both partners. This symbiosis is used agriculturally—especially in Asia—where Azolla with Anabaena serves as a biofertilizer and animal feed. Different Azolla–Anabaena combinations are adapted to various environments and can influence crop yields. Cultivating Azolla–Anabaena before and after rice cropping adds fixed nitrogen, phosphorus, organic carbon, and other nutrients to the soil when the biomass decomposes, enhancing rice growth.
The Azolla–Anabaena symbiosis has also been explored for phytoremediation. Together, they have been used to remove contaminants such as uranium and heavy metals (including mercury(II), chromium(III), and chromium(VI)) from polluted wastewater.
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