Heterocyst

Heterocysts or heterocytes are specialized nitrogen-fixing cells formed during nitrogen starvation by some filamentous cyanobacteria, such as Nostoc, Cylindrospermum, and Anabaena.

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.

For instance, NtcA controls the expression of several genes including HetR which is crucial for heterocyst differentiation.

[3] It is crucial as it up-regulates other genes such as hetR, patS, hepA by binding to their promoter and thus acting as a transcription factor.

In such a relationship, the bacteria do not respond to the availability of nitrogen, but to signals produced by the plant for heterocyst differentiation.

[8] The Azolla plant undergoes photosynthesis and provides fixed carbon for the Anabaena to use as an energy source for dinitrogenases in the heterocyst cells.

[8] In return, the heterocysts are able to provide the vegetative cells and the Azolla plant with fixed nitrogen in the form of ammonia which supports growth of both organisms.

In Asia, Azolla plants containing Anabaena species are used as biofertilizer where nitrogen is limiting[8] as well as in animal feed.

[9] When the Azolla-Anabaena plants die and decompose, they release high amounts of fixed nitrogen, phosphorus, organic carbon, and many other nutrients into the soil, providing a rich environment ideal for the growth of rice crops.