Heterocyst

Cells of a cyanobacterial filament differentiate into heterocysts due to alterations to their environment.
Their development is complete after around 20 hours at 30C and this process is reversible up until about 9-12 hours if the cells can find a viable cource of fixed nitrogen.

Mututal interdependance between vegetative and heterocystous cells of a cyanobacterial filament.
Organic compounds (E.g. Formaldehyde) are transported into the heterocysts, in exchange for fixed nitrogen (E.g. Glutamine).

The microanaerobic environemnt within heterocyct cells is important for the function of the nitrogenase enzyme (which is inhibited by the presence of oxygen).
The thick cell wall and presence of oxygen neutralising enzymes helps to create this oxygen free environment for the function of nitrogenase.

This nitrogen fixation means that heterocysts form symbioses with plants, mosses, fungi and lichen.
One symbiois is the Gunnera-Nostoc symbiosis - between the genus of plant Gunnera and the Nostoc cyanobacteria.
This symbiosis provides the cyanobacterial filament cells with a source of fixed carbon created by the plants photosynthesis in exchange for a source of fixed nitrogen from the heterocysts.
This means that Gunnera can grow in areas of poor nitrogen avalibility.

Gene rearrangment within cyanobacteria causes the expression of nif genes which are nitrogen fixing genes which code for the creation of nitrogenase enzymes.
Other features of heterocysts include: three cell walls, degredation of PSII, upregulation of glycolytic enzymes