Microbiology of Soils
A heterocyst is

a terminally differentiated cell that fixes nitrogen
the progenitor of cyanobacterial vegetative cells
a type of spore
a cell that carries out oxygenic photosynthesis

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Microbiology of Soils
Nitrogen fixation

both (a) and (b)
especially changes nitrogen compounds, mostly amines such as NH2
fix the free nitrogen (N2) by which it should not be usable by plants
changes the free nitrogen (N2) to a form usable by plants

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