Microbiology of Soils
A heterocyst is

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

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Microbiology of Soils
Nitrogen fixation

both (a) and (b)
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
especially changes nitrogen compounds, mostly amines such as NH2

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