Microbiology of Soils
A heterocyst is

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

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Microbiology of Soils
Nitrogen fixation

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

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