Microbiology of Soils
A heterocyst is

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

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Microbiology of Soils
Nitrogen fixation

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

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