Microbiology of Soils
A heterocyst is

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

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Microbiology of Soils
Nitrogen fixation

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

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