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
both (a) and (b)
fix the free nitrogen (N2) by which it should not be usable by plants

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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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