Microbiology of Soils
The transformation of nitrates to gaseous nitrogen is accomplished by microorganisms in a series of biochemical reactions. The process is known as

nitrogen fixation
denitrification
nitrification
ammonification

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Microbiology of Soils
A heterocyst is

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

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