Answer:
Organisms aren't able to use nitrogen in its N2 form. So, converting it to ammonium is critical for organisms to be able to absorb it.
Step-by-step explanation:
The structure of atmospheric nitrogen makes it relatively unreactive. So, when bacteria "fix" the nitrogen, it makes it into a more reactive form that allows organisms to use it. Nitrogen is essential in the building of certain biomolecules, which are the basis of life.