What is the most readily useable form of nitrogen by microorganisms?

Question

Quizplus · Accepted Answer

Microorganisms readily use ammonium ion (NH4+) as a source of nitrogen for building biomolecules, as it requires less energy to assimilate than nitrate (NO3-) or nitrite (NO2-). Dinitrogen (N2) must be fixed into a usable form, while nitrate and nitrite must be reduced before they can be assimilated. Nitric oxide (NO) is not a form of nitrogen that microorganisms can use for growth.

What Is the Most Readily Useable Form of Nitrogen by Microorganisms