air is about 78% nitrogen. nitrogen chemicals are needed for life. nitrogen is a necessary part of proteins, dna, and rna. in plants, nitrogen is needed for photosynthesis and growth. however, living things cannot use the elemental nitrogen in the air for these things. nitrogen fixation is needed to change the nitrogen in air (n2) into forms that can be used by life. most nitrogen fixation is done by microorganisms called bacteria. these bacteria have an enzyme that combines n2 with hydrogen gas (h2) to make ammonia (nh3).