Why can animals not use nitrogen gas directly from the air?
Animals cannot use nitrogen gas () directly from the atmosphere because they lack the specific enzymes required to break the extremely strong chemical bonds that hold the nitrogen atoms together.12
Molecular nitrogen is characterized by a very high bond energy, making it relatively inert and chemically stable under ordinary conditions. Because of this stability, higher life-forms are unable to convert atmospheric nitrogen into the reactive forms necessary for biological processes, such as the synthesis of DNA, RNA, and amino acids.132
Instead, all animals depend on nitrogen fixation—a process primarily performed by specialized bacteria and archaea—to convert atmospheric nitrogen into usable compounds like ammonia, nitrates, or nitrites. These compounds are absorbed by plants, and animals subsequently obtain the nitrogen they need by consuming those plants or other organisms. This movement of nitrogen through the environment is known as the nitrogen cycle.32