Living organisms can incorporate nitrates and ammonia into their metabolism.
- Nitrates can be assimilated by plants and some bacteria as a source of nitrogen for amino acids and nucleic acids.
- Ammonia can also be directly incorporated by organisms, particularly through processes like ammonification and assimilation.
Nitrites are often intermediate products in the nitrogen cycle and are typically converted to nitrates or ammonia rather than directly used by most organisms.
Nitrogen gas (N₂) is generally not directly incorporated into metabolism by most organisms, but it can be fixed into a usable form (like ammonia) by certain bacteria (nitrogen-fixing bacteria).
So, the best answers are nitrates and ammonia.