The nitrogen cycle in an aquaponics system has three living organisms.

• A. Fish, plants, and humans
• B. Plants, bacteria, and algae
• C. Fish, insects, and algae
• D. Fish, plants, and bacteria

Answer: (D)

In an aquaponics system, the nitrogen cycle depends on the collaboration of three living components: fish, bacteria, and plants. Fish produce waste that contains ammonia, which is toxic in high amounts. Beneficial bacteria carry out nitrification, first converting ammonia to nitrite, then to nitrate, a form plants can use. Plants uptake nitrate as a nutrient, removing nitrogen from the water and keeping the system balanced. This trio — fish as the nitrogen source, bacteria for conversion, and plants as the sink — forms the core of the cycle. While algae or other organisms can be present and play supporting roles, insects or humans aren’t part of the essential nitrogen-cycling trio in a standard aquaponics setup.