What processes in water produce carbon dioxide and what process uses it?

• A. fertilization produces it and diffusion uses it
• B. reproduction produces it and spawning uses it
• C. respiration produces it and photosynthesis uses it
• D. photosynthesis produces it and photosynthesis uses it

Answer: (C)

In aquatic systems, the gas carbon dioxide comes mainly from respiration. Organisms break down organic material to release energy, and CO2 is released as a byproduct into the water. Photosynthesis, performed by algae, aquatic plants, and some bacteria, uses carbon dioxide along with light and water to produce sugars and oxygen. So the process that produces carbon dioxide is respiration, and the process that uses it is photosynthesis. This balance helps explain why CO2 levels and pH in a tank can shift between day and night: photosynthesis consumes CO2 during the day, while respiration continues, potentially increasing CO2 and lowering pH at night. (Fertilization and spawning are reproductive processes and don’t describe CO2 production or usage; diffusion is a physical gas-exchange process, not the biological use of CO2 in making organic matter.)