Question

Why would it be hard to find the ideal CO2 level if the light intensity were very low?

Answer 1

Because at very low light the rate of photosynthesis is limited by photons, not by CO2. When light is the limiting factor plants (or algae) cannot use additional CO2 to increase carbon fixation, so changing CO2 concentration has little or no effect on growth or photosynthetic rate. That makes it hard to detect an “ideal” CO2 level — the response curve is essentially flat in the low‑light region, measurement noise and small biological variability swamp any tiny differences, and other factors (temperature, nutrients, stomatal behavior or diffusion in water) can obscure any effect.

In practice that means:
- You won’t see benefit from raising CO2 until light intensity is high enough that CO2 becomes limiting.
- Experiments under low light give poor resolution of the CO2 optimum; to find an ideal CO2 level you should test at higher, non‑light‑limiting illumination (or at several light levels).
- For aquaria/greenhouses under low light it’s usually unnecessary to add CO2 because plants can’t use it.