I mean, a little, but the closed nature of the greenhouse is still the bigger problem. 1000 cubic meters of air only has something like 150 grams of carbon in

I mean, a little, but the closed nature of the greenhouse is still the bigger problem. 1000 cubic meters of air only has something like 150 grams of carbon in it; even if your plants in question are 95% water, you're still only talking a few kilograms of plant growth before you completely exhaust the air in an enclosed space.