the photosynthetic rate of aquatic plants in a test tube can be determined by collecting and measuring the amount of oxygen that gasses out of the water. if bicarbonate, the source of co2 for aquatic plants, is added to the water, the rate of oxygen production increases. if co2 is fixed by the calvin cycle, but oxygen is evolved by the light reactions, how can an increase in the co2 supply increase the rate of oxygen evolution.