Investigation of limiting factors — practice question

The unicellular green alga Chlorella, which is a photosynthetic protoctist, was first examined for its possible use as a food source. Although producing it on a large scale turned out not to be economical, the many health benefits of Chlorella mean that it is now mass-produced and collected for use as a health food supplement. Fig. 1.1 shows cells of Chlorella. In one investigation of Chlorella productivity, the carbon dioxide concentration was changed to find out how it affected the light-independent stage of photosynthesis. A suspension of Chlorella cells was illuminated with a bench lamp. For $200$ seconds, the suspension was given carbon dioxide at a concentration of $1\%$. The carbon dioxide concentration was then lowered to $0.03\%$ for a further $200$ seconds. The concentrations of RuBP and GP (PGA) were recorded at regular intervals. Throughout the investigation, the temperature of the suspension was kept at $25\,^{\circ}\text{C}$. The results are shown in Fig. 1.2.