Who observed that when chlorella cells are exposed to continuous light, the rate of  photosynthesis was less as compared to those exposed in intermittent light?

• Warburg discovered that a decrease in photosynthesis by increasing the oxygen concentration. Oxygen is the competitive inhibitor of the carbon dioxide fixation by RuBisCo, which initiates photosynthesis. Furthermore, oxygen stimulates photorespiration which reduces photosynthetic output. These two mechanisms working together are responsible for the Warburg effect. So, this option is incorrect.
• Arnon discovered the photosynthetic phosphorylation. So, this option is incorrect.
• Samuel Ruben discovered that oxygen, which is released during photosynthesis came from a water molecule. So, this option is incorrect.
• Emerson observed that when Chlorella cells are exposed to continuous light, the rate of photosynthesis was less as compared to those exposed to the intermittent light. 

So, the correct answer is option B.