Brown algae (Phaeophyceae) are multicellular marine organisms commonly found in coastal waters. A student investigated the photosynthetic activity of brown algae under controlled laboratory conditions and recorded the following observations:

• The brown algae appear olive-brown in colour when illuminated under white light.
• At low carbon dioxide concentrations, the rate of carbon fixation is low.
• Above a certain carbon dioxide concentration, the rate of carbon fixation plateaus and remains constant, even if the carbon dioxide concentration is increased further.

Which of the following statements correctly explains these observations?