Which process by phytoplankton contributes to global oxygen production and carbon cycling?

• A. Cellular respiration
• B. Chemosynthesis
• C. Fermentation
• D. Photosynthesis

Answer: (D)

Photosynthesis is the process by which phytoplankton use sunlight to convert carbon dioxide and water into organic matter, releasing oxygen in the process. This makes them a major source of oxygen for both the atmosphere and oceans, while also tying into the carbon cycle by taking in CO2 and turning it into biomass. Some of that carbon ends up stored in long-term sinks when organic matter settles to the deep ocean. The other processes don’t fit this role: cellular respiration consumes oxygen to release energy, fermentation is an anaerobic way to harvest energy, and chemosynthesis uses chemical energy rather than light. So photosynthesis best explains how phytoplankton contribute to global oxygen production and carbon cycling.