Which of the following organisms releases molecular oxygen (O₂) into water?

Phytoplankton are microscopic plant-like organisms that float near the surface of oceans, lakes, and rivers. They play a vital role in maintaining the balance of oxygen in water and the atmosphere. Through the process of photosynthesis, phytoplankton use sunlight, carbon dioxide (CO₂), and water (H₂O) to produce glucose for their energy and release molecular oxygen (O₂) as a byproduct. This oxygen dissolves in water, supporting aquatic life such as fish, crustaceans, and other organisms that depend on dissolved oxygen for survival.

The basic photosynthesis reaction in phytoplankton is similar to that of green plants on land:
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

This means six molecules of carbon dioxide and six of water produce one molecule of glucose and six molecules of oxygen in the presence of sunlight and chlorophyll pigments. Since phytoplankton live in the upper sunlit zone of water bodies, they can effectively capture solar energy and continuously produce oxygen.

In contrast, organisms like E. coli and Salmonella are bacteria that do not perform photosynthesis. They may even consume oxygen during respiration. Amoeba, a single-celled protozoan, also respires using oxygen but does not release it into the environment.

Phytoplankton are therefore considered the primary producers of aquatic ecosystems. Scientists estimate that about 50–80% of the Earth’s total oxygen originates from phytoplankton activity in the oceans. Their role is essential not just for aquatic organisms but for the global oxygen cycle. Understanding phytoplankton helps us appreciate how microscopic life contributes to sustaining life on Earth.