Which of the following is the major biological source of dissolved oxygen in Earth's atmosphere?

• A. Decomposition of organic matter by fungi
• B. Volcanic outgassing
• C. Respiration by terrestrial animals
• D. Photosynthetic activity of aquatic algae and cyanobacteria

Answer: (D)

Verified step by step guidance

1. Understand the role of photosynthesis in producing oxygen: Photosynthesis is a process where organisms like plants, algae, and cyanobacteria use sunlight, carbon dioxide, and water to produce glucose and oxygen. The oxygen released during photosynthesis contributes significantly to Earth's atmospheric oxygen.
2. Recognize the major contributors to photosynthesis: Aquatic algae and cyanobacteria are primary producers in aquatic ecosystems. They perform photosynthesis on a large scale, releasing oxygen into the water and subsequently into the atmosphere.
3. Eliminate incorrect options: Decomposition of organic matter by fungi does not produce oxygen; it breaks down organic material into simpler compounds. Volcanic outgassing releases gases like carbon dioxide and sulfur dioxide, not oxygen. Respiration by terrestrial animals consumes oxygen rather than producing it.
4. Focus on the correct option: Photosynthetic activity of aquatic algae and cyanobacteria is the major biological source of dissolved oxygen in Earth's atmosphere. These organisms are responsible for a significant portion of global oxygen production.
5. Connect the concept to Earth's oxygen cycle: The oxygen produced by aquatic algae and cyanobacteria during photosynthesis is released into the water and eventually diffuses into the atmosphere, maintaining the balance of oxygen essential for life on Earth.