Which process is primarily responsible for producing the oxygen present in Earth's atmosphere?

• A. Anaerobic fermentation by bacteria
• B. Volcanic outgassing
• C. Respiration by animals
• D. Oxygenic photosynthesis by cyanobacteria and plants

Answer: (D)

Verified step by step guidance

1. Understand the question: The problem asks which process is primarily responsible for producing the oxygen in Earth's atmosphere. This requires knowledge of biological processes that release oxygen.
2. Recall the concept of oxygenic photosynthesis: This is the process by which cyanobacteria, algae, and plants use sunlight to convert carbon dioxide and water into glucose and oxygen. The oxygen is released as a byproduct.
3. Compare the options: Anaerobic fermentation by bacteria does not produce oxygen; volcanic outgassing releases gases like carbon dioxide and sulfur dioxide, not oxygen; respiration by animals consumes oxygen rather than producing it.
4. Focus on oxygenic photosynthesis: Cyanobacteria were among the first organisms to perform oxygenic photosynthesis, leading to the accumulation of oxygen in Earth's atmosphere billions of years ago. Plants continue this process today.
5. Conclude that oxygenic photosynthesis is the correct answer: It is the primary biological process responsible for the oxygen present in Earth's atmosphere, as it directly releases oxygen as a byproduct.