By what process did cyanobacteria contribute to the oxygenation of the Earth's atmosphere?

• A. Respiration
• B. Nitrogen fixation
• C. Fermentation
• D. $\text{Photosynthesis}$

Answer: (D)

Verified step by step guidance

1. Understand the role of cyanobacteria in Earth's history: Cyanobacteria are a group of photosynthetic bacteria that played a crucial role in the oxygenation of Earth's atmosphere.
2. Identify the process associated with oxygen production: Photosynthesis is the process by which cyanobacteria convert carbon dioxide and water into glucose and oxygen, using sunlight as energy.
3. Differentiate between the processes: Respiration, nitrogen fixation, and fermentation do not produce oxygen as a byproduct. Respiration consumes oxygen, nitrogen fixation involves converting nitrogen gas into ammonia, and fermentation is an anaerobic process.
4. Recognize the significance of photosynthesis: The oxygen produced by cyanobacteria through photosynthesis accumulated in the atmosphere, leading to the Great Oxygenation Event.
5. Conclude the contribution: Cyanobacteria's photosynthesis was the primary process that increased atmospheric oxygen, transforming Earth's environment and enabling the evolution of aerobic life forms.