How did early photosynthetic organisms such as cyanobacteria affect Earth's atmosphere?

• A. They increased atmospheric oxygen through the process of oxygenic photosynthesis.
• B. They caused a significant drop in atmospheric carbon dioxide by absorbing it completely.
• C. They released large amounts of methane, leading to global warming.
• D. They decreased atmospheric nitrogen by converting it into ammonia.

Answer: (A)

Verified step by step guidance

1. Understand the role of early photosynthetic organisms like cyanobacteria in Earth's history. Cyanobacteria are known for performing oxygenic photosynthesis, which produces oxygen as a byproduct.
2. Recall the chemical equation for oxygenic photosynthesis: ${\mathrm{6CO}}_2+{\mathrm{6H}}_{2}O\to C_6{H}_{12}{O}_6+{\mathrm{6O}}_2$. This process converts carbon dioxide and water into glucose and oxygen using sunlight.
3. Recognize that cyanobacteria were responsible for the Great Oxygenation Event (GOE), which significantly increased atmospheric oxygen levels and transformed Earth's atmosphere, making it suitable for aerobic organisms.
4. Evaluate the incorrect options: Cyanobacteria did not absorb all atmospheric carbon dioxide, nor did they release methane or decrease nitrogen levels. Their primary impact was the production of oxygen.
5. Conclude that the correct answer is: Early photosynthetic organisms such as cyanobacteria increased atmospheric oxygen through the process of oxygenic photosynthesis.