📘 IAS Prelims 2025 — GS-I Q.40 | Classroom Explanation

(Environment & Ecology)

📌 Question

Q.40) With reference to the planet Earth, consider the following statements:

I. Rain forests produce more oxygen than that produced by oceans.
II. Marine phytoplankton and photosynthetic bacteria produce about 50% of the world’s oxygen.
III. Well-oxygenated surface water contains several folds higher oxygen than that in atmospheric air.

Which of the statements given above is/are correct?

(a) I and II
(b) II only
(c) I and III
(d) None of the above statements is correct

✅ Correct Answer: (b)

🧑‍🏫 Classroom Explanation

This question tests global oxygen production, marine ecology, and basic environmental chemistry, an area where UPSC frequently overturns popular myths.

🔍 Statement-wise Analysis

❌ Statement I: Incorrect

• A common misconception is that rainforests are the “lungs of the Earth”.
• In reality:

• Oceans produce more oxygen than terrestrial rainforests
• Tropical rainforests consume almost as much oxygen as they produce due to respiration and decomposition

• Prochlorococcus, a marine photosynthetic bacterium:

• Produces up to 20% of Earth’s oxygen alone
• More than all tropical rainforests combined

➡️ Statement I is incorrect.

✅ Statement II: Correct

• Scientific estimates show:

• ~50% (or more) of global oxygen production comes from oceans

• Major contributors:

• Marine phytoplankton
• Photosynthetic bacteria
• Seaweeds and algae

➡️ Statement II is correct.

❌ Statement III: Incorrect

• Oxygen in air vs water:

• Atmospheric air contains ~210 mg O₂ per litre
• Well-oxygenated surface seawater contains only ~8 mg O₂ per litre

• Even the most oxygen-rich water has far less oxygen than air

➡️ Statement III is incorrect.

🧮 Logical Elimination (UPSC Style)

• Statement I → ❌
• Statement II → ✅
• Statement III → ❌

➡️ Correct option: (b) II only

🎯 Final Answer

✅ Correct Answer: (b)

📦 Info Box — High-Value UPSC Takeaways

🌊 Oceanic Oxygen Production

• Oceans are the primary oxygen producers on Earth
• Microscopic organisms outperform forests in oxygen generation

🧪 Oxygen in Water vs Air

• Dissolved oxygen in water is:

• Much lower than atmospheric oxygen
• Critical for aquatic life but never exceeds air levels

⚠️ UPSC Myth Busters

• Rainforests ≠ biggest oxygen source
• More photosynthesis ≠ more net oxygen (respiration matters)
• Oxygen-rich water ≠ oxygen-rich compared to air

📍 GS Mapping

GS Paper I (Prelims)
Theme: Environment & Ecology
Topic: Biosphere, Oceans, Oxygen Cycle
Nature: Conceptual + Data-based