UPSC Prelims 2025 Q40: Environment & Ecology – Biogeochemical Cycles & Earth Systems
- (a) I and II
- (b) II only
- (c) I and III
- (d) None of the above statements is correct
Answer: (b) II only
Explanation
Oxygen Sources in the Atmosphere:
| Oxygen Source | Contribution | Statement | Verdict |
|---|---|---|---|
| Rain Forests | ~28% of global O₂ production | I: Rainforests produce more O₂ than oceans | FALSE - Oceans dominate at 50-80% |
| Marine Phytoplankton + Photosynthetic Bacteria | 50-80% of global O₂ | II: These produce ~50% | TRUE |
| Surface Ocean Water dissolved O₂ | 5-12 mg/L dissolved | III: Surface water contains several folds higher O₂ than atmospheric air | FALSE |
Why Statement I is false: Oceans produce 50–80% of Earth's oxygen through phytoplankton and cyanobacteria.
Rainforests produce ~28% — significant, but not more than oceans.
Why Statement III is false: Atmospheric air contains ~21% oxygen by volume.
Surface ocean water contains only 5–12 mg/L dissolved oxygen — roughly 0.001% by volume.
Water has FAR LESS accessible oxygen than air.
Prochlorococcus — a marine cyanobacterium — is the single most productive oxygen-generating organism on Earth.
Answer: (b) II only.
Why each option is right or wrong
- (a) Incorrect. Statement I is false — oceans produce far more O₂ than rainforests. Rainforests are vital carbon sinks but not the dominant oxygen source.
- (b) CORRECT. Only Statement II is accurate — marine phytoplankton and photosynthetic bacteria produce approximately 50–80% of Earth's oxygen.
- (c) Incorrect. Statement III is false — dissolved O₂ in surface water (5–12 mg/L) is dramatically lower than O₂ in air (~21%). Water does not have higher oxygen than air.
- (d) Incorrect. Statement II is definitively correct based on established oceanographic research.
Key point
Oceans (phytoplankton + cyanobacteria) = 50–80% of Earth's O₂ — more than rainforests. Dissolved O₂ in water << O₂ in air.
Remember
Oceans (phytoplankton) make 50–80% of Earth's oxygen — more than rainforests. Dissolved water O₂ is far less than air O₂.
Common mistakes
- The Amazon = lungs of the Earth myth: Rainforests absorb CO₂ but oceans produce more oxygen. The phrase is a metaphor for carbon absorption, not O₂ dominance.
- Thinking dissolved O₂ in water is higher than in air — actually the reverse. Fish breathe ~10 mg/L; we breathe ~210 mg/L equivalent.
- Forgetting photosynthetic bacteria (cyanobacteria). Prochlorococcus alone generates an estimated 20% of the oxygen we breathe.
Exam tip
O₂ sources: Oceans first (50–80%), rainforests second. This is counterintuitive and UPSC tests it frequently. The Amazon = 'lungs of the Earth' refers to CO₂ absorption, not O₂ production dominance.
Sources
- NCERT Class XI — Biology (Photosynthesis in Higher Plants)
- NOAA — Ocean's Role in Oxygen Production
- National Geographic — Prochlorococcus: The Tiny Organism Powering the Planet