Prelims 2025 · Q40
MediumWith 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 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?
Answer & explanation
Answer: (b) II only
The oceans, through photosynthesis by plankton and some bacteria, generate about half or more of the oxygen on Earth, so rain forests do not out-produce them: NOAA notes that one marine bacterium alone makes more oxygen than all tropical rainforests combined. Water can hold only a small amount of dissolved oxygen compared with air, so surface water never has several times more oxygen than the atmosphere.
- ✗ I Roughly half of Earth's oxygen production comes from the ocean (IUCN says more than half), and NOAA notes that the marine bacterium Prochlorococcus alone produces up to 20% of the biosphere's oxygen, a higher share than all the tropical rainforests combined.
- ✓ II NOAA says roughly half of the oxygen production on Earth comes from the ocean, mostly from oceanic plankton: drifting plants, algae and some bacteria that can photosynthesize.
- ✗ III NCERT notes that the amount of dissolved oxygen is fairly low compared with the amount of oxygen in air, which is why aquatic organisms breathe much faster than land ones. Water is therefore poorer in oxygen than air, not richer.
Remember · The ocean makes about half or more of Earth's oxygen (mostly phytoplankton); water holds far less oxygen than air.
📘 Read it in NCERT: Class 10 Science, Ch 5 (practise this chapter)
Sources
- IUCN (with IOC-UNESCO): Ocean deoxygenation: Everyone's problem (foreword) ↗ “The ocean generates more than half the oxygen we breathe.”
- NCERT Class 10 · Science, Chapter 5 “Since the amount of dissolved oxygen is fairly low compared to the amount of oxygen in the air, the rate of breathing in aquatic organisms is much faster than that seen in terrestrial organisms.”
- NOAA National Ocean Service: How much oxygen comes from the ocean? ↗ “Scientists estimate that roughly half of the oxygen production on Earth comes from the ocean. The majority of this production is from oceanic plankton — drifting plants, algae, and some bacteria that can photosynthesize. … this little bacteria produces up to 20% of the oxygen in our entire biosphere . That’s a higher percentage than all of the tropical rainforests on land combined.”
Question and answer: UPSC's official GS Paper I (2025, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 1 Oct 2026 (how we verify). Permalink ·