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 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 ✓ UPSC's answer
- (c) I and III
- (d) None of the above statements is correct
Why the answer is (b)
• Statement II is correct: marine phytoplankton, algae and photosynthetic bacteria such as Prochlorococcus generate roughly half of the oxygen produced on Earth each year.
• Statement I is wrong: rainforests produce a large amount of oxygen but their own decomposition and respiration consume almost as much; on balance the oceans, not forests, are the larger net source.
• Statement III is wrong: air is about 21% oxygen, whereas even well-oxygenated surface water holds only about 8–10 mg per litre — far less than air, which is why fish need gills and aquatic hypoxia is a concern.
• The popular claim that the Amazon is 'the lungs of the Earth' overstates its net contribution.
• Hence II only, option (b).
Why the other options are wrong
- (a) I and II
- Statement I is wrong: oceans, not rainforests, are the larger net oxygen source.
- (c) I and III
- Both I and III are wrong.
- (d) None of the above statements is correct
- Statement II is correct: marine photosynthesis yields about half the world's oxygen.
Asked in the GS Paper I of the UPSC Civil Services Preliminary Examination 2025, held on 25 May 2025. Question and answer key: Union Public Service Commission. Explanation: UPSC Answer Check.