UPSC Prelims 2025 · Question 20 of 100

UPSC Prelims 2025 question on Rainforest vs Ocean Oxygen

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 photo-synthetic 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?

  1. I and II
  2. II only
  3. I and III
  4. None of the above statements is correct
Show answer

Answer: B. II only

Verdict

Only Statement II is correct → Option (b).

Statement by statement

Statement I – INCORRECT: Scientists estimate that roughly half of the oxygen production on Earth comes from the ocean — primarily from drifting plants, algae, and photosynthetic bacteria. One particular species, Prochlorococcus, produces up to 20% of the oxygen in our entire biosphere — a higher percentage than all tropical rainforests on land combined. Hence Statement I is not correct.

Statement II – CORRECT: More than half of the oxygen we breathe comes from marine photosynthesizers like phytoplankton and seaweed. Marine phytoplankton and photosynthetic bacteria produce about 50% (or more) of the world's oxygen. Hence Statement II is correct.

Statement III – INCORRECT: Oxygen content is highest at the surface, but well-oxygenated surface water may only contain around 8 mg O2/l, while air contains 210 mg O2/l. Surface water has far less oxygen than atmospheric air, not several folds higher. Hence Statement III is not correct.

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