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UPSC Prelims 2025 GS Paper 1, question 40

Topic: Environment and ecology

Q40

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?

  1. (a) I and II
  2. (b) II onlyCorrect answer
  3. (c) I and III
  4. (d) None of the above statements is correct

Solution.

Answer: (b) II only

  1. Statement I is incorrect: marine ecosystems (oceans) produce significantly more oxygen globally than terrestrial rainforests.
  2. Statement II is correct: marine phytoplankton, kelp, algal plankton, and photosynthetic bacteria (such as Prochlorococcus) generate approximately 50% to 70% of the world's oxygen.
  3. Statement III is incorrect: atmospheric air contains about 210,000 ppm (21% by volume, or ~300 mg/L) of oxygen, whereas even well-oxygenated surface water holds only about 8 to 14 mg/L (less than 1%) of dissolved oxygen.