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. A. I and II
2. B. II only
3. C. I and III
4. D. None of the above statements is correct

Answer: B

Explanation

Statement I is incorrect. Scientists estimate that roughly half of the Earth’s oxygen is produced by the oceans, primarily by marine plankton. Tropical rainforests, while significant, produce less oxygen than the oceans. Statement II is correct. Marine phytoplankton and photosynthetic bacteria, such as Prochlorococcus, are indeed responsible for producing approximately 50% of the world’s oxygen through photosynthesis. Statement III is incorrect. While dissolved oxygen is highest in surface waters, the concentration of oxygen in water is significantly lower than in atmospheric air. For example, well-oxygenated surface water might contain around 8 mg O2/l, whereas atmospheric air contains about 210 mg O2/l. Therefore, only statement II is correct. Understanding global biogeochemical cycles and the role of different ecosystems in maintaining Earth’s atmosphere is a fundamental aspect of Environment & Ecology for UPSC.