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
(c) I and III
(d) None of the above statements is correct

The correct answer is (b) II only.

Explanation

Statement I: Incorrect. This is a common misconception. While rainforests are highly productive, they are also mature ecosystems. Most of the oxygen they produce through photosynthesis is consumed by the respiration of the vast number of organisms (animals, fungi, bacteria) that live there, including the trees themselves at night. Overall, rainforests are roughly oxygen-neutral. The oceans are the primary net producers of oxygen.
Statement II: Correct. This is a scientifically accepted fact. Tiny marine organisms called phytoplankton, along with photosynthetic bacteria (cyanobacteria), live in the upper layers of the ocean and perform photosynthesis. It is estimated that these microscopic organisms are responsible for producing 50% to 80% of the oxygen in the Earth’s atmosphere.
Statement III: Incorrect. This is factually wrong. Atmospheric air is about 21% oxygen (or 210,000 parts per million, ppm). In contrast, even cold, “well-oxygenated” surface water can only hold a tiny fraction of that amount, typically around 10-14 mg/L (which is 10-14 ppm). Therefore, the air contains thousands of times more oxygen by concentration than water does.