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Environment · Ocean Ecosystem & Zonation (Photic, Aphotic)

UPSC Prelims 2025, Question 40

The concept being tested: Atmospheric oxygen sources and production mechanisms.

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 higher folds oxygen than that in atmospheric air. Which of the statements given above is/are correct?

Options

  1. AI and II
  2. BII onlyCorrect answer
  3. CI and III
  4. DNone of the above statements is correct

The answer: II only

Marine phytoplankton, along with photosynthetic bacteria such as cyanobacteria, are estimated to generate roughly 50 percent of the oxygen present in Earth's atmosphere, a figure widely cited by NOAA and marine science bodies — this makes the claim about marine phytoplankton and photosynthetic bacteria correct. The claim about rain forests producing more oxygen than oceans fails because although tropical rainforests carry out enormous gross photosynthesis, a mature rainforest's own respiration and the decomposition of its litter consume nearly all the oxygen produced, leaving negligible net export; oceanic phytoplankton, cycling far faster, are the larger net atmospheric source, not rainforests. The claim about well-oxygenated surface water holding several-fold more oxygen than air inverts basic gas solubility: oxygen dissolves poorly in water, so even fully oxygenated surface water holds only a few milligrams of dissolved oxygen per litre, a small fraction of the concentration present in the same volume of atmospheric air.

What UPSC was testing

UPSC is testing whether aspirants can debunk the popular 'rainforests are the lungs of the earth' claim — a forest's net oxygen contribution is largely offset by its own respiration and decomposition, whereas fast-cycling ocean phytoplankton are the actual dominant source; getting either the rainforest or the dissolved-oxygen statement wrong is enough to fall into one of the three incorrect options.

How often this comes up

This concept has appeared 5 times in UPSC Prelims General Studies Paper I between 2015 and 2025 — in 2018, 2021, 2022 and 2025.

The option-by-option autopsy for this question — why each of the other three is wrong, and which neighbouring concept each one was built from — is in the app.