UPSC Prelims 2025 Q40: Earth's Oxygen — Oceans vs Rainforests
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?
- I and II
- II only
- I and III
- None of the above statements is correct
Answer: (b) II only
Statement-wise explanation
- I: incorrect — NOAA estimates that about half of Earth's oxygen comes from the ocean. All land plants together make up the rest, and tropical rainforests are only one part of that land share, so they cannot out-produce the oceans. Mature rainforests also use up nearly all the oxygen they make: respiration and the decay of dead plants consume it again. The net contribution of the Amazon is therefore close to zero, which is why the popular claim that it supplies '20% of the world's oxygen' is a myth.
- II: correct — NOAA states that about half of Earth's oxygen comes from the ocean. Most of it is made by oceanic plankton, meaning drifting algae and photosynthetic bacteria. A single cyanobacterium, Prochlorococcus, is estimated to produce up to 20% of the oxygen in the biosphere. The statement correctly names both groups and gives the accepted figure of about 50%.
- III: incorrect — The statement has the comparison backwards. Atmospheric air is about 20.9% oxygen, or roughly 209 ml per litre. Air-saturated water holds only about 5–9 ml per litre, depending on temperature and salinity. FAO gives 5.3 ml per litre at 30°C, about 0.5%. Air therefore holds roughly 25–40 times more oxygen than even well-oxygenated surface water. This is why fish need gills that pull oxygen from water very efficiently.
How to eliminate
Statement III can be ruled out with basic science. Gases dissolve poorly in water, and air is about one-fifth oxygen. If water held several times more oxygen than air, fish would have an easier time breathing than land animals, and the opposite is true. Ruling out III removes the two options that include it, leaving 'I and II', 'II only' and 'none correct'. Statements I and II nearly contradict each other: if oceans produce about half of Earth's oxygen, rainforests, which are only part of the land share, cannot produce more. So the choice comes down to whether you trust the 50% ocean figure, the one widely cited by NOAA and the UN. Fact to remember: about half of Earth's oxygen comes from the ocean, mostly from phytoplankton and cyanobacteria such as Prochlorococcus. The 'Amazon gives 20% of our oxygen' line is a myth.
Know more
Where Earth's oxygen comes from
Nearly all free oxygen comes from photosynthesis. Organisms use sunlight to turn CO₂ and water into sugar and release O₂. Production splits roughly evenly between land and sea.
| Source | Share of oxygen production | Key point |
|---|---|---|
| Marine phytoplankton and cyanobacteria | About 50% (NOAA) | Diatoms, dinoflagellates and Prochlorococcus, the smallest photosynthetic organism, which alone makes up to 20% |
| Land plants (all forests, grasslands, crops) | The other half | Tropical rainforests are the biggest gross producers on land |
| Rainforests' net contribution | Close to zero | Respiration and decay of plant matter use up almost all the O₂ the forest makes |
Gross vs net. A forest that is neither growing nor shrinking takes in about as much oxygen as it releases. Only carbon that gets locked away leaves a net oxygen surplus, for example organic matter buried in ocean sediments. Rainforests still matter, but as carbon sinks, rainfall generators and biodiversity hotspots, not as the world's 'lungs'.
Oxygen in water vs air
- Air: about 20.9% O₂ (about 209 ml per litre).
- Seawater: about 4–9 ml per litre at saturation. Cold, less salty water holds more. Warm, salty water holds less.
- Ocean warming lowers oxygen solubility. Together with nutrient runoff, this causes deoxygenation and dead zones (hypoxic areas).
Related exam points
- Cyanobacteria (blue-green algae) produced the oxygen of the Great Oxidation Event about 2.4 billion years ago.
- Phytoplankton also drive the biological carbon pump, which sinks carbon to the deep ocean.
- Dissolved oxygen (DO) and BOD are standard measures of water quality.
NCERT Concepts to Revise
- Blue-green algae: Statement II names 'photosynthetic bacteria'. Cyanobacteria are exactly these marine oxygen-producers.
- Amazon rainforest: Statement I rests on the popular claim that the Amazon is the planet's oxygen supplier.
- Tropical evergreen forest: Statement I compares tropical rainforests with oceans. Knowing how these dense, layered forests cycle carbon and oxygen helps judge the claim.