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? 

(a) I and II 

(b) II only 

(c) I and III 

(d) None of the above statements is correct

Ans-b

Explanation

The ocean generates 50 percent of the oxygen we need, absorbs 30 percent of all carbon dioxide emissions and captures 90 percent of the excess heat generated by these emissions. It is not just ‘the lungs of the planet’ but also its largest ‘carbon sink’ – a vital buffer against the impacts of climate change.  

Hence statement 1 is incorrect

Scientists estimate that roughly half of the oxygen production on Earth comes from the ocean. The majority of this production is from oceanic plankton — drifting plants, algae, and some bacteria that can photosynthesize. One particular species, Prochlorococcus, is the smallest photosynthetic organism on Earth. But this little bacteria produces up to 20% of the oxygen in our entire biosphere. 

Hence statement 2 is correct

Well-oxygenated surface water may only contain around 8 mg O2/l, while the air contains 210 mg O2/l.

The oxygen content in ocean water is highest at the surface for two primary reasons: this is where oxygen from the atmosphere dissolves into the ocean, and it is also the location where oxygen is produced by phytoplankton during photosynthesis. While respiration occurs in surface waters as well, the rate of oxygen production through photosynthesis exceeds the rate of oxygen consumption via respiration. It is important to note that despite the higher levels of dissolved oxygen at the surface, the concentration of oxygen in water is still significantly lower than that found in the air.

Hence statement 3 is incorrect

Read: Previous Year UPSC Environment Questions (PYQs) With Explanation 2025