The acidification of oceans is increasing. Why is this phenomenon a cause of concern? 1) The growth and survival of calcareous phytoplankton will be adversely affected. 2) The growth and survival of coral reefs will be adversely affected. 3) The survival of some animals that have phytoplanktonic larvae will be adversely affected. 4) The cloud seeding and formation of clouds will be adversely affected. Which of the statements given above is /are correct?

The correct answer is D. All four statements are valid consequences of ocean acidification, which happens as seawater absorbs extra CO₂ and its carbonate ion concentration falls.

• Statement 1 is correct. Calcareous phytoplankton such as coccolithophores and foraminifera build calcium carbonate shells. Lower carbonate saturation makes calcification harder and can weaken or dissolve these shells.
• Statement 2 is correct. Reef-building corals secrete aragonite skeletons. Slower calcification and higher dissolution weaken reef structure and growth.
• Statement 3 is correct. Many shellfish and other marine animals pass through a planktonic larval stage with a calcifying shell. These early stages are very sensitive to pH change, so survival suffers even when adults cope.
• Statement 4 is correct but needs care. Some phytoplankton produce dimethyl sulphide (DMS), which oxidises into aerosols that act as cloud condensation nuclei. Acidification can alter phytoplankton communities and DMS output, so cloud formation may be affected. This is a plausible feedback that is still being studied, not a settled quantitative effect.

Core concept: ocean acidification and its effect on marine calcifiers and ocean-climate feedbacks. For Mains GS-III (environment and climate change), link it to the ocean as a carbon sink and to the limits of natural absorption.

Trap: aspirants often dismiss statement 4 as unrelated to oceans, or assume only corals are affected. Both lead to choosing A or B.