Ocean acidification is primarily caused by which factor, and what is a key impact on calcifying organisms?

• A. Rising ocean temperatures are the sole cause of acidification, with no change in pH.
• B. Increased salinity directly dissolves carbonate shells.
• C. Natural seasonal cycles alone drive long-term ocean acidification.
• D. Increased atmospheric CO2 dissolves in seawater, lowering pH and reducing carbonate ions needed for calcification.

Answer: (D)

The main idea is how atmospheric CO2 changes seawater chemistry and what that does to organisms that build calcium carbonate shells or skeletons. When more CO2 from the air dissolves in seawater, it forms carbonic acid, which releases hydrogen ions and lowers pH. This shift also reduces the concentration of carbonate ions and lowers the calcium carbonate saturation state of the water. With less carbonate available and a harder-to-sustain saturation state, calcifying organisms struggle to precipitate CaCO3, leading to slower calcification and, in waters that are undersaturated, potential dissolution of existing shells. So the key impact is weaker shells and slower growth for calcifying species, making them more vulnerable. The other statements misstate the mechanism: acidification is driven by CO2 dissolving in seawater, not by temperature alone, salinity, or seasonal cycles driving long-term changes.