Short answers, pulled from the story.
Marine biogenic calcification is the process by which marine organisms produce and deposit calcium carbonate minerals to build skeletal structures such as shells, skeletons, and coral reefs. Corals, mollusks, foraminifera, and certain plankton all rely on it to create structures used for protection, support, and shelter.
Corals, mollusks, echinoderms, crustaceans, foraminifera, coccolithophores, red algae in the genus Corallinales, and some calcifying cyanobacteria all take part in marine biogenic calcification. Coccolithophores alone may account for up to 70 percent of the calcium carbonate formed in the global ocean, while corals contribute about 10 percent.
Ocean acidification lowers seawater pH, which reduces the availability of carbonate ions and makes it harder for calcifying organisms to build and maintain their skeletons or shells. Recent studies that isolated saturation state from pH changes point to saturation state as the more important factor in shell formation.
Marine biogenic calcification is central to the biological carbon pump, since calcium carbonate produced by marine organisms adds ballast to sinking particles and helps carry carbon to the deep ocean and seafloor. The related calcium carbonate counter pump also releases CO2 back into the surface ocean and atmosphere over a timescale of 100-1,000 years.
Carbonate skeletons evolved independently at least 28 times within the eukaryotic domain, with skeletal evolution occurring separately in foraminiferans and echinoderms. The Cambrian Period marked a major watershed, when mineralized skeletons appeared across many different groups and diversified gradually over about 25 million years.
By the end of the century, ocean acidification could reduce mussel calcification by about 25 percent and oyster calcification by about 10 percent, and global aquaculture shellfish production already contributes about US$29.2 billion to the world economy. Coral reefs carry an estimated average economic value of about US$490 per hectare annually, with reefs in Hawaiʻi and the Philippines contributing roughly US$360 million and at least US$1.06 billion each year respectively.