Coccolithophores such as this
Emiliania huxleyi are tiny, but account for a major portion of the biological activity in the ocean. In the past 220 years, these creatures, which make their own calcium carbonate protective shells, have been steadily increased the amount of armor it makes in response to heightened ocean acidity.
The acidity, say researchers, is linked to the ocean’s increasing absorption of carbon dioxide. Though certain calcium producers, such as coral and oysters, are threatened by acid levels, it seems coccolithophores have been adapting to changing conditions.
http://www.noc.soton.ac.uk/nocs/news.php?action=display_news&idx=457 SOURCE: National Oceanographic Centre UK