Coral is a great CO3 absorber. CO2 can acidify the ocean water, which lowers calcification rates affecting the ability to grow carbonate skeletons. Combined with an increase of CO2 in the atmosphere contributing to a global increase in temperature, which coral is extremely sensitive to, and an increase of CO2 anywhere is bad news.
Source: http://people.uncw.edu/szmanta/BIO%20585%20CoralReef%20Field...