Read here. There exists corals in several locations already thriving in open waters that possess the attributes of sea water under a condition of CO2 levels 2 to 3 times higher than today. The empirical evidence suggests that corals are fully capable of adapting to a wide range of conditions, including much higher levels of atmospheric CO2.
"...the two researchers report that "today, several reefs, including Galapagos, areas of Pacific Panama, and Jarvis (southern Line Islands), experience levels of aragonite saturation equivalent to that predicted for the open ocean under two times and three times pre-industrial CO2 levels"....."Probably the most important deduction to flow from these observations is the observable fact, in the words of Cohen and Holcomb, that "naturally elevated levels of inorganic nutrients and, consequently, high levels of primary and secondary production, may already be facilitating high coral calcification rates in regions with naturally high dissolved CO2 levels," which further suggests that earth's corals, with their genetically-diverse symbiotic zooxanthellae, are likely well equipped to deal successfully with whatever increase in the air's CO2 content will ultimately result from the burning of fossil fuels before other energy sources become viable..."
Additional sea coral postings.