How does rising atmospheric CO2 affect marine organisms?

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Blue Hole, Lighthouse Reef, Coast of Belize
Gischler, E., Shinn, E.A., Oschmann, W., Fiebig, J. and Buster, N.A. 2008. A 1500-year Holocene Caribbean climate archive from the Blue Hole, Lighthouse Reef, Belize. Journal of Coastal Research 24: 1496-1505.

Working with a six-meter-long ocean sediment core and a shorter 80-cm-long core extracted from the bottom of the Blue Hole of Belize's Lighthouse Reef (17.5°N, 87.5°W), the authors measured the δ18O of the fine-grained calcium carbonate sediments contained therein. Then, they report that "after a δ18O high [(indicative of colder temperatures)], probably reflecting the end of the Migration Period Pessimum (AD 400-800), there is a long δ18O low during the Medieval Warm Period from AD 1000-1400, another δ18O high around AD 1500, representing the Little Ice Age, followed by a δ18O drop." And based on a graphical representation of their results, the MWP was likely cooler than the last two to three decades of the 20th century.