How does rising atmospheric CO2 affect marine organisms?

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Cariaco Basin off the Venezuelan Coast
Goni, M.A., Woodworth, M.P., Aceves, H.L., Thunell, R.C., Tappa, E., Black, D., Muller-Karger, F., Astor, Y. and Varela, R. 2004. Generation, transport, and preservation of the alkenone-based U37K' sea surface temperature index in the water column and sediments of the Cariaco Basin (Venezuela). Global Biogeochemical Cycles 18: 10.1029/2003GB002132.

Based on the degree of unsaturation of certain long-chain alkenones synthesized by haptophyte algae contained in a sediment core retrieved from the eastern sub-basin of the Cariaco Basin (1030'N, 6440'W) on the continental shelf off the Venezuelan central coast, Goni et al. determined that the highest sea surface temperatures at that location over the past 6000 years "were measured during the Medieval Warm Period (MWP)," which they identified as occurring between AD 800 and 1400. From the graph of their results reconstructed below, it is further evident that peak MWP temperatures were approximately 0.35C warmer than peak Current Warm Period temperatures, and that they were fully 0.95C warmer than the mean temperature of the last few years of the 20th century.