FIG. 49.14

What is the effect of increased atmospheric CO₂ and reduced ocean pH on skeleton formation in marine algae?

BACKGROUND It is well established that atmospheric CO₂ levels are increasing, which in turn decreases the pH of ocean water. In the late 1990s, experiments showed that a decrease in pH affected the ability of some marine organisms to build skeletons made of calcium carbonate (CaCO₃). The German biologist Ulf Riebesell and his colleagues carried out experiments to investigate whether ocean acidification affects algae called coccolithophorids, which account for the majority of carbonate precipitation in the open ocean.

HYPOTHESIS From the effects of decreased pH on CaCO₃ skeleton formation in other marine organisms, Riebesell hypothesized that increasing CO₂ would interfere with skeleton formation in coccolithophorids.

EXPERIMENT Riebesell and his colleagues studied two species of coccolithophorids, Emiliania huxleyi and Gephyrocapsa oceanica. They grew both species in the laboratory under conditions of increasing CO₂, from pre-industrial levels, to present-day levels, and then to levels predicted for the future. They measured pH and rates of calcification and used scanning electron microscopy to observe skeleton formation.

RESULTS With increasing CO₂ and decreasing pH, they observed decreasing rates of calcification as well as deformed and incomplete coccolithophorids under the scanning electron microscope.

CONCLUSION The results support Riebesell’s hypothesis: Ocean acidification interferes with normal skeleton formation in marine plankton.

FOLLOW-UP WORK This work has been extended to investigate the effects of ocean acidification on skeleton formation in corals. For example, the Mediterranean coral Oculina patagonica grows normally at pH 8.2. When grown at pH 7.4, however, the coral skeletons dissolve. Upon restoration of normal conditions, the corals resume skeleton formation. This experiment shows that corals might survive ocean acidification, although the ecosystem benefits provided by reefs would be lost.

SOURCES Riebesell, U., et al. 2000. “Reduced Calcification of Marine Plankton in Response to Increased Atmospheric CO₂.” Nature 407: 364–367; Fine, M., and D. Tchernov. 2007. “Scleractinian Coral Species Survive and Recover from Decalcification.” Science 315:1811–1811.