structures in the marine biota. Marine calcifying organisms include many taxonomic groups and occupy diverse ecological niches. Important examples include photosynthetic primary producers (e.g., coccolithophores and coralline algae), zooplankton (e.g., pteropods), mollusks (e.g., clams, mussels, and oysters), crustaceans (e.g., crabs and lobsters), and animals that harbor photosynthetic symbionts (e.g., reef-building corals, some planktonic foraminfera). In most of these organisms, CaCO3 is the principal constituent of the “hard part.” But in some organisms, only a part of the exoskeleton is calcified (e.g., the calcite ossicles of sea stars), while, in others, calcium carbonate is integrated into an organic exoskeleton structure (e.g., lobster and crab shells). These CaCO3 structures are most often in the form of calcite, aragonite, high-magnesium calcite, or a mixture of these mineral forms, and the mineral form may change through the development of the organism (Politi et al., 2004; see also Box 2.3).

Most calcifying organisms studied so far show a decrease in calcification or shell weight (either a slower rate of calcification or a decrease in the mass of CaCO3 per individual) in response to elevated CO2 and reduced pH. This is the best-documented and most widely observed biological effect of the acidification of seawater. It has been reported in a range of organisms, including coccolithophores, foraminifera, mussels, urchins, oysters and other bivalves, corals, and coralline algae (e.g., see Fabry et al. 2008b; Ries et al., 2009). In some organisms, a significant reduction in calcification was observed for a decrease in pH of 0.2-0.4 units, in the range predicted to occur over the next century; in others, a significant effect was only observed under more severe acidification. A few studies have shown that some calcifying organisms are insensitive to seawater acidification, or even increase calcification over the range of pH projected for the next century (Ries et al., 2009; Wood et al., 2008; Miller et al., 2009). In coccolithophores, the effect can be complicated by the increase in growth rate caused by high CO2 (see below), such that the calcification rate per cell may increase while the ratio of inorganic to organic cellular carbon may decrease (Iglesias-Rodriguez et al., 2008). Note that increased calcification is not necessarily an indication of increased health of the organism, and there is some early evidence that other processes can be affected by the strain introduced by accommodation of the increased CO2 (Wood et al., 2008). It is also possible that species that live in environments where pH and CO2 concentrations are variable may be more tolerant of the overall increase in acidity, though this hypothesis has not been tested.

In some groups of organisms, a decrease in calcification is associated with more frequent malformations of the carbonate structures (e.g., coccolithophores; Riebesell et al., 2000; Langer et al., 2006), smaller and thinner shells in foraminifera (Moy et al., 2009) and mollusks (Miller et al., 2009; Talmage and Gobler, 2009), slower shell extension rates (e.g., mollusks;

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