Miller et al., 2009), and weakened shells (e.g., barnacles, mollusks) (Bibby et al., 2007; Clark et al., 2009; McDonald et al., 2009; Tunnicliffe et al., 2009). In reef-building corals, which have been studied most extensively, a wide range of responses has been observed, but on average, a doubling of preindustrial atmospheric CO2 concentration resulted in about a 10-60% decrease in calcification rates (Langdon and Atkinson, 2005; see also Figure 3.1 as an example, section 4.1, and Appendix C). In some species, seawater acidification led to reduced rates of larval development and increased larval mortality (e.g., echinoderms) as a result of the instability of the nascent calcified structures which are often less well crystallized than the mature form. It must be noted, however, that the physiological role of calcification is not always clear. For example, laboratory cultures of coccolithophores that have lost the ability to calcify grow at normal rates (Rost and Riebesell, 2004). Some species of corals can grow well in cultures without precipitating aragonite, even though the very structure of a coral reef depends on the precipitation of the mineral (e.g., Fine and Tchernov, 2007).

The spontaneous precipitation of CaCO3 in seawater requires a high degree of supersaturation of the mineral (i.e., Q>>1) (which is proportional to the carbonate ion [CO32–] concentration when pressure, temperature, and calcium ion concentration are kept constant). Within organisms, this is achieved by controlling Ω at the site of calcification at a value generally higher than that of seawater (e.g., Al-Horani et al., 2003; Furla et al., 1998; Bentova et al., 2009) through a process that involves pumping of various ions into specialized cellular compartments. Despite the fact that organisms control internal Ω with these processes, calcification has been observed to correlate well with the external value of Ω in many experiments and several taxa (see Figure 3.1); therefore, the external Ω and CO32– concentration may serve as indicators of the calcification response caused by acidification. There are several hypotheses regarding the correlation between external Ω and biological calcification; for example, acidification of the external medium may increase the energetic cost of calcification in some organisms. The energetic cost of calcification should depend on the underlying biochemical mechanisms, which are presently not well understood and are likely to differ widely among taxonomic groups. Marubini and others (2008) addressed multiple hypotheses to explain why acidification causes a decrease in coral calcification rates and suggested that decreases in intracellular or extracellular pH, or shifts in the buffering capacity of the calcifying fluid were likely. A recent study on a temperate coral provides evidence that the calcification response reflects changes in the proton pumping capacity, which is necessary to maintain the high saturation states of the internal calcifying fluid (Cohen et al., 2009). This is supported by results from an analytical survey of calcifica-

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