Using living corals collected from Okinawan coral reefs, laboratory experiments were performed to investigate the relationship between coral calcification and aragonite saturation state (W) of seawater at 25∞C. Calcification rate of a massive coral Porites lutea cultured in a beaker showed a linear increase with increasing W aragonite values (1.08-7.77) of seawater. The increasing trend of calcification rate (c) for W is expressed as an equation, c = aW + b (a, b: constants). When W was larger than ~4, the coral samples calcified during nighttime, indicating an evidence of dark calcification. This study strongly suggests that calcification of Porites lutea depends on W of ambient seawater. A decrease in saturation state of seawater due to increased pCO 2 may decrease reef-building capacity of corals through reducing calcification rate of corals.Keywords: coral, calcification, aragonite, saturation state, cultured experiment As a result of coral calcification (Eq. (2)), the W aragonite of seawater is altered through releasing CO 2 in surrounding environments. The elevated CO 2 in marine environments lowers pH through decreasing CO 3 2-concentration and leads to a decrease in W of seawater.The relationship between coral-reef calcification and W of seawater has been reported through field observations. Broecker and Takahashi (1966) observed that precipitation rate of CaCO 3 on the Bahama Bank was proportional to the degree of supersaturation, as a secondorder reaction. Smith and Pesret (1974) pointed out that coral calcification in the lagoon of Fanning Island was affected by the carbonate mineral saturation state. On a coral reef flat of Okinawa, Ohde and van Woesik (1999) also observed that calcification rate increased with the increase in W aragonite of seawater during daytime. From a global viewpoint associated with atmospheric CO 2 budget, Kleypas et al. (1999) strongly pointed out that coral calcification responds to W of seawater.On the other hand, the relationship between coral calcification and W aragonite of seawater was also studied through laboratory experiments under controlled conditions. Gattuso et al. (1998) observed a nonlinear increase in calcification rate of Stylophora pistillata with increasing W of seawater with a plateau in the rate when saturation state was larger than 3.90. Marubini and Atkinson (1999) also observed that calcification rate of Porites compressa decreased with decreasing W aragonite of seawater. However, only a few studies have provided clear relationship between coral calcification and seawater W.In order to elucidate the relationship between coral
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