Effect of light and temperature on calcification and strontium uptake in the scleractinian coral Acropora verweyi

Strontium thermometry has been suggested as a powerful tool for reconstructing seawater surface temperature (SST). In corals, an inverse relationship between SST and skeletal Sr/Ca ratios has been found. However, this ratio might also vary with calcification, which in turn is dependent on light and temperature. The aim of this study was to improve our knowledge of the uptake of Sr2+ as a function of light and temperature in the scleractinian coral Acropora verweyi. Two experiments were performed in which nubbins were acclimated over 4 wk either to 3 light intensities (100, 200 and 400 μmol m–2 s–1) or to 3 temperatures (20, 25, and 29°C) and growth rates were monitored. At the end of the 4 wk, nubbins were incubated, under the above light levels and temperatures, in individual beakers containing seawater spiked with the radiotracer 85Sr. Parallel incubations were carried out in dark beakers, in order to compare rates of Sr2+ incorporation in light and dark. The results obtained showed that growth rates were significantly higher under high light (0.16 ± 0.01 and 0.27 ± 0.01% d–1 for 100 and 400 μmol photons m–2 s–1) and under high temperature (0.06 ± 0.01% d–1 at 20°C to 0.35 ± 0.03% d–1 at 29°C). Rates of Sr2+ incorporation into the coral skeleton were also higher under high light (32.4 ± 3.0, 72.9 ± 13.5 and 91.2 ± 9.0 nmol (g dry weight, DW)–1 d–1, for corals cultured at 100, 200 and 400 μmol m–2 s–1 respectively) and high temperature (89 ± 14, 224 ± 38 and 436 ± 58 nmol (g DW)–1 d–1 for corals cultured at 20, 25 and 29°C respectively). Rates of Sr2+ uptake were also 2 to 3 times lower in the dark than in the light, comparable with the incorporation of calcium. Our results finally show a strong correlation between Sr2+ uptake and growth rates. Strontium uptake therefore follows the same pattern as calcium uptake, both ions being regulated by the calcification biochemistry.