Effect of<Emphasis Type="Italic"> p</Emphasis>CO<Subscript>2</Subscript> and temperature on the boron isotopic composition of the zooxanthellate coral<Emphasis Type="Italic"> Acropora</Emphasis> sp.

Culture experiments were carried out with Acropora sp. (a branching scleractinian coral) in seawater at two pCO₂ conditions (438 and 725 µatm) and two temperatures (25 and 28 °C) in order to establish the pH and temperature dependence of the boron isotopic composition of the skeleton. A clear pCO₂ effect, but no temperature effect, on the coral boron isotope composition is seen. For corals cultured at normal pCO₂ (438 µatm), the ¹¹B of the skeleton was 24.0±0.2 at 25 °C, and 23.9±0.3 at 28 °C. The values of ¹¹B measured for corals cultured at higher pCO₂ (725 µatm) were lower: 22.5±0.1, and 22.8±0.1 at 25 and 28 °C, respectively. The ¹¹B of corals cultivated at both high and normal pCO₂ conditions are consistent with a dominant pH control, and are very close to that calculated from theoretical considerations. Thus, the corals do not seem to significantly alter ambient seawater for calcification with respect to pH. Co-variation between boron and carbon isotope values is explored.Communicated by: Guest Editor A. Grottoli     

Isotopic composition of the delta-18O--delta-13C from the otoliths of reef fish from Taiaro (Tuamotu, French Polynesia): isotopic and biological implications

Nuclei (larval stage) and outer parts (adult stage) of fish otoliths from the Taiaro closed lagoon (French Polynesia) and adjacent ocean have been analysed for the C-O isotopic compositions. delta 18O values of the nuclei of both populations indicate that isotopic equilibrium is reached. This implies that the lagoonal fish population has done its complete biological cycle in the lagoon and represents an adaptation in a closed system. delta 18O values of the outer parts show a slight isotopic disequilibrium (< 0.2@1000) interpreted in term of vital effect. All the delta 13C values exhibit a strong isotopic disequilibrium related to metabolic activity.     

Microanalysis of C and O isotopes of azooxanthellate and zooxanthellate corals by ion microprobe

We have determined the ¹⁸O and ¹³C values of azooxanthellate (Lophelia pertusa) and zooxanthellate (Porites lutea) corals at a micrometer scale using an ion microprobe (SIMS—secondary ion mass spectrometry). In P. lutea, centers of calcification are small (10 to 15 m) and difficult to locate during measurements. In L. pertusa, they are large (50 m) and arranged in lines of centers of calcification. Our results show that centers of calcification in L. pertusa have a restricted range of variation in ¹⁸O [-2.8±0.3 (PDB)], and a larger range in ¹³C [14.3 to 10.9 (PDB)]. Surrounding skeletal fibers exhibit large isotopic variation both for C and O (up to 12), and ¹³C and ¹⁸O are positively correlated. The C and O isotopic compositions of the center of calcification deviate from this linear trend at the lightest ¹⁸O values of the surrounding fibers. Ion microprobe results on P. lutea demonstrate also a large range of variation for the ¹⁸O values (up to 10). No correlation is found with C isotopes that exhibit, in comparison with L. pertusa, a small range of variation (2). This variation of ¹⁸O at a micrometer scale is probably the result of two processes: (1) an isotopic equilibrium calcification with 1 pH unit variation in the calcification fluid as indicated by direct measurements of coelenteron pH in the coral Galaxea fascicularis (Al-Horani et al. 2003) and (2) a kinetic fractionation. The ¹³C apparent disequilibrium in P. lutea may be the result of mixing between metabolic CO₂ (respiration) and dissolved inorganic carbon (DIC) coming directly from seawater.