Résumé
The constant release of carbon in the atmosphere from human activity induces ocean acidification, with potential threat for calcifying organisms such as coccolithophores. The effect of acidification on Coccolithophores is not yet clearly understood: Culture experiments appears to show complex results and may induce opposite conclusions on the calcification response of the algae to an increase of pCO2 at the species level (for E. huxleyi compare Riebesell (2000) and Iglesias-Rodriguez et al. (2008)) or depending of the species (Langer et al., 2006). Natural oceanic environment offers an excellent laboratory to test the response of coccolithophores to various chemical settings. It is for example possible to sample a large spectrum of alkalinity or of carbonate saturation state conditions in the present surface ocean. Here we present estimation of the degree of calcification of important coccolithophore taxa in 170 water samples collected in diverse oceanic settings (Patagonian Shelf, Southern Indian Ocean, and Tropical Pacific Ocean). In this water collection, temperature, alkalinity, and pH ranged from 3 to 31° C, 2108 to 2418 μmole kg-1 and 7.6 to 8.2 respectively. The calcite weight of the coccoliths and of the coccospheres of Gephyrocapsa sp and of Emiliania huxleyi was estimated using automated pattern recognition and automated morphometry software. The results indicate that the degree of calcification of these species strongly depends on alkalinity, temperature and calcite saturation state. In area of high alkalinity, high temperature, and high calcite saturation state, these coccolithophores secrete the heaviest coccospheres and coccoliths. The facts that in the Chilean upwelling, where pH was the lowest, the mean coccolith weights are in the average of the global distribution and that the mean lightest and mean heaviest coccoliths are found both at higher pH, indicate that the effect of pH on its studied range is not a critical parameter that can be studied solely, but in conjunction with other physico-chemical parameter. The coccolithophore standing stocks are not dependant from the degree of calcification of the cells. An exponential equation, combining 7 physico- chemical parameters, explains more than 3/4 of the variance in weight of the coccoliths of these 170 samples. This indicates that calcification is extremely dependent on the carbonate system and that its different components have to be considered in a combined way. Iglesias-Rodriguez, M. D., et al, Science, 336-340, 2008. Langer, G., M. , et al., G3, 7, Q09006, 2006. Riebesell, U., et al., Nature, 407, 364-367, 2000.