| Summary: | About 30% of the carbon dioxide produced by human activities is absorbed by the ocean leading to a decrease of seawater pH and saturation state of calcium carbonate (CaCO3). The subsequent ocean acidification is likely to result in profound changes in marine ecosystems, in particular among the marine calcifiers. Coccolithophorides together with foraminifera produce more than 90% of the pelagic carbonate in the modern ocean. Culture experiments have shown that the response of coccolithophores to pH varies between and within species, thus complicating our understanding of the overall impact biological response on the carbon cycle and feedbacks on climate. Indeed, different sensitivities to increase dissolved CO2 and decrease seawater pH, and their consequences on calcification exist among coccolithophores, but they are still not fully described nor quantified. Calcareous coccoliths are formed inside the cell in an internal vesicle called the coccolith vesicle. The pH inside the coccolith vesicle would be a key parameter in determining calcite precipitation and therefore coccolith formation. Currently the coccolith vesicle pH cannot be accurately measured and thus estimates have to be based on indirect geochemical evidences. The capacity of the coccolith vesicle to regulate pH allowing for calcite precipitation could explain the resilience of somecoccolithophores to ocean acidification. To further explore this hypothesis, two strains of E. huxleyi were grown in batch cultures under three different pH conditions to assess their response to changing seawater pH. Physiological parameters including growth rate, POC and PIC were examine, in addition to assessing changes in the vesicle pH by measuring B/Ca and δ11B in coccolith calcite and evaluate the potential of boron for paleo-pH reconstruction. Environ 30% du dioxyde de carbone produit par des activités humaines est absorbé par l’océan menant à une diminution de pH d’eau de mer et de l’état de saturation de carbonate de calcium (CaCO3). L’acidification des océans ...
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