The anthropogenic increase in atmospheric CO2 partial pressure (pCO2) causes a decrease in seawater pH and changes in carbonate chemistry called ocean acidification (OA). All the marine species could be impacted by OA but calcifying organisms are known to be the most sensitive. This PhD thesis focused on the physiological responses of coralline algae (red calcareous algae) and the slipper limpet Crepidula fornicata (gastropod mollusc) exposed to current and future pCO2 conditions. Coralline algae showed variable species-specific responses to OA with a lack of effect in Corallina officinalis and a 80 % decrease in diel net calcification in Lithothamnion corallioides (maerl). Conversely, C. fornicata was highly tolerant to OA. Although net calcification decreased due to external shell dissolution, no metabolic depression has been shown under high pCO2. Size of early life stages (embryos and larvae) decreased with increasing pCO2 but there were no effect on larval respiration and survival. These calcifying species are of major ecological importance in coastal ecosystems from the Channel and Northeastern Atlantic. Changes induced by OA on the physiology of these key species are likely to impact energy and matter fluxes in the ecosystems they dominate.
