Marine organisms inhabiting environments where pCO(2)/pH varies naturally are suggested to be relatively resilient to future ocean acidification. To test this hypothesis, the effect of elevated pCO(2) was investigated in the articulated coralline red alga Corallina elongata from an intertidal rock pool on the north coast of Brittany (France), where pCO(2) naturally varied daily between 70 and 1000 mu atm. Metabolism was measured on algae in the laboratory after they had been grown for 3 weeks at pCO(2) concentrations of 380, 550, 750 and 1000 mu atm. Net and gross primary production, respiration and calcification rates were assessed by measurements of oxygen and total alkalinity fluxes using incubation chambers in the light and dark. Calcite mol % Mg/Ca (mMg/Ca) was analysed in the tips, branches and basal parts of the fronds, as well as in new skeletal structures produced by the algae in the different pCO(2) treatments. Respiration, gross primary production and calcification in light and dark were not significantly affected by increased pCO(2). Algae grown under elevated pCO(2) (550, 750 and 1000 mu atm) formed fewer new structures and produced calcite with a lower mMg/Ca ratio relative to those grown under 380 mu atm. This study supports the assumption that C. elongata from a tidal pool, where pCO(2) fluctuates over diel and seasonal cycles, is relatively robust to elevated pCO(2) compared to other recently investigated coralline algae.