Effects of elevated p CO 2 on the metabolism of a temperate rhodolith Lithothamnion corallioides grown under different temperatures

Coralline algae are considered among the most sensitive species to near future ocean acidification. We tested the effects of elevated p CO 2 on the metabolism of the free‐living coralline alga L ithothamnion corallioides (“maerl”) and the interactions with changes in temperature. Specimens were collected in N orth B rittany ( F rance) and grown for 3 months at p CO 2 of 380 (ambient p CO 2 ), 550, 750, and 1000 μatm (elevated p CO 2 ) and at successive temperatures of 10°C (ambient temperature in winter), 16°C (ambient temperature in summer), and 19°C (ambient temperature in summer +3°C). At each temperature, gross primary production, respiration (oxygen flux), and calcification (alkalinity flux) rates were assessed in the light and dark. Pigments were determined by HPLC . Chl a , carotene, and zeaxanthin were the three major pigments found in L . corallioides thalli. Elevated p CO 2 did not affect pigment content while temperature slightly decreased zeaxanthin and carotene content at 10°C. Gross production was not affected by temperature but was significantly affected by p CO 2 with an increase between 380 and 550 μatm. Light, dark, and diel (24 h) calcification rates strongly decreased with increasing p CO 2 regardless of the temperature. Although elevated p CO 2 only slightly affected gross production in L . corallioides , diel net calcification was reduced by up to 80% under the 1,000 μatm treatment. Our findings suggested that near future levels of CO 2 will have profound consequences for carbon and carbonate budgets in rhodolith beds and for the sustainability of these habitats.