Acid-base balance and metabolic response of the sea urchin Paracentrotus lividus to different seawater pH and temperatures

Purpose: In order to better understand if the metabolic responses of echinoids could be related to their acid-base status in an ocean acidification context, we studied the response of an intertidal sea urchin species, Paracentrotus lividus, submitted to low pH at two different temperatures. Methods: Individuals were submitted to control (8.0) and low pH (7.7 and 7.4) at 10°C and 16°C (19 days). The relation between the coelomic fluid acid-base status, the RNA/DNA ratio of gonads and the individual oxygen uptake were studied. Results: The coelomic fluid pH decreased with the aquarium seawater, independently of temperature, but this explained only 13% of the pH variation. The coelomic fluid showed though a partial buffer capacity that was not related to skeleton dissolution ([Mg²⁺] and [Ca²⁺] did not differ between pH treatments). There was an interaction between temperature and pH on the oxygen uptake (V_O2) which was increased at pH 7.7 and 7.4 at 10°C in comparison with controls, but not at 16°C, indicating an upregulation of the metabolism at low temperature and pH. However, gonad RNA/DNA ratios did not differ according to pH and temperature treatments, indicating that even if maintenance of physiological activities has an elevated metabolic cost when individuals are exposed to stress, they are not directly affected during short-term exposure. Long-term studies are needed in order to verify if gonad production/growth will be affected by low pH seawaters exposure.