* These authors contributed equally to this work.The increase in atmospheric CO 2 due to anthropic activities results in an acidification of the surface waters of the oceans. The impact of these chemical changes differs according to the considered organisms. The intertidal rocky shores may harbor organisms pre-adapted to the upcoming changes as they already face tidal pH and temperature fluctuations. In order to cope with the changes in seawater pH, these organisms possess different mechanisms involved in acid-base regulation. Some organisms present a higher buffer capacity than seawater, among which echinoderms. The properties of this buffer capacity and the factors influencing it were investigated in the sea urchin Paracentrotus lividus and in the starfish Asterias rubens, both species occurring in the intertidal zone of the North Atlantic and the North Sea, respectively. Buffer capacity is partly due to the coelomocytes present in the coelomic fluid and, in P. lividus, it is also due to a compound which contributes to a higher buffer capacity of the coelomic fluid of this species compared to that of the starfish. The effect of a decreased seawater pH (in the scope of predicted future ocean acidification) on this buffer capacity in P. lividus was investigated. A gradual increase of the buffer capacity was recorded when the seawater pH was decreased. Moreover, the comparison of different echinoderm species showed that Euechinoidea present a very high buffer capacity while Cidadroidea (other sea urchins), starfish and holothurians have a lower one. This can be explained either by the presence of the compound only in Euechinoidea, linked to differences in the respiratory machinery, or by metabolic differences between the various classes of echinoderms.
Increased atmospheric CO2 concentration is leading to changes in the carbonate chemistry and the temperature of the ocean. The impact of these processes on marine organisms will depend on their ability to cope with those changes, particularly the maintenance of calcium carbonate structures. Both a laboratory experiment (long-term exposure to decreased pH and increased temperature) and collections of individuals from natural environments characterized by low pH levels (individuals from intertidal pools and around a CO2 seep) were here coupled to comprehensively study the impact of near-future conditions of pH and temperature on the mechanical properties of the skeleton of the euechinoid sea urchin Paracentrotus lividus. To assess skeletal mechanical properties, we characterized the fracture force, Young's modulus, second moment of area, material nanohardness, and specific Young's modulus of sea urchin test plates. None of these parameters were significantly affected by low pH and/or increased temperature in the laboratory experiment and by low pH only in the individuals chronically exposed to lowered pH from the CO2 seeps. In tidal pools, the fracture force was higher and the Young's modulus lower in ambital plates of individuals from the rock pool characterized by the largest pH variations but also a dominance of calcifying algae, which might explain some of the variation. Thus, decreases of pH to levels expected for 2100 did not directly alter the mechanical properties of the test of P. lividus. Since the maintenance of test integrity is a question of survival for sea urchins and since weakened tests would increase the sea urchins' risk of predation, our findings indicate that the decreasing seawater pH and increasing seawater temperature expected for the end of the century should not represent an immediate threat to sea urchins vulnerability.
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