MEPS 419:95-108 (2010)  -  DOI: https://doi.org/10.3354/meps08841

ABSTRACT: Estuarine organisms are exposed to periodic strong fluctuations in seawater pH driven by biological carbon dioxide (CO₂) production, which may in the future be further exacerbated by the ocean acidification associated with the global rise in CO₂. Calcium carbonate-producing marine species such as mollusks are expected to be vulnerable to acidification of estuarine waters, since elevated CO₂ concentration and lower pH lead to a decrease in the degree of saturation of water with respect to calcium carbonate, potentially affecting biomineralization. Our study demonstrates that the increase in CO₂ partial pressure (pCO₂) in seawater and associated decrease in pH within the environmentally relevant range for estuaries have negative effects on physiology, rates of shell deposition and mechanical properties of the shells of eastern oysters Crassostrea virginica (Gmelin). High CO₂ levels (pH ~7.5, pCO₂ ~3500 µatm) caused significant increases in juvenile mortality rates and inhibited both shell and soft-body growth compared to the control conditions (pH ~8.2, pCO₂ ~380 µatm). Furthermore, elevated CO₂ concentrations resulted in higher standard metabolic rates in oyster juveniles, likely due to the higher energy cost of homeostasis. The high CO₂ conditions also led to changes in the ultrastructure and mechanical properties of shells, including increased thickness of the calcite laths within the hypostracum and reduced hardness and fracture toughness of the shells, indicating that elevated CO₂ levels have negative effects on the biomineralization process. These data strongly suggest that the rise in CO₂ can impact physiology and biomineralization in marine calcifiers such as eastern oysters, threatening their survival and potentially leading to profound ecological and economic impacts in estuarine ecosystems.

KEY WORDS: Hypercapnia · Ocean acidification · Calcification · Shell structure · Energy metabolism · Oxygen consumption · Mollusks