Elevated level of carbon dioxide affects metabolism and shell formation in oysters Crassostrea virginica (Gmelin)

Estuarine organisms are exposed to periodic strong fluctuations in seawater pH driven by biological carbon dioxide (CO sub(2)) production, which may in the future be further exacerbated by the ocean acidification associated with the global rise in CO sub(2). Calcium carbonate-producing marine species such as mollusks are expected to be vulnerable to acidification of estuarine waters, since elevated CO sub(2) 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 CO sub(2) partial pressure (pCO sub(2)) 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 CO sub(2) levels (pH ~7.5, pCO sub(2) ~3500 mu atm) caused significant increases in juvenile mortality rates and inhibited both shell and soft-body growth compared to the control conditions (pH ~8.2, pCO sub(2) ~380 mu atm). Furthermore, elevated CO sub(2) concentrations resulted in higher standard metabolic rates in oyster juveniles, likely due to the higher energy cost of homeostasis. The high CO sub(2) 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 CO sub(2) levels have negative effects on the biomineralization process. These data strongly suggest that the rise in CO sub(2) 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.