Increased Food Resources Help Eastern Oyster Mitigate the Negative Impacts of Coastal Acidification

SIMPLE SUMMARY: The concentration of CO(2) in the atmosphere has increased dramatically since the Industrial Revolution because of human activities including burning of fossil fuels. The oceans absorb almost one-third of atmospheric CO(2), which has led to a decrease in the pH of seawater, a process known as ocean acidification. Animals that build shells from calcium carbonate, such as the eastern oyster, are especially vulnerable to this reduction in pH. Over the last decade, many studies have demonstrated alterations in development and a reduction in shell growth in bivalves exposed to ocean acidification, while less attention has been paid to the mechanisms that enable these animals to survive. This study evaluated the challenges of oysters surviving in low pH conditions and identified potential processes involved in resilience to acidification, such as whether food availability or changes in trophic resources enhance resilience. Findings showed that oysters exposed to low pH had increased energetic demands to cope with acidification. Oysters supplemented with abundant food resources performed much better under acidification (had less mortality and greater growth). Furthermore, oysters demonstrated an ability to alter the food uptake process and optimize energy intake. Results suggest that if oysters have the energetic means to perform adaptive mechanisms, they may be successful under future ocean acidification. ABSTRACT: Oceanic absorption of atmospheric CO(2) results in alterations of carbonate chemistry, a process coined ocean acidification (OA). The economically and ecologically important eastern oyster (Crassostrea virginica) is vulnerable to these changes because low pH hampers CaCO(3) precipitation needed for shell formation. Organisms have a range of physiological mechanisms to cope with altered carbonate chemistry; however, these processes can be energetically expensive and necessitate energy reallocation. Here, the hypothesis that resilience to low pH is related to energy resources was tested. In laboratory experiments, oysters were reared or maintained at ambient (400 ppm) and elevated (1300 ppm) pCO(2) levels during larval and adult stages, respectively, before the effect of acidification on metabolism was evaluated. Results showed that oysters exposed to elevated pCO(2) had significantly greater respiration. Subsequent experiments evaluated if food abundance influences oyster response to elevated pCO(2). Under high food and elevated pCO(2) conditions, oysters had less mortality and grew larger, suggesting that food can offset adverse impacts of elevated pCO(2), while low food exacerbates the negative effects. Results also demonstrated that OA induced an increase in oyster ability to select their food particles, likely representing an adaptive strategy to enhance energy gains. While oysters appeared to have mechanisms conferring resilience to elevated pCO(2), these came at the cost of depleting energy stores, which can limit the available energy for other physiological processes. Taken together, these results show that resilience to OA is at least partially dependent on energy availability, and oysters can enhance their tolerance to adverse conditions under optimal feeding regimes.