Costs and mechanisms associated with resilience to acidification in marine bivalves

The unprecedented flux of CO2 into the ocean and the resulting chemical reactions has led to a reduction in pH, carbonate concentration, and saturation states of calcium carbonate, known as ocean acidification (OA). These conditions make it more difficult to precipitate biogenic calcium carbonate to mineralize shells because of the reduction in available carbonate ions. This represents a serious and growing threat to the future of commercially and ecologically important species, such as the northern quahog (Mercenaria mercenaria) and eastern oyster (Crassostrea virginica). But clams and oysters are found in heterogeneous coastal environments and are already exposed to reductions in pH surpassing predictions for the decrease in open ocean pH for the end of the century (with pH dropping below 7 under ambient conditions). These bivalves have shown high levels of resilience to fluctuations in pH and a capacity to respond to altered carbonate chemistry. However, the accelerated pace of these changes requires additional understanding of how or if species and populations will be able to acclimate or adapt to such swift environmental alterations. Future acidification might result in reduction in average pH, changes in the scale of variability, more occurrences of extreme acidification, and less periods of relief, exceeding thresholds of tolerance.

Thus far, the majority of studies have focused on the physiological effects of elevated pCO2 on bivalve larvae. While important, this leaves a substantial gap in knowledge of the molecular mechanisms of resilience to elevated pCO2 or the effect of acidification on different life history stages. To fill this gap in our understanding, this dissertation aims to uncover the mechanisms of resilience to elevated pCO2 in clams and oysters at different stages of their life.

This study combined physiological assays with ‘omic’ approaches (transcriptomics, genomics, proteomics) to assess the susceptibility of clams and oysters to acidification and the factors conferring resilience. Mechanisms enabling bivalves to respond to elevated pCO2 (from the organism level to individual genes) were investigated, taking into consideration the potential costs of resilience to elevated pCO2. Gene silencing experiments (RNAi) and chemical inhibition were used to confirm the protective role of candidate genes (perlucin and carbonic anhydrase, respectively) associated with resilience to elevated pCO2. While there were consequences for surviving under stressful acidification conditions, demonstrated by a marked reduction in immunity, depletion of energy resources, and inability to remineralize damaged shell, M. mercenaria and C. virginica, having already been exposed to natural fluctuations in pH and carbonate chemistry for generations, appear to be capable of implementing strategies to mitigate the negative impacts of elevated pCO2 (acclimation). While acclimation can be costly, the potential for adaptation was also investigated, and there was evidence to suggest genetic selection for OA-resilient genotypes enabling clams and oysters to persist under future climate regimes.

Schwaner C., 2023. Costs and mechanisms associated with resilience to acidification in marine bivalves. PhD thesis, State University of New York at Stony Brook, 467 p. Thesis (restricted access).

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