Proteomic and Transcriptomic Responses Enable Clams to Correct the pH of Calcifying Fluids and Sustain Biomineralization in Acidified Environments

Seawater pH and carbonate saturation are predicted to decrease dramatically by the end of the century. This process, designated ocean acidification (OA), threatens economically and ecologically important marine calcifiers, including the northern quahog (Mercenaria mercenaria). While many studies have demonstrated the adverse impacts of OA on bivalves, much less is known about mechanisms of resilience and adaptive strategies. Here, we examined clam responses to OA by evaluating cellular (hemocyte activities) and molecular (high-throughput proteomics, RNASeq) changes in hemolymph and extrapallial fluid (EPF-the site of biomineralization located between the mantle and the shell) in M. mercenaria continuously exposed to acidified (pH similar to 7.3; pCO(2) similar to 2700 ppm) and normal conditions (pH similar to 8.1; pCO(2) similar to 600 ppm) for one year. The extracellular pH of EPF and hemolymph (similar to 7.5) was significantly higher than that of the external acidified seawater (similar to 7.3). Under OA conditions, granulocytes (a sub-population of hemocytes important for biomineralization) were able to increase intracellular pH (by 54% in EPF and 79% in hemolymph) and calcium content (by 56% in hemolymph). The increased pH of EPF and hemolymph from clams exposed to high pCO(2) was associated with the overexpression of genes (at both the mRNA and protein levels) related to biomineralization, acid-base balance, and calcium homeostasis, suggesting that clams can use corrective mechanisms to mitigate the negative impact of OA.

Automated summary

This study examined the cellular and molecular changes in northern quahogs (Mercenaria mercenaria) continuously exposed to acidified and normal conditions. The results showed that under ocean acidification conditions, clams were able to increase intracellular pH and calcium content, which was associated with the overexpression of genes related to biomineralization, acid-base balance, and calcium homeostasis.