The reduction in pH from atmospheric inputs of CO2 (ocean acidification, OA) threatens marine calcifiers, including the eastern oyster (Crassostrea virginica), that precipitate biogenic CaCO3 for shell formation. Recent investigations have demonstrated that alterations in gene expression enable bivalves to respond to episodic low pH. Evidence generated from several studies highlighted the importance of upregulating genes related to biomineralization, ion transport, and acid-base balance such as carbonic anhydrase (CA) genes. Two experiments were designed to evaluate the effect of acidification on calcification processes and to probe the specific role of CA in oyster resilience to low pH. First, adult oysters were exposed to eight months of chronic acidification stress (pH similar to 7.3, pCO(2) similar to 3300 ppm) or control conditions (pH similar to 7.9, pCO(2) similar to 500 ppm) before shells were artificially damaged and shell repair monitored. Results showed a dramatic decrease in shell regeneration after chronic high pCO(2) exposure (only 30% of oysters regrew any shell) suggesting that mechanisms that promote calcification under high pCO(2) conditions may not be sustainable for extended periods of time. To further explore these mechanisms, a second experiment was designed by focusing on the role of CA in mitigating acidification stress. Here, adult oysters received an injection of acetazolamide in dimethyl sulfoxide (DMSO) to inhibit CA or DMSO (control) before rearing in control (pH similar to 8.1, pCO(2) similar to 340 ppm) or acidified (pH similar to 7.3, pCO(2) similar to 3300 ppm) conditions. After three weeks, oyster shells were damaged and shell repair monitored. Oysters incubated at low pH seawater with CA inhibition had the least amount of shell regeneration at the end of 21-day regrowth period. Interestingly, oysters were able to increase intracellular pH (pH(i)) of hemocytes under low pH conditions; however, this ability was significantly diminished with CA inhibition. Results highlight the role of CA in maintaining calcification under low pH conditions by establishing an intracellular environment favorable to calcium carbonate precipitation.
