Shellfish CO2 excretion is modulated by seawater carbonate chemistry but largely independent of pCO2.

Four species of shellfish, blue mussel (Mytilus galloprovincialis), Pacific abalone (Haliotis discus hannai), zhikong scallops (Chlamys farreri), and Pacific oyster (Crassostrea gigas), were exposed to decoupled carbonate system variables to investigate the impacts of different seawater carbonate parameters on the CO₂ excretion process of mariculture shellfish. Six experimental groups with two levels of seawater pH (pH 8.1 and pH 7.7) and three levels of total alkalinity (TA = 1000, 2300, and 3600 μmol/kg, respectively) were established, while pH 8.1 and TA = 2300 μmol/kg was taken as control. Results showed that the CO₂ excretion rates of these tested shellfish were significantly affected by the change in carbonate chemistry (P < 0.05). At the same TA level, animals incubated in the acidified group (pH 7.7) had a lower CO₂ excretion rate than those in the control group (pH 8.1). In comparison, at the same pH level, the CO₂ excretion rate increased when seawater TA level was elevated. No significant correlation between the CO₂ excretion rate and seawater pCO₂ levels (P > 0.05) was found; however, a significant correlation (P < 0.05) between CO₂ excretion rate and TA-DIC (the difference between total alkalinity and dissolved inorganic carbon) was observed. Blue mussel has a significantly higher CO₂ excretion rate than the other three species in the CO₂ excretions per unit mass of soft parts, with no significant difference observed among these three species. However, in terms of CO₂ excretion rate per unit mass of gills, abalone has the highest CO₂ excretion rate, while significant differences were found between each species. Our studies indicate that the CO₂ buffering capacity impacts the CO₂ excretion rate of four shellfish species largely independent of pCO₂. Since CO₂ excretion is related to acid–base balancing, the results imply that the effects of other carbonate parameters, particularly the CO₂ buffering capacity, should be studied to fully understand the mechanism of how acidification affects shellfish. Besides, the species difference in gill to soft parts proportion may contribute to the species difference in responding to ocean acidification. [ABSTRACT FROM AUTHOR]