The effect of total alkalinity on growth performance and calcification in juvenile Pacific abalone Haliotis discus hannai

A 45-day trial was conducted to study the effect of seawater total alkalinity (TA) level up- and downregulation on the growth performance and calcification of Haliotis discus hannai Ino, while seawater pH was maintained at pH(NBS) = 8.1. Although seawater was not acidified, the results showed that TA downregulation caused a significant reduction (P < 0.05) in the somatic tissue growth of juvenile abalone, while TA upregulation significantly increased growth performance (P < 0.05). Similar to the impacts of pH reduction, TA downregulation also induces a decline in CO2 buffering capacity, which may be the reason why somatic tissue growth was reduced, as lowered CO2 buffering capacity was reported to shift the acid-base balancing of abalone. Parts of the periostracum layer weremissing and exposed the inner shell layers of the individuals from the TA-downregulated group. Scanning electron microscopy (SEM) results showed calcium carbonate densely deposited onto the inner shell in the control and TA-upregulated groups, while sparsely deposited calcium carbonate was observed in the TA-downregulated group. The C: N ratio in the shell of individuals from the TA-downregulated group was significantly lower than that of the other two groups, indicating that less inorganic carbon was added to the shell. As a result, abalone grew lighter and thinner shells in TA-downregulated seawater. Although seawater was not acidified, TA downregulation also caused a reduction in the calcium carbonate saturation state (Omega), which induced the erosion of the surface shell and the interruption of calcium carbonate generation. In conclusion, although seawater pH remained at ambient levels, the lowered CO2 buffering capacity and Omega induced by seawater TA downregulation also showed a detrimental effect on the growth and calcification of Pacific abalone. The impact of ocean acidification on the growth of abalone should not be assessed using only seawater pH and/or pCO(2) but rather taking into account all of carbonate chemistry, particularly the CO2 buffering capacity. Abalone cultivation is suggested to be carried out in seawater with a higher level of CO2 buffering capacity and Omega, which can be achieved through integrated culture with seaweed or increasing the seawater TA level.