CO₂ and calcification processes in fish

All teleost fish calcify and produce structures known as otoliths confined by saccular epithelia in the inner ear. These structures enable detection of acceleration/movement, equilibrium, pressure changes and sound vibrations. In addition, all marine teleosts produce CaCO₃ precipitates in the intestinal lumen to serve osmoregulation. The precipitation reaction in the intestinal lumen of marine fish is vital for their survival and the precipitated CaCO₃, which they void to the environment, is a substantial contribution to the inorganic carbon cycle. Both calcification processes are enhanced by exposure to elevate environmental CO₂, also at levels relevant for short-term climate change, although species differences in sensitivity exists. The present chapter summarizes what we know about the mechanisms of both calcification processes and how elevated CO₂ affects them and their related functions. Fish produced CaCO₃ is generally Mg-rich and relatively soluble compared to other biogenic CaCO₃. Consequently, the fate of voided CaCO₃ is relatively uncertain. Although fish produced CaCO₃ has been observed in sediments at shallow depths it appears that the majority of the voided CaCO₃ dissolves and contributes to elevated titratable alkalinity with depth in ocean waters.