Carbonic Anhydrase and Metazoan Biocalcification: A Focus on Molluscs

Carbonic anhydrase is a super-family of metallo-enzymes (containing α, β, γ, ζ and δ-CA families) that catalyse the reversible hydration of carbon dioxide. Among their numerous functions, CAs - in particular that of the α-CA family - are known to play a key role in biocalcification processes, i.e., the ability to deposit calcium carbonate crystallites in a controlled manner to form exoskeletons. In the gastropod mollusc Haliotis tuberculata – the European abalone – we identified two CA transcripts, htCA1 and htCA2, in the mantle, the calcifying organ responsible for shell formation from an extracellular organic matrix and a mixture of inorganic ions. Because these two transcripts are specifically expressed in the mantle, this suggests that the two corresponding CA isoforms may be directly involved in shell formation. In the present paper, whole mount in situ hybridization experiments performed on larval stages of H. tuberculata reveal the expression of htCA1 in cells associated with the statocysts, the sensory organs for gravity, while htCA2 is not expressed in these cells. We compile the activity and expression data for these two CAs in H. tuberculata and discuss these results in an evolutionary context using a simplified phylogeny from compiled CA sequence data of several metazoans. This shows that the evolution of this protein super-family has a complex history with origins at the dawn of the Phanerozoic.