The Implication of Carbonic Anhydrase in the Physiological Mechanism of Penetration of Carbonate Substrata by the Marine Burrowing Spongecliona Celata(demospongiae)

The marine burrowing sponge Cliona celata contains measurable carbonic anhydrase activity when assayed with a modified electrometric method. 2. When clionid tissues are extracted and centrifuged in isotonic sucrose, the majority of the enzyme activity is found in the mitochondrial size fraction, indicating the enzyme is membrane- or particle-bound. 3. Much of the enzyme activity can be released into solution with sonication. Treatment with isobutanol, which dissociates lipoprotein complexes, releases all of the enzyme activity into solution. 4. From a range of buffer solutions, 0.025 M trisaminomethane (pH 8.4-8.7) was chosen as the optimal buffer for the assay of clionid carbonic anhydrase. 5. There is a positive correlation between the excavating activity of the sponge (mg of CaCO3 removed per day) and the level of carbonic anhydrase in sponge tissues. In addition, enzyme activity is concentrated in the cortical tissues of gamma-form C. celata. It is these tissues that are most likely to come in contact with fresh substrata in the form of shell fragments. 6. Sodium acetazolamide is capable of inhibiting the activity of clionid carbonic anhydrase in vitro with no apparent inhibition of the overall metabolism of the overall metabolism of the sponge. 7. Acetazolamide-induced inhibition of the sponge's carbonic anhydrase activity markedly reduces the ability of alpha-form C. celata to excavate calcium carbonate chips from the valves of Mercenaria mercenaria. 8. Similar concentrations of acetazolamide provoke no papillary contraction or reduction in oxygen consumption in vivo, indicating the reduction in excavation rate is not due to the ability of the sponge to detect the acetazolamide and shut down the flow of sea water through its choanosome. It appears, therefore, that the primary mechanism for the dissolution of calcium carbonate by C. celata involves a shift in the carbonate solubility product in the microenvironment of the etching cell, mediated through the activity of the enzyme carbonic anhydrase.