Figure 7
Working model for chemical dissolution of calcium carbonate by an excavating sponge. Protons are received in exchange for calcium ions (potentially via calcium chelators) at the water sponge interface. Protons are moved through the sponge via the transport cells until reaching the etching cells. They are then incorporated in vesicles (pH 4 to 5), which travel towards filopodia. Subsequently, the vesicles discharge their content through exocytosis, which leads simultaneously to the dissolution of calcium carbonate and disintegration of the etching cell. The new space is then rapidly colonised by new filopodia and so on. Newly freed bicarbonate ions are converted to CO2, a reaction made faster with carbonic anhydrase enzymes, which passively diffuses out of the sponge.
