Two scenarios for the potential function of scl-CA1 and scl-CA2 in spicule formation. In both, scl-CA1 catalyses the formation of mainly metabolic CO2 to HCO3- within the sclerocytes, and then transports it to the extracellular calcification site by a biacrbonate transporter. Here, scl-CA2 could also produce HCO3- from CO2 diffused into the extracellular space (left). Alternatively, scl-CA2 could catalyse the reverse reaction, in order to remove protons that were formed by the reaction of HCO3- to carbonate. The CO2 could then diffuse into the sclerocyte and again serve as substrate for sclCA1. In addtion to metabolic CO2, DIC (in form of HCO3-) might be taken up from the seawater, which could involve the activity of another CA, as had been suggested for corals . The DIC transport form and mechanisms within the sponge tissue are yet unknown.