Abstract. The CaCO3 deposits of aquatic plants may be intra-, inter- and extracellular. Calcification is mainly the result of photosynthetic CO2 or HCO−3 assimilation. This raises the local pH and CO2−3 concentration resulting from shifts in the dissolved inorganic carbon equilibrium, due to either net CO2 depletion as in Halimeda or localized OH− efflux (or H+ influx) as in Chara. The plant cell wall may be important in CaCO3 nucleation by acting as an epitaxial substratum or template, or by creating a microenvironment enriched in Ca2+ compared to Mg2+. Hypotheses on the reason for the lack of calcification in many aquatic plants are presented.