Ammonia and biological deposition of calcium carbonate

THE mechanism for the formation of CO₃^2-, the reacting species in CaCO₃ formation in biological systems which deposit CaCO₃, is not known. The involvement of carbonic anhydrase in the biological formation of CaCO₃ (refs. 1 and 2) suggests that CO₃^2- arises from the dissociation of HCO₃-. Hodges and Lörcher^3,4 have summarized the major questions concerning the origin of CO₃^2- in the avian eggshell-forming system and have outlined a model, originally proposed by Diamantstein⁵, in which the CO₃^2- of eggshell CaCO₃ arises in the shell gland lumen from HCO₃- which is produced intracellularly by shell gland carbonic anhydrase. As emphasized by Hodges and Lörcher and also recently by Simkiss⁶, a major question remaining with the mechanism of CO₃^2- formation is the fate of the proton formed by HCO₃- dissociation. We propose that this proton may be neutralized by the reaction, NH₃ + H⁺⇌NH₄⁺. We also feel that NH₃ acts in this manner in CaCO₃ deposition by certain land snails^7,8. The neutralization of protons by NH₃ has precedence as a biological phenomenon. For example, protons are neutralized by NH₃ in the mammalian kidney during metabolic acidosis⁹ and this reaction may also serve to regulate the acidity of sweat¹⁰. The effect of NH₃ in the biological deposition of CaCO₃ is summarized by the equation: This overall reaction has also been considered as a model for the geochemical deposition of CaCO₃ in certain circumstances¹¹.