The characteristics and biological relevance of inorganic amorphous calcium carbonate (ACC) precipitated from seawater

The importance of amorphous calcium carbonate (ACC) as a potential phase in the biomineralization of marine calcifiers is increasingly being reported, particularly as the presence of ACC has been observed or inferred in several major groups. Here, we investigate the structure and conditions required to precipitate ACC from seawater-based solutions, with emphasis on the co-influence of the carbonate system (pH, dissolved inorganic carbon (DIC) concentration), seawater Mg/Ca ratio and the presence of amino acids. We find that Mg2+ and the presence of aspartic acid, glutamic acid, and glycine strongly inhibit ACC precipitation. Moreover, we were unable to precipitate ACC from seawater with a carbonate chemistry within the range of that thought to characterise the calcification site of certain marine calcifiers (i.e. DIC <6 mM, pH <9.3), although substantial modification of the seawater Mg/Ca ratio (Mg/Casw) allowed precipitation at a reduced DIC, with the implication that this could be an important component...