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The role in CaCO3 crystallization of an acid Ca2+-binding polysaccharide associated with coccoliths of Emiliania huxleyi.

Coccoliths of the unicellular marine alga Emiliania huxleyi are formed intracellularly in a specialized vesicle. Closely associated with the CaCO3 crystals of the coccoliths is an acid Ca2+-binding polysaccharide. The latter is considered to fulfil a regulatory function in CaCO3 crystallization. In this study it is demonstrated that the coccolith polysaccharide is able to inhibit CaCO3 precipitation in vitro. The degree of inhibition is dependent on the nature of the cations bound to the acid groups of the polysaccharide. After substitution of Na+ by Ca2+ ions the polysaccharide is far less effective in inhibiting CaCO3 precipitation. 
 
 
 
The coccolith polysaccharide contains two types of acid groups: uronic acids and sulphate esters. Only the uronic acids are responsible for the inhibition of CaCO3 precipitation. Desulphated polysaccharide inhibits precipitation to the same extent as the native molecule whereas the carboxyl-reduced polysaccharide is unable to block CaCO3 crystallization. 
 
 
 
Inhibition of CaCO3 precipitation can be suppressed by the presence of ethanol. Presumably the conformation of the molecule is altered under these conditions. 
 
 
 
Alginic acid [poly(mannuronic-iduronic acid)] and poly(galacturonic acid) also inhibit CaCO3 precipitation. Inhibition of precipitation is effected by the coccolith polysaccharide and alginic acid at comparable concentrations. The concentration of poly(galacturonic acid) needed to obtain the same result is about 20-times higher. Contrary to the coccolith polysaccharide, both poly(uronic acid)s inhibit CaCO3 precipitation in the absence as well as in the presence of ethanol. These results suggest that conformational changes of the coccolith polysaccharide may play a role in CaCO3 crystallization in vivo.