The Role in CaCO₃ Crystallization of an Acid Ca²⁺‐Binding Polysaccharide Associated with Coccoliths of Emiliania huxleyi

Abstract

Coccoliths of the unicellular marine alga Emiliania Izuxleyi are formed intracellularly in a specialized vesicle. Closely associated with the CaC0, crystals of the coccoliths is an acid CaZ +-binding polysaccharide. The latter is considered to fulfil a regulatory function in CaC03 crystallization. In this study it is demonstrated that the coccolith polysaccharide is able to inhibit CaCO, 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 CaC0, 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 CaCO, precipitation. Desulphated polysaccharide inhibits precipitation to the same extent as the native molecule whereas the carboxyl-reduced polysaccharide is unable to block CaCO, crystallization.Inhibition of CaC0, 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(ga1acturonic acid) also inhibit CaC0, precipitation. Inhibition of precipitation is effected by the coccolith polysaccharide and alginic acid at comparable concentrations. The concentration of poly(ga1acturonic acid) needed to obtain the same result is about 20-times higher. Contrary to the coccolith polysaccharide, both poly(uronic acid)s inhibit CaC0, 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 CaCO, crystallization in vivo.

Polysaccharide from coccoliths (CaCO3 biomineral)

“…The driving force is increased, due to a decrease of solubility. Contrary to the suggestion of Borman et al [12], conformational changes in the polysaccharide are not responsible for the change in growth inhibition. In the experimental procedure used by Borman et al [12] inhibition is measured as the increase in induction time t, i.e.…”

“…The latter parameter is lowered to 40.4 min upon addition of 5% ethanol. However, as [tm] values for the control experiments range between 36.1 min and 77.5 min, this lowering of [tJ by ethanol is not significant, and the previously described effect of ethanol [12] may be ascribed to a direct stimulation of CaC03 precipitation. The driving force is increased, due to a decrease of solubility.…”

“…One pool of calcium is very firmly bound to sulphate esters and resists even EDTA treatment [12]. A second pool of calcium is bound to uronic acids and can be exchanged with the Ca2+ in the solution.…”

“…In an article by Borman et al [12], it was noted that the inhibitory effect of coccolith polysaccharide on the precipitation of CaC03 was completely suppressed by addition of 5% ethanol. In our system, using calcium oxalate monohydrate seed crystals, addition of 5% ethanol did not significantly alter the inhibition of crystal growth or the stimulation of agglomeration by the polysaccharide (see Table 1).…”

“…The side chain contains several neutral sugars and two types of acidic groups: uronic acids and sulphate esters [lo, 111. One polysaccharide molecule contains approximately 60 uronic acid groups; these are capable of binding Ca2* preferentially from a medium also containing Na' and Mg2+ [9,12]. It has been shown that the polysaccharide inhibits CaCOJ precipitation [12].…”