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.

Coccoliths are delicate calcified structures produced by marine unicellular algae. In the species Emiliania huxleyi the calcium carbonate (mostly calcite) is closely associated with a complex, acidic polysaccharide which binds calcium ions specifically, interferes with the in vitro crystallization of calcium carbonate, and appears to be bound to a positively charged protein before the crystallization process is finished. Ultra-high resolution electron microscopy of the coccoliths reveals that the crystallographic structure differs in different parts of the constituent calcite elements. The synthesis of the coccoliths takes place intracellularly, and when this process is ended the coccoliths are extruded and incorporated into the so-called coccosphere surrounding the cell. Transmission electron microscope studies reveal the localization of polysaccharides in the calcifying organelle by means of cytochemical staining technique. The results are combined in a putative scheme describing coccolithogenesis.

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Coccoliths of Emiliania huxleyi (Lohmann) Hay and Mohler, a unicellular calcifying alga, consist of calcite closely associated with an acidic, Ca2+‐binding polysaccharide. This polysaccharide is thought to play a regulatory role in coccolith synthesis by interfering with CaCO3 crystallization. Here we show that the polysaccharides from three different strains, A 92, L and 92 D, all inhibit the precipitation of CaCO3 in vitro to the same extent. The monosaccharide compositions of the A 92 and L polysaccharide are similar. The 92 D material, however, deviates from the other two: it contains significantly lower amounts of methylated sugars and ribose, and elevated levels of rhamnose and galactose. It also contains antigenic determinants not detected in the A 92 and L polysaccharides. In contrast to the latter two macromolecules the 92 D polysaccharide migrates as two bands upon polyacrylamide gel electrophoresis, possibly resulting from complexing with small amounts of protein. The coccolith polysaccharide from L cells, cultured at an elevated growth rate, also migrates as two bands. This phenomenon is due to an increase in molecular size distribution. The results suggest that certain properties of the molecule may be subject to variation without interfering with its function.

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