Biocalcification by the Marine Alga Emiliania huxleyi (Lohmann) Kamptner

Abstract

Emiliania huxleyi (Lohmann) Kamptner, belonging to a group of marine unicellular algae, the Coccolithophoridae, possesses a cell wall containing calcified structures called coccoliths. These coccoliths contain a water‐soluble acid polysaccharide. The polysaccharide contains ester sulphate and uronic acid groups and binds Ca2+ preferentially from a medium also containing Na+ and Mg2+. It is thought to perform a regulatory function in the calcification process. In the present paper we describe a series of studies in vivo intended to give a preliminary characterization of a number of metabolic steps leading to coccolith formation. In view of the putative role of the acid polysaccharide in coccolith synthesis, we have concentrated on the incorporation of polysaccharide precursors by calcifying cells. As a second approach we investigated the differences between calcifying cells and cells of the same species that have lost the capability to produce coccoliths (non‐calcifying cells).Calcifying cells were offered radioactive calcium, bicarbonate, galactose and sulphate in the light. Most of the calcium label and a small part of the label of galactose and bicarbonate was incorporated into extracellular coccoliths. The label of galactose was detected in at least seven of the constituent monosaccharides of the coccolith‐associated polysaccharide. Radioactive sulphate was also incorporated into the extracellular coccolith polysaccharide. In contrast to the label of bicarbonate and galactose that of calcium reached a steady‐state concentration intracellularly within 2 h. A technique was developed for the isolation of intracellular CaCO3, presumably coccoliths in statu nascendi. About 10% of the label of bicarbonate and galactose was incorporated into an intracellular acid polysaccharide, resembling the coccolith‐associated macromolecule. Non‐calcifying cells fail to take up Ca2+ and galactose. We demonstrated that non‐calcifying cells produce an acid polysaccharide, resembling the coccolith‐associated macromolecule. The former polysaccharide is shed into the surrounding medium and can be labelled by offering the non‐calcifying cells radioactive bicarbonate.