PHAEOCYSTIS GLOBOSA (PRYMNESIOPHYCEAE) COLONIES - HOLLOW STRUCTURES BUILT WITH SMALL AMOUNTS OF POLYSACCHARIDES

In both field samples and cultures the total amount of sugar and of ca rbon in colonies of Phaeocystis globosa was correlated with colony sur face area, suggesting a hollow structure. A conceptual model based on biochemical data and on the assumption that the mucus occurs as a laye r of a Bred thickness, irrespective of colony size, predicts that the thickness of the layer is 7 mu m. A confocal laser scanning microscope image of fluorescently labelled mucus confirmed this view of the colo ny structure. The measured contents of carbon and sugar per cell (incl uding mucus) were constant for all colony sizes. Cells in laboratory c ultures contained 122 pg C, which is twice the value for cells obtaine d at a field station in the North Sea (57 pg). In contrast, sugar per cell was higher in the field than in exponentially growing cultures. T herefore the percentage of sugar carbon relative to total carbon of co lonies in the field was higher (19-35%) than that in cultures (10%). H ighest values were found at low ambient nutrient concentrations, proba bly due to the presence of storage glucans in the cells that are produ ced under nutrient limitation. The functional role of Phaeocystis colo nies in the pelagic system has to be re-evaluated because (i) biomass estimates have previously often been based on the assumption of a colo ny filled with mucus while in reality it is a hollow structure with an aqueous lumen, and (ii) in contrast to earlier reports it is shown th at only a small amount of carbohydrate is needed to build the colony s tructure.