In both field samples and cultures the total amount of sugar and
of
carbon in colonies of Phaeocystis globosa was correlated with
colony
surface area, suggesting a hollow structure. A conceptual model based on
biochemical data and on the assumption that the mucus occurs
as a layer of a fixed thickness, irrespective of colony size, predicts
that
the thickness of the layer is 7 μm. A confocal laser scanning
microscope image of fluorescently labelled mucus confirmed this view of
the
colony structure. The measured contents of carbon and sugar
per cell (including mucus) were constant for all colony sizes. Cells in
laboratory cultures contained 122 pg C, which is twice the value for
cells obtained 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. Therefore the percentage of sugar carbon relative to total
carbon of colonies in the field was higher (19–35%) than that in
cultures (10%). Highest values were found at low ambient nutrient
concentrations, probably due to the presence of storage glucans in the
cells that are produced under nutrient limitation. The functional role
of
Phaeocystis colonies in the pelagic system has to be re-evaluated
because (i) biomass estimates have previously often been based on the
assumption of a colony 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 that only a small amount of carbohydrate is
needed to build the colony structure.