RELEASE OF EXTRACELLULAR PRODUCTS BY PHYTOPLANKTON WITH SPECIAL EMPHASIS ON POLYSACCHARIDES

Organic substances are released from phytoplankton cells during all phases of growth. Carbohydrates, especially polysaccharides, sometimes comprise 80-90% of the total extracellular release. Nutrient status profoundly affects the amount and composition of the exudate. Severe N- and P-limitation favours the release of carbohydrates that do not contain these elements. Increase in the medium N/P ratio above the classical Redfield ratio of 16 increases the production of extracellular polysaccharides in laboratory cultures. The type and amount of polysaccharide excreted and the effects of nutrient limitation are often highly species-specific. The production by different species can vary by a factor of up to 50. The rate of release is best specified either as a relative rate of release (PER, percent of total carbon fixation) or as a specific rate. The absolute rate of release for the diatom Chaetoceros affinis was higher in the exponential than in the stationary phase, whereas the PER was 5 times higher in the stationary phase. The mechanisms for excretion in algae are still largely unknown. Both simple and facilitated diffusion may be involved for small molecules and more complex mechanisms for macromolecules. The chemical structures of the extracellular polysaccharides are often species-specific. For example, the marine diatoms C. affinis and C. curvisetus both produce a polysaccharide containing residues of rhamnose, fucose and galactose, but there are major differences in structure. Accumulation of gelatinous masses due to extracellular release by algae was observed in the Adriatic Sea during the summers of 1988-1991. Several species of diatoms and dinoflagellates are implicated, and the jelly is composed of polysaccharides and/or proteoglycans. Blooms of Phaeocystis in the southern part of the North Sea form gelatinous colonies with localised concentrations of up to 7 mg/l. These contain 55-90% of polysaccharidic mucilage.