THE ROLE OF SURFACE-ACTIVE CARBOHYDRATES IN THE FLOCCULATION OF A DIATOM BLOOM IN A MESOCOSM

A study was undertaken to evaluate the role of exocellular polysaccharides in the flocculation of a marine diatom bloom in a large tank mesocosm. Surface-active organic matter was extracted from 1.0 mu m-filtered tank water by bubble adsorption each day for 7 days of the experiment. In agreement with past studies, particles (3-51 mu m equivalent spherical diameter) were readily formed by bubbling and became concentrated in the foam. At the beginning of the bubbling (0-0.5 h), both particles and surface-active carbohydrates were extracted at high rates; however, these rates dropped off steeply after about 0.5 h of bubbling. The rate of particle formation by bubbling could be modeled fairly well by second order kinetics. The extracted, surface-active material was enriched in deoxysugars and galactose, while the residual material was enriched in glucose. Extracted surface-active carbohydrates reached a maximum of 33% of the total dissolved sugars (<1.0 mu m) by the end of the log growth phase. The concentration of extracted surface-active carbohydrates, when normalized to chlorophyll, was strongly correlated (r(2) > 0.99) with particle stickiness (alpha). In addition, the concentration of surface-active carbohydrates was well correlated (r(2) = 0.91) with the concentration of transparent exopolymer particles (TEP) in the tank, and it was demonstrated that TEP could be copiously formed by bubbling of 1.0 mu m-filtered seawater. The finding of a highly surface-active, deoxysugar-rich polysaccharide male;rial that can be rapidly (<0.5 h) and selectively extracted by bubble adsorption is significant, as it is apparent that this material played important roles in particle stickiness and TEP formation in the tank, and thus it may, at times, play similar roles in particle aggregation in the sea.