The role of surface‐active carbohydrates in the formation of transparent exopolymer particles by bubble adsorption of seawater

Abstract

Experiments were performed to examine the role of surface‐active polysaccharides in the formation of transparent exopolymer particles (TEP) by bubble adsorption in seawater. Filtered (1.0 µm, 0.45 µm) and ultrafiltered (0.1 µm, 100 kDa, and 10 kDa) seawater samples were bubbled in a glass foam tower. The neutral sugar composition, concentration of TEP, and concentration of surface‐active carbohydrates in generated foam samples were determined. Three different surface seawater samples (Monterey Bay, California; Shannon Point, Anacortes, Washington; and East Sound, Orcas Island, Washington) were used in the experiments. Significant concentrations of pre‐existing and new TEP were extracted into foam by bubble adsorption. Newly generated TEP accounted for 28–52% of the TEP collected in the foam samples by bubbling 1.0‐µm‐filtered samples. Neutral sugar composition analyses of foam samples derived as a function of bubbling time indicated that two types of surface‐active carbohydrates were extracted by bubble adsorption: highly surface‐active carbohydrates, which were extracted initially, and less surfaceactive carbohydrates. As in our previous mesocosm study (Mopper et al. 1995), highly surface‐active polysaccharides were enriched in deoxysugars (fucose and rhamnose), whereas the less surface‐active polysaccharides and residual (bubble‐stripped) water were glucose rich. In addition, the highly surface‐active fraction was strongly enriched in covalently bound sulfate. The concentrations of TEP and surface‐active carbohydrates that were extracted into the foam both decreased sharply with decreasing filter size used to filter samples prior to bubbling, in agreement with theoretical considerations. The results of this study suggest that bubble adsorption of sulfate‐rich surface‐active carbohydrates is an important pathway for the formation of TEP in surface waters, especially during algal blooms.