DIFFERENTIAL AGGREGATION OF DIATOMS

The relative species composition of diatom flocs formed in the presence of 2 diatom species was studied in a series of laboratory experiments. The aim of this study was to investigate whether species composition in flocs reflected initial species composition or whether certain species were enriched in aggregates. In each experiment, 2 species with differing aggregation behavior were combined and aggregated. These experiments elucidate several mechanisms controlling aggregation of diatoms. Cells of Nitzschia angularis aggregated preferentially due to their high cell stickiness, which is a function of cell surface properties. Species such as Chaetoceros gracilis cause aggregation indirectly, by generating transparent exopolymer particles (TEP) which, in turn, scavenge cells into aggregates. Thalassiosira weissflogii did not form flocs itself, but was included in flocs formed by other species. Differential aggregation may lead to species succession during the bloom, and to sequential sedimentation. Flocculation behavior Like that initiated by C. gracilis will most Likely result in 1 mass sedimentation pulse, scouring of the water column and the end of the bloom.