THE ROLE OF PARTICULATE CARBOHYDRATE EXUDATES IN THE FLOCCULATION OF DIATOM BLOOMS

Diatom blooms are frequently terminated by mass aggregation of cells i nto large, rapidly sinking aggregates. It has been hypothesized that t ransparent exopolymer particles (TEP), abundant particles formed from the polysaccharides exuded by living cells, may be essential for this mass flocculation processes. We investigated the abundance of TEP and their role in the aggregation of diatoms in laboratory cultures and du ring a natural diatom bloom off California. TEP and dissolved carbohyd rates accumulated appreciably over the growth cycle of Chaetoceros gra cilis in the laboratory. The flocculation of C. gracilis in a laborato ry flocculator was dominated by TEP, not cells, and large flocs, consi sting predominantly of particulate polysaccharides, formed at a rate m ore than an order of magnitude higher than predicted by coagulation th eory for cells alone. The frequency of interparticle attachment was th ree orders of magnitude higher for TEP than for cells. The pattern of flocculation of a natural diatom bloom was similar to that of laborato ry cultures. Prior to bloom flocculation the abundance and total quant ity of TEP and the concentration of particulate carbohydrates increase d, while dissolved carbohydrate concentrations decreased. During the f locculation stage TEP aggregated into fewer, but much larger particles and concentrations of dissolved carbohydrates decreased further. The percentage of diatom cells which were attached to TEP increased during the flocculation period from 3 to 90% and TEP formed the matrix of al l the natural diatom aggregates observed. During the late flocculation stage the quantity of TEP and TEP aggregates did not increase further and concentrations of diatoms decreased, presumably because large flo cs sank out. Our findings indicate that TEP should be included in mode ls of particle aggregation in the ocean. The abundance, large size and high sticking coefficient of TEP make them essential to the aggregati on of diatom blooms. The extracellular release of polysaccharides by g rowing cells may be an adaptation for aggregation. The abiotic formati on of particulate organic matter (TEP) from dissolved organic matter ( DOC) may help to explain the extremely high turnover rates of DOC obse rved during blooms.