The role of particulate carbohydrate exudates in the flocculation of diatom blooms

Diatom blooms are frequently terminated by mass aggregation of cells into large, rapidly sinking aggregates. It has been hypothesized that transparent 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 during a natural diatom bloom off California. TEP and dissolved carbohydrates accumulated appreciably over the growth cycle of Chaetoceros gracilis in the laboratory. The flocculation of C. gracilis in a laboratory flocculator was dominated by TEP, not cells, and large flocs, consisting predominantly of particulate polysaccharides, formed at a rate more than an order of magnitude higher than predicted by coagulation theory for cells alone. The frequency of interparticle attachment was three orders of magnitude higher for TEP than for cells. The pattern of flocculation of a natural diatom bloom was similar to that of laboratory cultures. Prior to bloom flocculation the abundance and total quantity of TEP and the concentration of particulate carbohydrates increased, while dissolved carbohydrate concentrations decreased. During the flocculation 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 all 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 flocs sank out. Our findings indicate that TEP should be included in models of particle aggregation in the ocean. The abundance, large size and high sticking coefficient of TEP make them essential to the aggregation of diatom blooms. The extracellular release of polysaccharides by growing cells may be an adaptation for aggregation. The abiotic formation of particulate organic matter (TEP) from dissolved organic matter (DOC) may help to explain the extremely high turnover rates of DOC observed during blooms.