Role of hydrophobic extracellular polysaccharide of Aulacoseira granulata (Bacillariophyceae) on aggregate formation in a turbulent and hypereutrophic reservoir

The highest density of aggregates (1.4 x 10(3) L-1) in the hypereutrophic Barra Bonita Reservoir (mean depth of 10.2 m) coincided with the peak population density of the diatom Aulacoseira granulata (2.3 x 10(6) chains L-1), which was present in 94% of the aggregates for 22 consecutive months. To investigate why this diatom, which produces very small amounts of free extracellular polysaccharide (EPS) and transparent exopolymer particles (TEP), occurs so frequently in natural aggregates, we carried out aggregation experiments with cultures of A. granulata with and without its free EPS and TEP. Stickiness (alpha) was higher in exponential than stationary growth, either with or without EPS, but the highest value was obtained in cultures without EPS (0.37). During the stationary growth phase, alpha decreased, both with and without EPS. Replacement of natural polymer carbohydrates by A. granulata free EPS (>14 kiloDalton [kD] < 0.4 mu m), or adding cultured diatom to reservoir water, produced aggregates larger than the natural ones. The role of EPS in these aggregation characteristics was investigated by analyzing its monosaccharide composition. The proportion of terminal monosaccharides significant in the aggregation was 74.5%, 56.1% of them consisting of hydrophobic deoxy-sugars fucose and rhamnose which increase the stickiness. Early in the growth, only hydrophobic-adhering films are responsible for fast aggregation by chain-to-chain collision. With aging cells, aggregation was mainly by the trapping effect of free EPS and TEP, both formed by released adherent films. We hypothesize that aggregates have an important role in seeding A. granulata in environments with significant advective transport, such as reservoirs.