MEPS 155:77-88 (1997)  -  doi:10.3354/meps155077

New measurements of phytoplankton aggregation in a flocculator using videography and image analysis

Waite A, Gallager S, Dam HG

Diatom aggregation can increase the sedimentation rate of the intense pulses of carbon formed during diatom blooms. Laboratory studies of aggregation dynamics have been troubled by the unavoidable disruption of aggregates in laboratory flocculators due to subsampling and electronic particle counting. Aggregate disruption prevents accurate measurement of aggregation rate and makes observation of particle structure impossible. This led us to develop a new non-disruptive method to quantify diatom aggregation using high-resolution video and image analysis. The system easily resolved cells of Chaetoceros neogracile 6 to 8 µm in diameter, and facilitated observations of aggregate morphology. C. neogracile made either ball-like or net-like aggregates depending on culture density and physiological state. Only the net-like aggregates reached a substantial size (>1 mm). A Si-limited batch culture experiment indicated that stickiness of C. neogracile increased significantly with increasing severity of nutrient limitation. This increase was not detectable by the subsampling and electronic particle counting method. It is likely that spine formation, cell-surface bound sugars, and transparent exopolymer particles all played a role in determining aggregate structure and changes in stickiness. The stickiness calculation model assumes cultures are initially suspensions of single cells which form doublets as they aggregate. This assumption was tested by comparing mean particle size in a culture with the percentage of cells in aggregates as determined using nearest neighbor distances, allowing us to assess the mean number of cells per aggregate. C. neogracile commonly formed aggregates containing anywhere between 2 and 10 cells showing that model assumptions can be violated under normal experimental conditions. A larger field of view was necessary to estimate the larger particle sizes typical of coastal diatom blooms.

Aggregation · Adhesion · Video · Image analysis · Diatom · Chaetoceros neogracile

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