Seasonal and diet changes in ultrastructure in the dinoflagellate Ceratium hirundinella

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Abstract
The ultrastructure of the dinoflagellate Ceratium hirundinella (O.F.Mull.) Bergh. was studied in the plankton of a small productive lake throughout one season of its growth and during one 24-h period of that season. The results were related to measured environmental variables. Samples for the seasonal study were taken every 4 weeks and fixed immediately upon removal from the lake and samples for the diel study were fixed in situ at depth. There was little difference in general ultra-structure of cells during the diel study or during the period of rapid growth but greater changes were apparent following excystment and prior to encystment. Most changes were observed within the chloroplasts, pyrenoids and storage products. Changes in chloroplast structure were shown to be related to the light conditions being experienced by the cells. Starch and lipid occurred in varying proportions depending on the stage of the life cycle. Starch and associated pyrenoid clusters varied with irradiance level over the 24-h period.