Intracellular chlorophyll a content is one of the many measurable parameters which displays a diel rhythm in marine phytoplankton. In asynchronous laboratory cultures of the diatom Skeletonema costatum, cellular chlorophylls a and c exhibit periodicities which closely follow the light-dark cycle and are not the result of cell division. The laboratory cultures also exhibit diel rhythms in cellular flourescence properties and carbon: chlorophyll a ratios. The occurrence of similar patterns of cellular flourescence, carbon: chlorophyll a ratios, and in situ flourescence in diatom-dominated natural phytoplankton communities suggests the possibility of diel rhythms in cellular chlorophyll a content in diatoms in the sea. The data also suggest that the observed periodicity in cellular chlorophyll content is regulated by the diel light cycle and that the co-occurrence of synchronous or phased cell division would only modify the observed periodicity.
In~oducfion
A major problem in understanding phytoplanktonic processes, interpreting field data, and building mathematical models to predict phytoplankton response is the numerous diel rhythms which have been detected in both laboratory cultures and natural phytoplankton communities. Diel periodicity has been reported in phytosynthesis-irradiance curves (Prezelin