A comparative study of the size-dependent organic composition of marine diatoms and dinoflagellates

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Abstract
Cellular chlorophyll a, protein, carbohydrate and lipid content was determined for eleven clones of centric marine diatoms (volume 89–1.47 × 107 μ3) and eight species of marine dinoflagellates (597–4.45 × 104 μ3) cultured under continuous illumination at 18°C and 20°C, respectively. In both groups the log of cellular concentrations of each constituent was directly related to the log of cell volume; diatoms generally had lower cellular concentrations than dinoflagellates of an equivalent volume. Diatom chlorophyll a, protein and lipid concentrations normalized to a unit cell volume (pg μ−3) decreased exponentially with increasing cell size; this decrease is a consequence of the diatoms' unique morphology restricting cell cytoplasm to a thin parietal layer within the frustule. Although dinoflagellates yield a wide range of cytoplasm concentrations, small dinoflagellates contained up to 3-fold higher cytoplasm concentrations of all constituents than diatoms of equal volume. The log of cellular caloric values, summed from the caloric equivalents of cellular protein, carbohydrate and lipid, was a linear function of log volume. Diatoms contained ca. half the caloric value of dinoflagellates of an equivalent volume. Although the evaluation of caloric content provides a basis for comparing the “nutritional value” of phytoplankton groups, evidence from the literature suggests subjective factors such as taste and digestibility are equally important in determining nutritional values of individual species.

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