The invariance of macromolecular composition with altered light limited growth rate of Amphidinium carteri (dinophyceae)

The effect of irradiance on the growth rate, macromolecular composition and photosynthetic carbon metabolism of Amphidinium carteri was studied in batch culture. Growth rate increased linearly with increasing irradiance up to a maximum growth rate of 0.04 h-1 at an irradiance of 80 μEm-2s-1. In contrast to a number of other studies on both prokaryotic and eukaryotic microorganisms, ours showed that cellular content of RNA, DNA, protein and carbohydrate of A. carteri were invariant with growth rate over the range μ=0.04 to 0.007 h-1. The invariant macromolecular composition was correlated with a constant modal cell volume. Chlorophyll and lipid per cell increased with decreasing irradiance. The distribution of [14C]-bicarbonate in the major end products of photosynthesis after incubation with isotope for 14% of a doubling time showed that the percentage carbon in the chloroform (lipids and pigments) fraction increased with decreasing irradiance while that of the trichloroacetic acid soluble (carbohydrate) fractions decreased. The percentage of isotope in the trichloroacetic acid insoluble (protein) fraction and methanol: water fraction (metabolites) remained constant. Because this species, under light-limited growth, differs from other organisms so far studied, more species must be similarly examined before nucleic acid content is used as an index growth rate in the field.