Chemostats were used to construct nutrient (vitamin B12) and energy budgets for the flagellate Monochrysis (Pavlova lutheri (Droop) Green) under varying conditions of nutrient and light limitation and luxury, the aim being to describe light limitation and luxury in terms analogous to those of the Cell Quota nutrient model. Measurements were made of steady-state dilution rate, biomass, cell quota, cell chlorophyll a, cell energy content, dark respiration and photosynthetic efficiency, under low irradiance (<13 W per m2).
Cell energy content was 0·54 joules per million cells and was not influenced by growth rate or nutrient/light status. Cell chlorophyll a was 25% less in the nutrient-limited cultures and did not show a clear trend with growth rate except in cultures receiving less than 4·3 W per m2. The effective specific extinction coefficient was 0·12 cm2 per μg chlorophyll. The photosynthesis/irradiance relation was linear. The mean slope (= efficiency) in samples from the light-limited chemostats was 0·225 and was independent of dilution rate. Samples from nutrient-limited chemostats showed a much lower slope, which decreased with decreasing dilution rate. These findings were confirmed by the chemostat kinetic data. Dark respiration varied with dilution rate in a hyperbolic manner and was greater in the nutrient- than the light-limited chemostat.
Light level had no effect on the subsistence quota when the nutrient was limiting, whereas under light-limiting conditions the subsistence quotas were enlarged in inverse proportion to the incident irradiance, thus showing a threshold rather than multiplicative interaction between light and the nutrient.
Energy absorption was very little reduced on nutrient limitation, the great reduction in overall efficiency being associated mainly with the reduction in photosynthetic efficiency.
The reciprocal of photosynthetic efficiency was shown to be analogous to a standardized nutrient cell quota, and an analogous coefficient of luxury could be defined as the ratio of the light-limited to nutrient-limited photosynthetic efficiency at the same growth rate. Thus, light limitation and luxury could be described in terms compatible with the nutrient model without having to introduce new concepts, and a practical criterion between light- and nutrient-limitation could be provided.