Modeling Carbon Cell Quota in Light-limited Phytoplankton

The carbon cell quota of light-limited microalgae often increases with average photon flux density. Models of light-limited microalgal growth do not account for this phenomenon, however. In this paper we present a model of the carbon budget of phytoplankton, which accounts for the variation in carbon cell quota with photon flux density. We distinguish two components in the cell, permanent biovolume and a transient carbon pool. Furthermore, we distinguish three processes: assimilation, maintenance and growth. Assimilation acts to increase the transient carbon pool, whereas maintenance and growth both act to decrease it. Maintenance represents a sink of carbon; in contrast, growth leads to an increase of permanent biovolume. We derive equations describing the carbon cell quota, the specific growth rate, photosynthesis and dark respiration. The model accurately describes data on the complete carbon budget of three widely different phytoplankton species. We show that the model is a generalization of the well-known Droop model for nutrient limited growth. This result indicates that light and nutrient limited growth may be less dissimilar than is generally thought.