Experimental and Theoretical Analyses of the Variation in Phosphorus Cell Quota of Phytoplankton.

In order to understand the mechanisms involved in the variation of phosphorus cell quota of phytoplankton during a batch culture system, Chaetoceros sp. was cultured and the variations of cell number, dissolved inorganic phosphorus and particulate phosphorus in the medium were monitored during the course of the experiment. A mathematical simulation model based on simple conventional equations, such as Michaelis-Menten's equation and Droop's equation, indicated inadequate fitness of the experimental data. The model was improved by incorporating 3 possible physiological processes; (i) the luxury uptake of phosphate, (ii) the lag time between growth and uptake, and (iii) the excretion of cellular organic phosphorus. The improved model explained the experimental data satisfactorily, and clarified the factors governing the variation of phosphorus cell quota as follows. (1) The phosphorus cell quota increase due to the extraordinary phosphate uptake during the luxury uptake period. (2) The phosphorus cell quota decreased with the intracellular consumption due to the accelerated growth that depends on the increased cell quota during the Michaelis-Menten period. (3) There was a balance between the uptake rate of phosphorus and the decreasing rate of phosphorus cell quota by the use for the cell division in the late Michaelis-Menten period. (4) The excretion of dissolved organic phosphorus contributed to ca. 50% of the decrease of cellular phosphorus in the late Michaelis-Menten period.