Diel rhythms of algal phosphate uptake rate in a P-limited cyclostat and their effects on interspecific competition

Abstract

The diel rhythms in the phosphate $(P_i)$ uptake rates for 3 species of green algae were examined in a P-limited cyclostat under a 12:12 h light/dark (L/D) cycle. For Ankistrodesmus convolutus and Chlorella vulgaris, the $P_i$ uptake rates based on the cell number increased during the daytime and decreased at night. In contrast, Chlamydomonas sp. exhibited the opposite uptake pattern. The cell densities also oscillated under a L/D cycle, dividing at a species-specific timing rather than continually dividing. In general, the cell densities exhibited an inverse relationship with the $P_i$ uptake rates. There were no allelopathic effects between species except for Chlamydomonas sp. on A. convolutus, reducing its growth about 30%. A competition experiment between A. convolutus and C. vulgaris in a P-limited cyclostat resulted in the dominance of C. vulgaris, regardless of the relative initial cell densities. C. vulgaris also dominated in a mixed culture with Chlamydomonas sp., irrespective of the initial seeding ratio and dilution rate. However, Chlamydomonas sp. and A. convolutus coexisted in the competition experiment with gradual decrease of Chlamydomonas sp., when equally inoculated. Mathematical expressions of the oscillations in the $P_i$ uptake rate were used to develop two types of competition models. The first model was based on Lotka-Volterra model and competition coefficients were calculated from the oscillation equations of $P_i$ uptake rates. The experimental results conformed to the model predictions. The second simulation model was based on Droop model, modified to reflect the effects of diel rhythms in $P_i$ uptake rates and cell division gates. The simulation results for each of the single species conformed reasonably well to the experimental data. The results of the competition experiments also matched the predictions of competition simulation, although the experimental competition processes were generally more delayed than the competition simulat...