Study on Microcystis aeruginosa growth in incubator experiments by combination of Logistic and Monod functions

A combination of Logistic and Monod functions was proposed in this paper to study Microcystis aeruginosa growth in incubator experiments. This enables the Microcystis aeruginosa growth dynamics to be better described in incubator experiments and its parameters to be calculated more accurately. This method was justified by the data from the experiment and applied to study the effect of prometryn on Microcystis aeruginosa growth. In the experiment, a different concentrations of prometryn (0, 50, 100 and 200 μg·L−1) were added to the culture medium; the algal cell density, concentrations of orthophosphate (PO43−-P) and ammonia nitrogen (NH4+-N) were measured. The results show that Microcystis aeruginosa growth with time can be well described using the Logistic function. The maximum algae densities of Microcystis aeruginosa corresponding to the four prometryn concentrations are 11.7 × 106, 8.1 × 106, 5.6 × 106 and 3.0 × 106 cells·mL−1, respectively. The derived formula for the specific growth rate, growth rate and inhibition rate using Logistic function agreed reasonably well with the measured data. It was found that variations of consumed nutrients concentrations (PO43−-P and NH4+-N) can also be well described by the Logistic function. A function that describes the relationship between algal densities and consumed nutrient (PO43−-P and NH4+-N) concentrations is also derived from the Logistic function. Combination of Monod and Logistic functions can better describe relationship between specific growth rates and nutrients concentrations compared to the use of Monod function alone. In general, the half saturation coefficient, Kc for PO43−-P (4.74 × 10−4, 1.99 × 10−3, 5.54 × 10−3 and 3.87 × 10−2 mg·L−1) and Kc for NH4+-N (1.80 × 10−3, 5.84 × 10−3, 5.23 × 10−3 and 1.06 × 10−2 mg·L−1) in Monod function increase with increasing prometryn concentrations, which indicates that the affinity of algae growth to PO43−-P and NH4+-N decrease with increasing prometryn concentrations. In addition, relationships between nutrients concentrations and time can be derived by combining of Monod and Logistic functions, which agree well with the measured data. It is concluded that the combined application of Monod and Logistic functions provides a promising and more robust method of studying algal growth in incubator experiments.