Effect of phosphorus stress on Microcystis aeruginosa growth and phosphorus uptake

Abstract

This study was designed to advance understanding of phosphorus regulation of Microcystis aeruginosa growth, phosphorus uptake and storage in changing phosphorus (P) conditions as would occur in lakes. We hypothesized that Microcystis growth and nutrient uptake would fit classic models by Monod, Droop, and Michaelis-Menten in these changing conditions. Microcystis grown in luxury nutrient concentrations was transferred to treatments with phosphorus concentrations ranging from 0-256 μg P∙L-1 and luxury nitrogen. Dissolved phosphorus concentration, cell phosphorus quota, P uptake rate and cell densities were measured at day 3 and 6. Results showed little relationship to predicted models. Microcystis growth was asymptotically related to P treatment from day 0-3, fitting Monod model well, but negatively related to P treatment and cell quota from day 3-6. From day 0-3, cell quota was negatively related to P treatments at <2 μg∙L-1, but increased slightly at higher P. Cell quota decreased greatly in low P treatments from day 3-6, which may have enabled high growths in low P treatments. P uptake was positively and linearly related to P treatment during both periods. Negative uptake rates and increases in measured culture phosphorus concentrations to 5 μg∙L-1 in the lowest P treatments indicated P leaked from cells into culture medium. This leakage during early stages of the experiment may have been sufficient to stimulate metabolism and use of intracellular P stores in low P treatments for rapid growth. Our study shows P regulation of Microcystis growth can be complex as a result of changing P concentrations, and this complexity may be important for modeling Microcystis for nutrient and ecosystem management.

Fig 1

Phosphorus concentrations in culture media in each of the phosphorus treatments on day 3 (a) and on day 6 (b).

Fig 2

Microcystis growth rates in each of the phosphorus treatments for the day 0–3 period (a) and day 3–6 period (b). The asymptotic line in Fig 3A is the best fit to the Monod model. The linear line in Fig 2b is the best model for the day 3–6 relationship.

Fig 3. Microcystis growth rates related to cell quota.

Growth rates for day 0–3 versus cell quota on day 3 are plotted in Fig 3a. Growth rates for day 3–6 are related to cell quota on day 6 in Fig 3b. The linear models in both figures were the best model and fit for these relationships. (Note the difference in Y-axis.)

Fig 4. The relationships between cell quota and phosphorus treatment.

Fig 4a and 4b show the results of piecewise linear regression for cell quota on day 3 and day 6, respectively. (Note the difference in Y-axis.)

Fig 5

The relationship between phosphorus uptake rates and phosphorus treatment for day 0–3 (a) and for day 3–6 (b). Linear models which were the best fit to the relationships. (Note the difference in Y-axis.)