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.
Highlights
Freshwater harmful algal blooms, often dominated by cyanobacteria, are speculated to increase with climate change and are expected to decrease the potable and recreational uses of water [1,2]
This study aims to advance understanding of Microcystis growth and phosphorus uptake kinetics in changing phosphorus concentrations by employing and evaluating multiple modeling approaches
The subculture for seeding was filtered under very low suction and carefully washed with half strength WC medium having no phosphorus to minimize carrying-over of surface adsorbed phosphorus into the flasks. This approach of washing and transfer to experimental flasks has been used by Tsukada et al [16], and we observed no ill effects of washing during the first 3 days of our experiment when Microcystis growth rates increased asymptotically with phosphorus treatment, as expected
Summary
Freshwater harmful algal blooms, often dominated by cyanobacteria, are speculated to increase with climate change and are expected to decrease the potable and recreational uses of water [1,2]. Microcystis aeruginosa (referred to as “Microcystis” in this paper for convenience) is a common constituent of harmful cyanobacterial blooms [3,4,5].
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