Effect of nitrogen and phosphorus on growth and microcystin production in three Microcystis species

Aim : The effects of nitrogen and phosphorus concentration on growth and microcystin production were investigated in three species of bloom-forming Microcystis isolated from two South Korean freshwater systems. Methodology : Three species of cyanobacteria were collected from Yeongchun Dam and Ankei Dam in Kyungpook Province, South Korea. Culture experiments were conducted at 25 +/- 1 degrees C under cool white fluorescent light (ca. 50 mu mol m(-2) s(-1)) in media with different concentrations of nitrogen (0 to 20 mg l(-1)) and phosphorus (0 to 5 mg l(-1)). Cell numbers were determined in a hemocytometer for calculation of growth rate. Microcystin was analysed using high pressure liquid chromatography. Results : The highest growth rate (mu(max)) and maximal microcystin production occurred at nitrogen concentrations of 10 and 20 mg l(-1) in all three species. The response to phosphorus concentration was more complex. The highest growth rate (mu(max)) of M. aeruginosa, M. ichthyobiabe and M. viridis occurred at phosphorus concentrations of 0.5 mg l(-1), 0.1 mg l(-1), and 3 mg l(-1), respectively. M.aeruginosa also had maximal microcystin production at 0.5 mg l(-1 )P. In contrast, Michthyoblabe and U vindis had high microcystin production at 0 mg l(-1) and at 5 mg l(-1) P (the highest tested concentration), and low microcystin production at 0.1 mg l(-1) P. Thus at 0.1 mg l(-1), Mdchthyoblabe had the highestgrowth rate but produced least amount of microcystin. The types of microcystins produced varied according to species and nutrient conditions. Interpretation : Microcystin production and growth in Microcystis species isolated from South Korea varied according to species and nutrient conditions. These species responded similarly to different nitrogen concentrations. but differently to different phosphorus concentrations.