Role of illumination intensity in microcystin development using Microcystis aeruginosa as the model algae

Microcystis aeruginosa (M. aeruginosa) is one of the most common genera of cyanobacteria in algal blooms. In the present work, the impact of the illumination intensity on the growth of M. aeruginosa has been studied and a grinding method for the extraction of intracellular microcystins (MCs) was developed. The variations of algal density, pH, total phosphorus (TP), and total nitrogen (TN) have been investigated during MCs' culturing period. Results showed that the extraction efficiency of MC-YR by the grinding method was 275% higher than the sonication method, and the extraction efficiencies of MC-RR and MC-LR by the grinding method were similar to the sonication method. The optimal illumination intensity for M. aeruginosa was found to be 19-38 μmol m^-2 s^-1 with suitable pH range of 7.5-10.5. Active release of extracellular MCs was not significantly observed when illumination intensities were ≤ 38 μmol m^-2 s^-1. Furthermore, the intracellular MC yields under different illumination intensities were found to be a relatively stable level. However, excess illumination intensity (≥ 47 μmol m^-2 s^-1) led to the lysis of algal cell and increased the concentrations of extracellular MCs, with MC-RR as the dominant compound. The calculated intracellular/extracellular MCs ratios for MC-RR, MC-LR, and MC-YR were 2.38 (N = 100, SD = 2.44), 2.68 (N = 64, SD = 3.48), and 1.25 (N = 30, SD = 1.64), respectively. Strong illumination intensity and cell lysis were found to be the two major factors influencing the release of extracellular MCs.