Effects of Light Intensity on Growth, Microcystin Production, and Release of Two Microcystis spp.

The impact of light intensity on microcystin (MC) production and release of different toxic strains remains unclear. This study examined two toxic strains of Microcystis aeruginosa (FACHB-1203 and FACHB-1322) under light intensities of 35, 70, and 105 μmol photons/m 2 ·s to assess growth, MC synthesis and release, and the transcription of mcyB and mcyH. Results showed that both strains achieved the highest cell density at 70 μmol photons/m 2 ·s at the end of cultivation. Intracellular total MC (TMC, sum of MC-LR, MC-YR, and MC-RR) concentration decreased with increasing light intensity. MC release rates increased under high light intensity for both strains, although extracellular TMC concentrations responded differently to light intensity. An increase in light intensity from 70 to 105 μmol photons/m 2 ·s shifted intracellular MC congeners from MC-RR and MC-YR toward the more toxic MC-LR for both strains and enhanced MC-LR and MC-YR release. A decrease in light intensity from 70 to 35 μmol photons/m 2 ·s shifted extracellular MC-RR toward MC-LR. Correlation analyses suggested that the decreased intracellular TMCs under high light intensity resulted from an increased MC release rate, driven by the increased transcription of mcyB and mcyH. These results enhance the understanding of the adaptive strategies employed by toxic cyanobacteria under fluctuating light intensities.