Hydrogen peroxide induces apoptotic-like cell death in Microcystis aeruginosa (Chroococcales, Cyanobacteria) in a dose-dependent manner

Abstract

We investigated the capability of Microcystis aeruginosa to cause apoptosis by pursuing morphological, molecular and physiological characteristics after exposure to H2O2. Microcystis proliferation was only weakly affected after exposure to 150 mu M H2O2 but cell numbers decreased dramatically after exposures of 250 and 325 mu M H2O2. Cells exposed to 250 and 325 mu M H2O2 were examined using transmission electron microscopy, and they exhibited membrane deformation and partial disintegration of thylakoids. Correspondingly, fluorescence imaging of DNA by Hoechst 33342 staining revealed the condensation of nucleoid chromatin. Moreover, cellular injury was concomitant with dramatic decreases in photosynthetic efficiency (ratio of variable fluorescence to maximum fluorescence [Fv/Fm], maximum electron transport rate [ETRmax]) and elevated caspase-3-like activity after exposure of 250 and 325 mu M H2O2. Terminal deoxynucleotidyl transferase Deoxyuridine 5-triphosphate nick end labelling (TUNEL) positive staining appeared in cells exposed to 250 mu M and 325 mu M H2O2, and the percentage staining increased with increasing H2O2 concentration. These data suggested that M. aeruginosa exposed to H2O2 underwent an apoptotic event. Additionally, cells exposed to H2O2 had increased cytoplasmic vacuolation and nontypical DNA laddering. Increased caspase-3-like activity was not inhibited in the presence of the synthetic caspase inhibitor carbobenzoxy-valyl-alanyl-aspartyl-[O-methyl]-fluoromethylketone. Therefore, H2O2 induced apoptotic-like cell death in a dose-dependent manner. Taken together, our results provided a novel mechanism for explaining cyanobacterial bloom dynamics in response to environmental stress. The results also contributed to the understanding of the origin and evolution of programmed cell death.We investigated the capability of Microcystis aeruginosa to cause apoptosis by pursuing morphological, molecular and physiological characteristics after exposure to H2O2. Microcystis proliferation was only weakly affected after exposure to 150 mu M H2O2 but cell numbers decreased dramatically after exposures of 250 and 325 mu M H2O2. Cells exposed to 250 and 325 mu M H2O2 were examined using transmission electron microscopy, and they exhibited membrane deformation and partial disintegration of thylakoids. Correspondingly, fluorescence imaging of DNA by Hoechst 33342 staining revealed the condensation of nucleoid chromatin. Moreover, cellular injury was concomitant with dramatic decreases in photosynthetic efficiency (ratio of variable fluorescence to maximum fluorescence [Fv/Fm], maximum electron transport rate [ETRmax]) and elevated caspase-3-like activity after exposure of 250 and 325 mu M H2O2. Terminal deoxynucleotidyl transferase Deoxyuridine 5-triphosphate nick end labelling (TUNEL) positive staining appeared in cells exposed to 250 mu M and 325 mu M H2O2, and the percentage staining increased with increasing H2O2 concentration. These data suggested that M. aeruginosa exposed to H2O2 underwent an apoptotic event. Additionally, cells exposed to H2O2 had increased cytoplasmic vacuolation and nontypical DNA laddering. Increased caspase-3-like activity was not inhibited in the presence of the synthetic caspase inhibitor carbobenzoxy-valyl-alanyl-aspartyl-[O-methyl]-fluoromethylketone. Therefore, H2O2 induced apoptotic-like cell death in a dose-dependent manner. Taken together, our results provided a novel mechanism for explaining cyanobacterial bloom dynamics in response to environmental stress. The results also contributed to the understanding of the origin and evolution of programmed cell death