Paraquat induces different programmed cell death patterns in Microcystis aeruginosa and Chlorella luteoviridis

Although programmed cell death (PCD) has been reported in phytoplankton, knowledge of the characterization of the PCD pathway and cascade process in different phytoplankton species is still limited. In this study, PCD progression in cyanobacterium Microcystis aeruginosa and green algae Chlorella luteoviridis by paraquat-induced oxidative stress was monitored. The results showed that paraquat-induced PCD in the two species belonged to the caspase-dependent pathway. Dose- and time-dependent PCD characteristics in the two strains under paraquat included the increase in caspase-like activity, DNA fragmentation, and chromatin condensation. However, the signaling pathway and cascade events of PCD in M. aeruginosa and C. luteoviridis differed. In M. aeruginosa, the free Ca2+ concentration was rapidly increased at 8 h, followed by a significant elevation of the reactive oxygen species (ROS) level at 24 h, and eventual cell death. In C. luteoviridis, the mitochondrial apoptosis pathway, revealed by the depolarization of the mitochondrial membrane potential at 1 h and increase in the ROS level and caspase-like activity at 8 h, might contribute to cell death. In addition, the dynamics of ROS levels and metacaspase activity were synchronized, suggesting that paraquat-triggered PCD was ROS-mediated in both M. aeruginosa and C. luteoviridis. These results provide insights into PCD patterns in prokaryotic cyanobacteria and eukaryotic green algae under similar stress.

Automated summary

This study found that programmed cell death (PCD) in cyanobacterium Microcystis aeruginosa and green algae Chlorella luteoviridis induced by paraquat involved the caspase-dependent pathway. However, the signaling pathway and cascade events of PCD differed between the two species. In M. aeruginosa, cell death was triggered by a rapid increase in free Ca2+ concentration followed by elevated reactive oxygen species (ROS) levels, while in C. luteoviridis, cell death was mediated by the mitochondrial apoptosis pathway along with increased ROS levels and caspase-like activity. Furthermore, the levels of ROS and metacaspase activity were synchronized in both species, implying that paraquat-induced PCD was ROS-mediated.