XVI International Botanical Congess

The phytoplankton assemblage in Lake Kinneret is dominated by a seasonal Peridinium gatunense bloom, which terminates in a sudden collapse. Intracellular generation of reactive oxygen species (ROS) was found at the end of the bloom and in batch cultures. Production of ROS depended on CO2 concentration. Catalase blocked cell death. A spike of H2O2 produced an oxidative burst and cell death. H2O2 concentration required to induce cell death was 20-fold higher in young cultures, but addition of older medium increased their sensitivity, suggesting production of a conditioning factor. Cell death was accompanied by protoplast shrinkage and DNA fragmentation, and was blocked by a cysteine protease inhibitor, which enhanced cyst formation. In summary, CO2 limitation initiates a cascade leading to ROS generation and terminating in programmed cell death.