XVI International Botanical Congess
The phytoplankton assemblage in Lake Kinneret is dominated by the spring bloom of P. gatunense which terminates in a sudden collapse. Chemical monitoring of the lake did not show significant depletion of phosphate or nitrogen. Measurements of pH and CO2 within patches of P. gatunense indicated a decrease in dissolved CO2 during the bloom, concomitant with a rise in antioxidant enzyme activity. Samples taken during different times of day indicated development of oxidative stress in the cells within the entire water column. A similar increase in production of reactive oxygen species (ROS) was detected in batch cultures of P. gatunense during the culture growth. The levels of ROS directly depended on the concentration of CO2. The rise in ROS decreased CO2 fixation, resulting in amplification of the oxidative stress to a point that triggered cell death. Scavenging of H2O2 by catalase prevented the accumulation of ROS and blocked cell death. On the other hand, an H2O2 spike created an oxidative burst 6-12 hours later, followed by cell death. The threshold concentration of H2O2 required to induce cell death was 20-fold higher in a 2 weeks old than in a 10 weeks old culture. Incubation of the young cells in a replenished medium from the older culture reduced the threshold H2O2 concentration required to induce cell death, suggesting production of a conditioning factor in the older culture. Morphological observations during cell death showed a progressive shrinkage of the ROS containing protoplasts and appearance of nuclear fragmentation. Formation of ROS and cell death (induced by either aging or by H2O2 treatment) were blocked by inhibitors of cysteine proteases. Inhibition of cell death enhanced cyst formation. In summary, our results show that during the bloom P. gatunense become CO2 limited and initiate an active cascade of events leading to generation of ROS, and terminating in the collapse of the population, due to programmed cell death. We propose that PCD may function to eliminate the less adapted cells, allowing only the most fitted organisms to produce progeny for the next season.