A programmed cell disintegration of Chlorella after heat stress

The small vegetative cells of a synchronous Chlorella pyrenoidosa culture subjected to a heat pre-treatment (46.5 °C for 1 h in the dark) and cultured again under continuous illumination thereafter started to disintegrate within hours. The breakdown processes of various cellular components were studied. Chlorophylls were degraded via photooxidation, started after a 2-h delay and completed within 8 h. However, no free radicals were released from the sites of photooxidation. DNA in nucleus and chloroplast disappeared along with chlorophyll pigments, but mitochondrial DNA appeared to decay at a much slower rate. In addition, a novel nuclease activity was detected in heat-treated cells undergoing DNA degradation. The decomposition of DNA rendered the disintegration process irreversible. Contrary to the programmed cell death of higher plants, these treated Chlorella cells failed to exhibit DNA laddering and massive protein degradation, but retained their cell membrane integrity. Thus, we observed an organized disintegration of Chlorella cells subjected to heat treatment, but in a way very different from the programmed cell death observed in many higher plants.