Abstract
Part I Siphonous green algae Codium edule P. C. Silva (Bryopsidales, Chlorophyta) has the highest covering ratio among the macroalgae on the coral reef of Nanwan bay in southern Taiwan, but its population in the subtidal region drastically decreases from July to September each year. The objective of this study was to find out whether the high temperature of summer could be the ground for this population decrease. Chlorophyll fluorescence measurements revealed that the circadian rhythm of photosynthesis was disrupted at a temperature as low as 32°C. TEM studies showed that incubation for 4 h at 35°C induced a decrease in turgidity accompanied by vacuole shrinkage and plasmolysis. The marked disintegrative changes, including damages in organelles such as chloroplasts and nuclei, occurred after about 8 h, at which time central vacuoles collapsed and the cell interior was then filled with numerous small vesicles. Fluorescence microscopic studies showed that nucleus could not be stained by DAPI, indicating the most chromatins were degraded after 8 h incubation at 35°C. Our results suggested that the rising of the temperature of seawater during the summer could be one of the major causes of the massive death of C. edule in the field. Part II To find out how microalgae cope with a period of high temperature stress, the small vegetative cells of Scenedesmus vacuolatus were first subjected to a heat treatment (46.5ºC for 1 h in the dark) and then cultured right away under continuous illumination and dark conditions separately. The population and cellular responses throughout the whole period of decline and recovery were examined. For continuous illuminated cultivation, the heat treatment induced several changes including inhibition of photosynthesis, condensation of nuclear chromatin, and cessation of cell multiplication. Subsequently, most of the apparent damages seemed to take place during illuminated cultivation at an intensity much lower than that used for normal growth. It caused complete chlorophyll degradation accompanied with disintegration of chloroplasts, partial inhibition of mitochondrial activity, and a significant change in cellular protein profile. For example, with features like active protein synthesis, intact cell membranes and undegraded chromatin indicating cells were alive. Proteomic analysis identified several homologous proteins. The results suggest that the proteases in the Clp family were likely to make one of the major contributions in the degradation and recovery processes of chloroplasts and mitochondria, DNA methylation might be responsible for the changes in protein profile, and Rho family GTPase might play major roles in regulation of a large number of cellular processes in recovering cells. ATP synthase β subunit down-regulated in bleached cells, indicate these cells maintained low level of biochemical activity, and mitochondria can supply ATP continuously. The recovery of bleached culture was started from these cells that had restored the ability of cell multiplication through a repair process after 24 h of light cultivation. Subsequently, there were more than 50% bleached Scenedesmus cells can regreen, and the regreen process start from a site in chloroplast and then spread gradually. Mitochondria and nuclear repair earlier than cell complete regreen. These bleached cells were still alive but entered a growth paused state to repair the damage. Thereafter, these cells could become green and reproductive again, and the number of recover cells increasing along with time. For continuous dark cultivation, these heat treated cells could make a fast repair and then entered a ‘stand-by’ state, which was characterized by condensed but undegraded chromatin, partially functional but morphologically altered chloroplasts, slowly decaying mitochondria, disappeared vacuoles, slightly shrunk protoplast and intact plasma membranes. These cells, though seemingly were undergoing cell injury, could quickly return to normal upon illumination. Cell death occurred only after a long period of darkness (>72 h). In conclusion, these studies showed that S. vacuolatus have heat tolerant ability for 46.5ºC. It can partially repaired cell damages that induced by heat stress during continuous dark cultivation, but fully recovery should be cultured in light condition subsequently.