RESPONSE OF MICROALGA CHLORELLA TO CHANGES IN LIGHT INTENSITY

Abstract

The importance of microalgae cultivation has been appreciated by more and more people as the global population and environmental problems increased significantly in recent years. Nevertheless, the mass cultivation of microalgae is still facing many challenges. More research needs to be done before human beings can really benefit from this new technology. One of the fundamental problems pertaining to the outdoor mass cultivation is the low efficiency of photosynthesis. On the basis of their observations, Lee and Low of this laboratory raised a hypothesis which suggests that low photosynthetic efficiency of outdoor microalgal cultures may well have resulted from the much slower downstream metabolism rate of microalgae compared to the fast fluctuation of the outdoor irradiance (Lee and Low 1991, 1992 and 1993 ). This project was therefore set up for testing the hypothesis. Laboratory and field experimental results that had been done within two years are presented in this thesis. In the first part of the thesis, the up to date research results and theories on the relationship of light and growth of microalgae were surveyed. The most recent findings about cytosolic carbon and nitrogen metabolism and its relationship with photosynthesis were particularly reviewed. In order to examine the Lee and Low hypothesis, outdoor semi-continuous culture, indoor batch and continuous cultivation were used. Physical, chemical and biochemical analysis were conducted. Experimental results were presented in a separate part. The growth parameters of three outdoor cultures and the respective photosynthetically active radiation (PAR) were very closely followed. By careful examination of the relationship between carbon and nitrogen output rates and the PAR on the days, interesting patterns of carbon and nitrogen response to PAR were found. The results suggested that output rates of carbon with respect to PAR on clear and cloudy days were similar, whereas output rates of nitrogen on clear day were much lower than expected. The different pattern of response to PAR resulted in difference in C/N ratio of cultures. Indoor batch and chemostat experiment results confirmed the outdoor findings. Protein synthesis responds much slower than carbohydrate when PAR shifts up. Possible mechanisms of this phenomenon were discussed. Nitrogen uptake rates of algae showed a hypobolic function of irradiance under light limited conditions. The lower saturation PAR of N uptake comparing to C may explain the poorer N response. Exposure to strong light may cause decrease in not only efficiency but also maximum capacity of the light limited N uptake. Based on the recent findings of Stitt (1990), poor N response to high PAR may also clue to the subtle balance between chloroplast and cytosolic metabolism. The concept of mean light intensity and its possible application on optimizing outdoor microalgal cultivation were discussed. Several suggestions regarding further research and applications of the current findings were made.