Study On The Effects Of Organic Carbon Sources On The Growth Of Microcystis Aeruginosa Under Different Nutrient Conditions

The ecological disasters, such as water blooms and red tide, are the key accounts that trouble the environmental quality of water. The inducing factors that leads to the large-scale outbreak of algae has been a hot topic in academic for years. The reasons for the water blooms could be attributed to the excess nitrogen and phosphorus content, however, it is not the only causing factor. It has been reported that many kinds of algae do not grow autotrophicly strictly. This paper studies the mixotrophic capability of Microcystis aeruginosa and the impact of the mechanism on its growth from the perspective of nutrient mode, focusing on the conditions under different nutrient levels and light intensities. Meanwhile, we expect our reasearch could be a scientific foundation for controlling the water blooms or red tide in the future. Firstly, glucose was tested as a basic theoretical research, and then, the sodium acetate, humic acid and urea, three kinds of typical carbon source in nature, were studied under different environmental conditions to investigate the growth and physiological ecology of Microcystis aeruginosa.The result implied that a certain initial amount of glucose would enhance the growth of the microalgae, which would be inhibited when exposed in high concentration. Moreover, the optimial concentration of glucose which promoted the growth of microalgae most varied in different nutrient conditions and light intensities; the tolerance of Microcystic aeruginosa to glucose at low light intensity was higher than that of at high light intensity, and the optimial concentration of glucose in dim light would be higher. Under the two light intensities, the optimial concentration of glucose both decreased when the nutrient condition improves. The cell abundance in the groups with 0-60mg/L glucose didn't have significant difference compared with the control. In addition, the content of cell chlorophyll a would reduce when grew mixotrophically; meanwhile, it also decreased when the initial concentration increased under any nutrient conditions or light intensities. The mixotrophic capability seemed to have close connection with light intensity, but there was less relations with the nitrogen and phosphorus content. The capability to utilize the glucose in low light intensity was far higher than that in high light intensity. There would be more promotion produced by glucose in poor-level medium than rich-level. The cell aboundance in autotrophic group increased along with the raising nutrition level in both high and low light intensity.The impact on the growth of Microcystic aeruginosa by different kinds of organic carbon sources varied under any nutrient levels. Taking glucose, humic acid, sodium acetate and urea, with equal amount of C, as organic carbon source, the organic carbon source which promoted biomass most was different under various nutrient levels. At low light intensity, the most optimial carbon source was glucose, sodium acetate and humic acid sequentially when exposed in poor and mid-level medium, but the group supplemented with humic acid showed more promotion than others when exposed in rich-level medium. The result also identified urea could mitigate the disadvantage produced by the poor nutrient level, but inhibit the growth under the mid and rich nutrient level. However, when light intensity was increased, urea could enhance the growth of Microcystic aeruginosa enormously, and come to be the best kind of organic carbon source in poor-level and mid-level medium, which implied that the capability of the microalgae to utilize urea increased at high light intensity and the organic carbon source which contained nitrogen would not be more helpful to the growth than other carbon source which didn't contain nitrogen. Moreover, it's significant that mixotrophic group grew better than autotrophic group in the adaptation period of the growth in any culture condition.Compared with the autotrophic group, chl a content of the mixotropic group supplemented with organic carbon source dropped previously, and their competence of photosynthesis also weakened. Howerver, urea could enhance the cell abundance as well as increasing the cell chl a content due to its nature of nitrogen-containing, because the increased nitrogen content could accelerate the composition of chl a. What's more, it's also revealed that the change trend of cell carbohydrate content was mainly consistent with that of cell chla a content, and the cell carbohydrate content of mixotrophic group was less, which could help to result in algae's floating aggregation. The effect of the same organic carbon source on the microalgae varied significantly between different nutrient levels. The cell nitrogen and phosphorus content also decreased in the groups supplemented these several typical carbon sources. The fluorescence characters of microalgae had also been influenced in different degrees when added different kinds of carbon source to the medium. The use efficiency of light(α) by each kind of organic carbon source was inhibited in rich-level medium at dim light intensity, but it showed reverse phenomenon in glucose-added group and sodium acetate-added group when exposed at high light intensity. Furthermore, the change trend of ETRmax in urea-added group couldn't accord with that ofαin the rich-level medium.