| It is generally anticipated that the present atmospheric CO2 concentration of 380 ppm would increase to be 750 ppm by the end of this century due to the increasing industrial combustion of fossil fuels and human activities. The significant impacts of increasing atmospheric CO2 on global climate change and ecosystem have been of general concern. As the CO2 is a substrate for plant photosynthetic process, increasing CO2 concentrations will importantly affect the physiology and growth of plant. In addition, the atmospheric CO2 is endothermic and insulating, thus elevated atmospheric CO2 will inevitably result in the increase of global air temperature. It is estimated that the global average temperature would increase by 5-6℃ in response to the doubling of the atmospheric CO2 concentration. Temperature is one of the key factors affecting enzymes activities of biochemical reactions in the plant. Therefore, increased temperatures resulting from elevated atmospheric CO2 will remarkably influence the physiology, growth, production and energy distribution of plants. Grazer-induced defenses in phytoplankton can prevent strong population fluctuations in bi-and tritrophic food chains and play an important role in maintaining the stability of community and trophic structure. Although the effects of rising CO2 and temperature on freshwater algae are extensively studied, it remains unclear how the inducible defenses of algae response to the rising CO2 and temperature. To elucidate the effects of elevated atmospheric CO2 and temperature on inducible defenses, the present thesis carries out the following studies with the freshwater phytoplankton Scenedesmus obliquus and zooplankton Daphnia magna:1. Effect of elevated CO2 on inducible defenses of S. obliquusWe cultured S. obliquus under the CO2 concentration (350 ppm) and the high concentration (750 ppm). The treatment is the group with filtrate from D. magna, while the control is the group without filtrate from D. magna. Light intensity, light cycle and incubation temperature were consistent throughout the experiment. We cultured S. obliquus for 9 days, and took samples every day. Samples were collected and then counted using a blood-cell counting chamber under a microscope to analyze the growth of S. obliquus. We measured photosynthesis parameters with Phyto-PAM and used electron microscope to observe cellular structure. The results showed that the effect of Daphnia water on the growth of S. obliquus was not significant under CO2 level of 350 ppm and 750 ppm. Elevated CO2 concentration promoted cell growth rate significantly (P<0.05). Cell density was increased by 28% in the control and treatment groups because of rising CO2 level. And induced colony formation could be strengthened. The pH values of the medium were lowered by rising CO2 concentretions, and it significantly decreased with time. The concentrations of total dissolved carbon (TC), total dissolved organic carbon (TOC) and dissolved inorganic carbon (IC) in the medium increased with elevated atmospheric CO2. However, there were no significant differences in the concentrations among TC, TOC and IC under the level of 350 ppm and 750 ppm with time. The efficiency of photosystem II ((?)PSII) in S. obliquus of the control was significantly increased by elevated CO2 in the early of the experiment, while there was no influence in the treatment. The observation with electron microscope showed that extracellular polysaccharide increased under 750 ppm CO2 concentration.2. Effect of elevated temperature and CO2 on induced colony formationWe combined atmospheric CO2 concentration (380 ppm,750 ppm) with air temperature (25℃,28 ℃,31℃), so there were 6 groups to culture SI obliquus. The treatment is the group with D. magna water, while the control is the group without D. magna water. Light intensity, light cycle and incubation temperature were consistent throughout the experiment. We cultured S. obliquus for 9 days, and took samples every day. Samples were collected and then counted using a blood-cell counting chamber under a microscope to analyze the growth of S. obliquus. The results showed that the cell growth promoted and cell density increased with elevating temperature under the CO2 level of 380 ppm, but there was no significant difference under 28 ℃,31 ℃. When elevated CO2 level to 750 ppm, the number of cells per particle was higher under 25 ℃ and 28 ℃ than that compared to corresponding temperature under 380 ppm CO2. There was no significant difference between 380 ppm and 750 ppm under 31 ℃. Whether in low or high CO2 concentration, the number of cells per colony was on the decline with rising temperatures. This indicated that colony formation in Scenedesmus should be negatively related to temperature. |
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