| Climate change leads to the rise of average waters temperature, at the same time, the concomitant aggravation of temperature fluctuation is likely to have more immediate effects on aquatic ecosystem. In this study, we discussed the influence of the temperature fluctuation on the induced defense of Scendesmus obliquus. In nature, the change of water temperature has certain periodicity, such as diurnal change, seasonal variation, etc. Global climate change leads to frequent and severe fluctuation in temperature. Frequent temperature fluctuations will affect aquatic ecosystem closely related to human’s survival and development, which also urgently need to pay attention to environment problems. Once aquatic ecosystem suffers heavily outside interference, which will affect the relationship between aquatic organisms and destroy the stability of community structure.Therefore, temperature fluctuation is an important environmental factor and studying its action has very vital significance. At present, there are most researches on the growth of the aquatic animals in the laboratory under the conditions of constant temperature, so we don’t know temperature fluctuations how affect the aquatic animals, which caused the people’s interests. Since the1970s, the impact of cyclical changes in temperature on the growth of aquatic animals has begun to get widely attention. From the current researches, the range of research object is very wide, almost covering all of groups of aquatic animals and plants. However, due to the differences in species, temperature settings, and so on, the differences between the results of studies are significant, but the fundamental response is similar.The experimental materials are filtered water from Daphnia magna and Scenedesmus obliquusas, and each experiment lasted for9days, and several temperature fluctuations range around22℃and25℃, respectively are22-28℃,23-27℃,24-26℃and18-26,20-24℃. At first, adding Daphnia magna water to Scenedesmus obliquusas, samples (2ml) were collected every other day and fixed in Lugol’s solution (2%). Cell density was determined using a blood-cell counting chamber under a microscope. Also, three different temperature fluctuation cycle (24h,48h,72h) were studied in the22to28℃, the result showed some differences. At22℃, we chose the two initial density, and the results also show that initial density affected induced defense in Scenedesmus obliquusas. Detailed results are as follows: 1) Different amplitude of fluctuating temperature, period and initial density affect cell density of Scenedesmus obliquusasWhen the average temperatures are22℃and25℃, in several different temperature fluctuating modes, the growth has no significant influence under the Daphnia magna water; but it can increase the Scenedesmus obliquus to form colonies, which is similar to the former. When temperature fluctuation cycle is not at the same time, the results also show that there is no significant effect on the growth; however, the initial density at22℃is low, adding the Daphnia magna water (treatment) contributes to the growth than those in the absence of Daphnia water. This shows that do not have a significant impact on the growth of S.obliquus in different temperature fluctuation amplitude and period. When its initial density is very low, S.obliquus in the presence of Daphnia water growed better than the control group, and appeared significant difference after statistics analysis. It may be that the nutrients lead to different results. Different nutrient covered the effect of Daphnia magna water.2) Different amplitude of fluctuating temperature, period and initial density affect the formation of Scenedesmus obliquusasThe mean number of cells per particle decreased under fluctuating temperature (20-24℃,18-26℃). The ANOVA statistical analysis show:three fluctuating temperature did not affect S.obliquus significantly within3days; at5th and7th day, under the condition of variable temperature (18-26℃), the number of cells in each group has significantly reduced than the other two temperature modes; at9th day, there is no significant difference between treatment and control.The mean number of cells per particle decreased under fluctuating temperature (24-26℃,23-27℃,22-28℃). By ANOVA statistical analysis, the Daphnia water can promote S.obliquus to form colonies; at the last two days (7, and9), it can be seen that fluctuating temperature (24-26℃,23-27℃,22-28℃) can decrease the the mean number of cells per particle in different levels (comparing to the constant temperature25℃).When the period of temperature fluctuation was longer than48h and72h, the mean number of cells per particle increased, that is, formed colonies. By the ANOVA statistical analysis:Daphnia water and period did not affect the mean number of cells per particle; at5and7day, as we can see, the mean number of cells per particle under the period of24h was significantly higher than those under the period of48h and72h. This shows that the extension of temperature fluctuation cycles have favorable impact on formation of S.obliquus.When initial density decreased, after nine days observation, the results showed that in the control group the mean number of cells per particle changed normally (with the increase of time, it will increase; under the condition of constant temperature (25℃), there are more the mean number of cells per particle, but in treatment group, since5days, the mean number of cells per particle was gradually reduced, and on the ninth day, the morphology of S.obliquus was dominated by unicells, almost can’t see the colonies.3) Different amplitude of fluctuating temperature, period and initial density affect the formation proportion of Scenedesmus obliquusasInitially, Scenedesmus morphology was dominated by unicells (about60%), and in the3th day, a rapid formation of four-celled colonies was observed, and the proportion of unicells decreased gradually; also appeared eight cells and irregular shape. Experimental results show that the Daphnia water can induce S.obliquus to form colonies.4) Different amplitude of fluctuating temperature, period and initial density affect polysaccharide of Scenedesmus obliquusasAt temperature fluctuations (24-26℃,23-27℃,22-28℃), the contents of both the soluble extracellular and the bound fractions of polysaccharides of S.obliquus cells decreased. At the temperature of25℃and22-28℃, the contents of both the soluble extracellular and the bound fractions of polysaccharides of S.obliquus cells increased significantly in the presence of Daphnia water than that in the control. The increasing of the fluctuation cycle could change polysaccharide content. Under the period of48h and72h, the bound fractions of polysaccharides of S.obliquus cells increase. The analysis of variance shows that the bound fractions of polysaccharides significantly increased with the increasing of period; but extracellular polysaccharide is not obvious. At the same time, Daphnia water can also make the contents of polysaccharide significantly higher than the control group. After9days, according to data, the period and Daphnia water had a certain effect, but the effect of different fluctuating temperatures is relatively weak. |
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