| Global climate change has resulted in an increase of the average water temperature, which is in favor of the establishment spring bloom cyanobacteria Microcystis dominance and the bloom forming. Meanwhile an increase in temperature fluctuation frequency and intensity has been brought, leading to a more immediate effect on water ecosystems. With fast physiological and photochemical process and small cells, planktonic algae may be more sensitive to short-term temperature fluctuations, and the responses of algae growth to the temperature fluctuations are different, so the temperature fluctuations may be the important factor, affecting Microcystis dominance establishment in response to elevated temperature. In this study, we investigated the effect of temperature fluctuations on different categories of Taihu dominant algae growth and physiological characteristics. Our experiments include laboratory experiment and field monitoring. As the study objects of laboratory experiment, cyanobacteria Microcystis aeruginosa, green algae Chlorella pyrenoidosa and Scenedesmus obliquus, diatom Cyclotella meneghiniana, cultured under monoculture and mixed culture. According to the actual situations of temperature fluctuations during spring in Taihu, we set different temperature fluctuation models, aiming to probe into the response mechanism of the growth, competition and physiological characteristics of planktonic algae to temperature fluctuations. Field monitoring during spring in Taihu was used to continuously monitor during the process of cyanobacterial dominance establish, then an analysis of different categories of algal photosynthetic characteristics and pigment content would be made to research the influence of temperature fluctuations on the dominant algae during spring in Taihu. Results are as follows:1. The effect of temperature fluctuation on the growth, photosynthetic activity and physiological characteristics of algae under different base temperaturesM. aeruginosa, C. pyrenoidosa, S. obliquus and C. meneghihiana, cultured under two temperature models:the constant temperature (15℃,20℃) and diurnal fluctuation temperatures (15±5℃,20±5℃) for15days, were determined their cell density every day and the last polysaccharide content of different forms. The results were that, the growth of Microcystis were significantly promoted by fluctuation temperature (15±5℃), but the influence for Chlorella took second place, and Scenedesmus has the minimum; when temperature was20±5℃, Cyclotella completely was not dominant, but Microcystis had a growth advantage in short term (3-6d), in contrast, Chlorella took a growth advantage in long term (more than7d), which may be a more complex intermediate state during the dominance thifts from green algae to cyanobacteria. The bound polysaccharide content of Microcystis at20±5℃decreased significantly, indicating that the relative long-term temperature fluctuations may not be conducive to the accumulation of bound polysaccharides of Microcystis. It can be speculated, temperature fluctuations under the base of temperature15℃were easy to promot Microcystis to grow, and at the basic temperature of20℃, relative short-time (15d) temperature fluctuations were benefit to the dominance establishment and maintain of Microcystis, and relative long-term temperature fluctuations may affect that of Microcystis by influenceing the accumulation of the bound polysaccharides of Microcystis.2. Effects of temperature fluctuation on algae growth and competitionWith temperature20"C as control group, and diurnal fluctuations20±5℃for the treatment group, we tested and observed the photosynthetic activity (Fv/Fm, Fv’/Fm’) and change of chlorophyll a content of M. aeruginosa and S. obliquus for28days under monoculture and mixed culture, and found that Fv/Fm, Chla of single Microcystis species and Fv’/Fm’ for single Scenedesmus significantly inhibited at diurnal fluctuating temperature20±5℃(P<0.05), the others had no significant effect. When it was mixed culture, the Fv/Fm for two algae in the treatment group were significantly higher than those in control group (P<0.001), but the effect for Microcystis worked significantly faster than Scenedesmus in the later stage of the experiment. Visibly, temperature20±5℃under mixed culture was better to promote maximum photosynthetic capacity for Microcystis, which also showed that the mixed culture could effectively simulate field environment. And compared with the laboratory study, Fv/Fm of cyanobacteria at low temperature in the field relatively was higher than that of green algae, suggesting that cyanobacteria were easier to exclude green algae in the field than laboratory. Therefore, we speculate that this may be one cause of the more competitive advantage of cyanobacteria in the field.3. Effects of different fluctuation cycles of M. aeruginosa and S. obliquus in mixed cultureIn order to study the influence of different periodic fluctuation of temperature on competition between Microcystis and Scenedesmus, we set cultivation of this experiment were monoculture and mixed culture, diurnal cycle as control group,6days (3days for high temperature,3days for low temperature) and10days (5days for high temperature,5days for low temperature) for a cycle as two different treatments, setting temperature difference in a cycle is6℃(20±3℃) with the average temperature in20℃. We found that:(1) When in monoculture, compared with the control group, Fv/Fm and Fv’/Fm’for two algae of two different treatment groups, and Scenedesmus Chla content showed no significant differences (P>0.05), only chlorophyll a formation for Microcystis under the treatment of10days as a cycle in the later experimental period (from the20th day) was significantly inhibited (P<0.05), so as the biomass of Microcystis during this period, which may be explained by that the low temperature phase of this way was not conducive to Microcystis cell division. But in general, photosynthetic activity of single species Microcystis and Scenedesmus was not affected by the temperature fluctuations with different cycles.(2):By co-culturing, Fv/Fm and Fv’/Fm’and Chla content of Microcystis and Scenedesmus had no significant difference between two different treatment groups and control groups, respectively (P>0.05), meaning that there were no differences in the responses of the photosynthetic capacity for two algae to different fluctuation models. And the10days’ cycle treatment increased Microcystis cell density on experimental segment (15-25d), indicating that this fluctuation under relative short-time could promote the growth and competition of Microcystis. In addition, only in10days’cycle treatment group, Chla ratio of Microcystis and Scenedesmus in co-culture on10-23days, was higher than that of control groups (P<0.001), which also showed Microcystis cultured by co-culturing would be better in the competition under short-term temperature fluctuation with a10days’cycle. In short, from monoculture and mixed culture results to know, when temperature difference was6℃at average temperature20℃, short-term temperature fluctuation with a10days’cycle would be more advantageous for Microcystis to compete with others.4. Field monitoring of the effects of temperature fluctuations in spring on the photosynthetic activity of algae and cyanobacteria dominanceFor the sake of revealing how the temperature fluctuations to affect the photochemical mechanism of Microcystis dominance establishment in Taihu, this experiment was continuous monitoring during the process of cyanobacterial dominance establishment during spring in Taihu, companied with determination of algal pigment content and photosynthetic properties of different algae (cyanobacteria, green algae and diatoms) by fluorescence spectrophotometer and Phyto-PAM. Results showed that, while temperature rose, there were positive correlations between diurnal temperature fluctuations and chlorophyll a, phycocyanin respectively. However, the correlation between the changes in non-cyanobacteria abundance and diurnal temperature difference was no significant. When temperature gradually rose to above20℃and diurnal temperature fluctuations were larger, cyanobacteria density was not high but photosynthetic activity was strong, which may lay a foundation for the occurrence of cyanobacterial bloom. Therefore, temperature with temperature fluctuations was lower during spring, cyanobacteria though not to become the dominant population in phytoplankton community, but it has the potential to rapidly grow so that the biomass could accumulate quickly, advancing the occurrence time of cyanobacteria bloom. |
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