| Water eutrophication has resulted in the frequent occurrence of cyanobacteria bloom.Microcystis bloom is a common type of cyanobacteria bloom,which is commonly caused by both toxic strain and non-toxic strain.Environmental changes will affect the competition of toxic strain and non-toxic strain in the water,which will influence the succession direction of cyanobacteria and the toxicity of algal bloom.It has not been fully understood how the temperature rise caused by greenhouse effect and UV radiation enhancement caused by ozone depletion will affect the toxic strain and non-toxic strain of Microcystis,thus affect the succession of algal bloom.Therefore,we cultivate Microcystis aeruginosa(toxic strain:FACHB 912&FACHB 905 and non-toxic strain:FACHB 469)under different solar UV radiation and tempreature in order to examine their physiological and biochemical characteristics.The main results are as follows:With the increase of UV radiation,the PSII maximum photochemical efficiency(F_v/F_m)of both strains decreased significantly,the PsbA protein content also decreased significantly,photosynthetic activity of the toxic strains decreased more significantly,and the toxic strains showed stronger recovery ability when transferred to growth light.In response to the increase of PAR and UVR radiation,the sustained non-photochemical quenching(NPQs)was rapidly induced in non-toxic strain and extracellular polysaccharides(EPS)content was higher,while superoxide dismutase(SOD)activity was higher in toxic strain.In addition,we found that toxic strain of Microcystis aeruginosa can increase the content of microcystins(MCs)under UV radiation.UV radiation and temperature rise alone can result in the inhibition of photosynthetic activity of both strains.Under the coupling effect,the toxic strain is significantly inhibited.When the two conditions are coupled,the inhibition of photosynthetic activity in toxic strains can be further deepened and the repair rate was decreased,but the light tolerance was still higher than that of non-toxic strain.The content of carotenoid(Car)was increased under UV radiation.The activity of SOD and catalase(CAT)was significantly increased under UV radiation,but the effect of temperature was inhibited.The toxic strain had higher EPS content,which decreased with increasing temperature,and increased with increasing temperature coupled with UV radiation.The synthesis of microcystin was promoted by increasing temperature,and the content of microcystin was further increased by UV radiation coupling effect.In the long-term PAR and UV radiation treatment,the photosynthetic activity of the non-toxic strain was significantly inhibited by UV irradiation during the first two days and began to recover on the third day.The F_v/F_m of non-toxic strain which under UV radation was the same as that of PAR radation on the sixth day.However,UV radiation did not show significant inhibition on the toxic strain.Under UV radiation,the relative content of allophycocyanin(APC/Chl a)and phycocyanin(PC/Chl a)were higher in both strains,but PC/Chl a was significantly higher in non-toxic strain than in toxic strain.In the same way,SOD activity in both strains increased under UV radiation and CAT activity of the toxic strain did not change significantly under the two radiation treatments.In addition,UV radiation also induced higher EPS content in both strains,which tended to stabilize with the extension of culture time.The content of MCs in toxic strain was promoted by UV radiation.Compared with non-toxic strain,the toxic strain has higher growth rate.In summary,the toxic strain and non-toxic strain of Microcystis aeruginosa can adapt to environmental changes through different physiological mechanisms.In this study,we found that the toxic strain may be more competitive under the background of enhanced UV radiation and global warming.This result may affect the toxicity of cyanobacteria blooms.The universality of the results needs to be further verified. |
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