The Effect Of UV-B Stress On The Inducible Defense And Interspecific Competition Of Scenedesmus Obliquus

Algae,as producer of the aquatic ecosystems,play an important role in the waters.However,freshwater algae and other organisms are greatly affected by the increasing UV-B radiation due to the serious loss of ozone and in recent years.Previous studies focused on the adverse effects of enhanced UV-B on algae,while the changes in the algae will inevitably affect the interspecific relationships,such as competition and predation.Therefore,our experiment focused on the effects of enhanced UV-B on the inducible defense and interspecific competition of Scenedesmus obliquus.Specifically,we analyzed the effects of increasing ultraviolet radiation on algae and their interspecific relationships.It was of great significance for enriching and developing the ecological theoretical knowledge of inducible defense and competition.We firstly explored the effects of different UV-B intensities on the inducible defense of S.obliquus.A total of three UV-B intensities(0 mW cm-2,0.03 mW cm-2,0.07 mW cm-2)were set up for the experiment,including the control group(without filtrate from Daphnia magna)and the treatment group at each UV-B intensity.In the treatment group(with filtrate from D.magna to induce anti-grazing defense),the light intensity and light cycle were kept in constant for a total of 9 days.We firstly analyzed the population abundance,population proportion changes,photosynthetic parameters,fructose synthesis and gene phosphoglucomutase(PGM),ADP-glucose pyrophosphorylase(AGPase),photosynthetic efficiency-related gene ribulose-1,5-bisphosphate carboxylase/oxygenase(Rubisco),gene expression of the heat shock protein(Hsp-70A).Secondly,the effects of enhanced UV-B on interspecific competition between S.obliquus and other green algae(Chlorella pyrenoidosa)or cyanobacteria(Microcystis aeruginosa)were investigated.Under the three different UV-B intensities,S.obliquus and eukaryotic algae C.pyrenoidosa were mono-cultured or co-cultured in the same ratio conditions respectively.S.obliquus and prokaryotic M.aeruginosa were cultured separately and co-cultured at a ratio of 1 to 5.The experiment would be finished until the competition reached stable.The main results of the study are as follows:1.Effect of UV-B on the induced anti-grazing defense of S.obliquusWith the increasing of UV-B intensity,the population growth rate and population density of S.obliquus were significantly decreased,and UV-B radiation also inhibited the formation of S.obliquus inducible defensive colony.In the absence of UV radiation,the algae in the treatment group responded to the grazing pressure from the D.magna and formed large proportions of multicellular colonies(mainly four-celled and eight-celled colonies).However,under the action of UV-B,the anti-grazing defense of the algae was inhibited.The average number of cells per particle decreased and the formation of eight-celled colonies was significantly suppressed.When the UV-B intensity was as high as 0.07 mW cm-2,the photosynthesis of S.obliquus was severely inhibited and the inhibition was more pronounced with the prolongation of culture time.Overall,UV-B inhibited the growth of the algae and induced defensive population and photosynthetic efficiency.Obviously,the enhancement of UV-B is not beneficial to the growth of algae,and it also destroys the predation relationship in water.In the absence of UV radiation,the expression levels of PGM and AGPase genes related to the synthesis of fucose in the treatment group were relatively high,indicating that PGM and AGPase genes were involved in the formation of the colony.In contrast,with the increase of UV-B radiation,the relative expression levels of the treatment of PGM and AGPase in the related genes of fucoidose was gradually decreased in the group,which impaired the synthesis of polysaccharides.It may be the reason for the inhibition of population formation.Under the UV-B,the relative expression of Rubisco,a key enzyme in photosynthesis,decreased in photosynthetic plants,resulting in a decrease in photosynthetic efficiency.Whereas in the filtrate group,the expression level was greater than that in the control group.This may be due to that they need to use more energy in the defensive process of the algae and provide as much substrate as possiblefor polysaccharide synthesis.When the intensity of UV-B was 0,0.03 mW cm-2,the filtrate may act as a stress factor,and the relative expression level of stress protein Hsp-70A gene was significantly increased,but there was no significant difference between the control group and the filtrate group.As the UV intensity was too high,the filtrate loses its role.2.Effect of UV-B on interspecific competitionUnder different intensities of UV-B radiation,S.obliquus and eukaryotic algae C.pyrenoidosa were cultured separately and in the same ratio co-cultivation conditions.The growth rate of C.pyrenoidosa cultured alone was faster than that of S.obliquus.C.pyrenoidosa was more susceptible to high UV-B than S.obliquus.In mixed cultures,UV-B intensity had no effect on competition between the two algae,and S.obliquus dominated in most cases.It may be related to resisting UV-B radiation by forming cell populations.Under different intensities of UV-B radiation,S.obliquus and prokaryotic algae M.aeruginosa was cultured alone and co-cultured at a ratio of 1:5.When cultured alone,the growth of M.aeruginosa and 5.obliquus were both affected by high and low UV light.At the same intensity of UV-B,M.aeruginosa grew more rapidly than S.obliquus,but the growth and photosynthesis of M.aeruginosa were more susceptible to UV-B than S.obliquus.In the mixed culture,the M aeruginosa had a competitive advantage without UV-B,probably as the mixed cultured M.aeruginosa released more toxins under the competitive pressure,thus inhibiting the growth of S.obliquus.Under the action of UV-B,M.aeruginosa was inhibited by UV-B,and S.obliquus was in a competitive advantage.It indicated that M.aeruginosa was more susceptible to UV than S.obliquus,which may be due to the absence of nuclear membrane and thick polysaccharide layer.UV-B was more likely to target the DNA of M.aeruginosa,thereby affect its growth,metabolism,and toxin genes,and thus reduce its competitive advantage.