| The large-scale and continuous outbreak of cyanobacterial harmful algal blooms seriously threatens the freshwater ecosystems and human health worldwide.Microcystis aeruginosa is one of the dominant species of cyanobacteria with the most frequent outbreaks in the world,it can allelopathically inhibit other organisms by releasing harmful secondary metabolites.It is known that the substances secreted by M.aeruginosa(exudates)during its exponential growth stage have strong allelopathic effect,but its toxic mechanism to the target organism,the components of allelochemicals and their influencing factors(such as temperature)are not very clear.In this study,the allelopathic mechanism of exponential exudates of M.aeruginosa on Scenedesmus quadricauda was systematically studied by using the method of multilevel and multi-omic integration analysis,and the influence of temperature change on the allelopathic effect of M.aeruginosa was investigated.The main results are as follows:1.The exudates of M.aeruginosa significantly inhibited the growth,chlorophyll a content,photosynthesis parameters and ATP content of S.quadricauda(p<0.05),thus indicated significant allelopathic effect.2.By comparing the differential transcriptome,proteome and metabolome of S.quadricauda under the treatment of M.aeruginosa exudates,it was found that the allelopathic mechanism of M.aeruginosa on S.quadricauda was mainly indicated by inhibition of chlorophyll synthesis,causing of DNA damage,and interfering with protein processing and degradation.For example,by inhibiting the transcription and translation of the key enzymes of chlorophyll synthesis(magnesium-chelatase and protochlorophyllide reductase),the content of these enzymes can be reduced,thus the content of coproporphyrinogen,the intermediate product of chlorophyll metabolism,can be reduced.Finally,the content of chlorophyll a in the cells of S.quadricauda decreased significantly.3.A total of 60 compounds were detected in the exudates of M.aeruginosa,of which fatty acyls(10 compounds),carboxylic acids and derivatives(7 compounds)and organooxygen compounds(4 compounds)were the highest.Based on the joint analysis of the structure and function of these compounds and the damage mechanism of S.quadricauda,potential harmful compounds such as palmitic acid and oleic acid were found.4.The change of temperature could significantly affect the growth and allelopathy intensity of M.aeruginosa and S.quadricauda.Comparing to higher temperature(25℃),the growth of the two strains decreased significantly at lower temperature(20 ℃).However,at lower temperature,the allelopathic effect of M.aeruginosa exudates on S.quadricauda was significantly enhanced.Further comparing the composition difference of the M.aeruginosa exudates under the two temperature treatments,it was found that the abundance of daminozide and tiludronic acid increased.5.The differential transcriptome,proteome and metabolome of M.aeruginosa under lower temperature were further analyzed.It was found that lower temperature inhibited the pathways related to energy production in M.aeruginosa cells,such as oxidative phosphorylation,photosynthesis-antenna proteins,porphyrin and chlorophyll metabolism,thus inhibiting cell growth.At the same time,it promoted the pentose phosphate pathway in M.aeruginosa cells which is related to secondary metabolites,thus was speculated to be related to the increase of allelopathic activity of M.aeruginosa.To sum up,this study found that chlorophyll synthesis,DNA replication and repair,and protein processing were the main targets of allelopathy of M.aeruginosa exudates on S.quadricauda.Oleic acid and other toxic compounds are found in the M.aeruginosa exudates as potential allelochemicals.And it was found that the growth of M.aeruginosa slowed down but the allelopathic effect was enhanced at lower temperature.These results provide important references for further understanding the chemical ecological toxicity of cyanobacterial blooms and their countermeasures. |
PDF Full Text Request