The Succession Of Algal Community Structure In Lakes And Characteristics Of Algicidal And Denitrification Of Actinomycetes

Excessive pollutants containing nitrogen,phosphorus and carbon are discharged into lake waters,which accelerates the eutrophication of lake waters worldwide,resulting in the formation of harmful algal blooms.Cyanobacteria harmful algal blooms are widely distributed in surface waters including lakes and reservoirs,and have become a serious threat to ecological environment,human health and economic development.Algae control methods using microorganisms have received increasing attention because of their advantages such as environmental friendliness and ecological properties.In this paper,the water quality and algal community structure and their coupling relationship in different lakes in Xi’an were investigated.Secondly,taking Microcystis aeruginosa,a typical cyanobacteria,as the target to control algae species,a strain of actinomyces Z1with efficient algicidal and nitrogen removal characteristics was selected to explore the algicidal mechanism of actinomyces Z1 and its nitrogen removal characteristics in water.Finally,the Z1 strains were injected into the lake collection water to further verify the removal effect and denitrification performance,and to explore the effect of the addition of actinomyces Z1 on the algae and bacterial community structure,aiming to provide a scientific basis for the control of algae in lake water bodies.The main research results are as follows:（1）The successional characteristics of the algal community structure and its interaction with water quality in lakes from July to December 2021 were investigated using traditional microscope counting method combined with ecological network analysis and structural equation modeling method of six urban lakes in Xi’an.The results illustrated that 1)Eight different phyla were identified.The Chlorophyta was dominant in the urban lake and Chlorella sp.was the dominant algal species throughout the study period.In addition,Synedra sp.were present in all six urban lakes,with the highest abundance in CL Lake in October（55.17%）.Mallomonas sp.appeared in higher abundance in CL Lake in August（66.67%）,while it did not appear in any other urban lakes.2)Ecological network analysis showed that positive correlations were dominant among the algal communities of six urban lakes,and CL lake had more modules than the other lakes,indicating a more complex interaction pattern and a more stable network.3)The algae-water quality correlations of RDA1 and RDA2 explained 47.35%of the total variance,T、TP、DO、NO₃^--N and NH₄⁺-N had significant effects on the algae community structure.Structural equation modeling（SEM）indicated that physical parameters（T,p H,and DO）had significant negative effects on cell density and algal community structure.（2）Microcystina aeruginosa was used as a targeted algal species,and a strain of actinomycetes with high efficiency in algicidal efficiency and nitrogen removal was selected from water environment.The results showed that 1)the algicidal mode of Actinomycete Z1 was indirect,that was secreting extracellular active substances.The cell-free supernatant had no effect on the algae of Chlorella vulgaris and Chlamydomonas obliquus.2)The chlorophyll fluorescence parameters of Fv/Fm and Y（II）were significantly inhibited after co-culture cell-free supernatant with Microcystis aeruginosa cells.Total ATP and intracellular ATP contents also showed a decrease.The antioxidant enzyme activities CAT and SOD increased and reached a peak on day 6,while the enzyme activities started to decrease,but were always higher than the control group.The changes of MDA also indirectly reflected the degree of damage to the algal cell membrane by their effects.It was also found that the Zeta potential on the surface of algal cells changed from the initial value of-37.95 m V to-15.78 m V,which increased the Zeta potential.3)The analysis of three-dimensional fluorescence spectra of algal lysis products illustrated that some components of humic acid-like substances decreased during algal lysis,and the fluorescence intensity of aromatic proteins and metabolites of dissolved new microorganisms increased.4)The results of denitrification in the process of algae lysis showed that the removal rate of TN could reach 88.1%.The NO₃^--N concentration decreased from 203.142 mg/L on the second day to 20.59 mg/L on the eighth day,and then leveled off.（3）From the perspective of practical application,the algicidal effect and denitrification performance of Actinomycete Z1 on algae in lake collected water bodies were further verified,and their effects on algal and bacterial community structure were explored.The results manifested that 1)The Chl-a content in the experimental group（Z1-T）decreased from the initial 125.73μg/L to 29.27μg/L on the 10th day,and the algal removal rate reached 76.7%.2)TN concentration generally showed a decreasing trend from 4.25 mg/L to 0.89 mg/L,with a removal rate of 79.16%.an increase in total nitrogen concentration was observed on day 2.The highest NO₃^--N concentration removal rate was81.4%.3)Total ATP content and ETSA showed a similar trend,increasing and then starting to decrease after day 8.4)Five different algal phyla were identified,and the number of Cyanophyta in the Z1-T group decreased significantly by 91.9%.Proteobacteria increased from 18.46%to 61.37%（CK-10）and 45.90%（Z1-T）.5)The results of LEf Se analysis and LDA branching tree showed that the Firmicutes was only a biomarker for CK-10.After 10 days of co-culture,Pseudoxanthobacter sp.,Flavobacterium sp.and Microbacterium sp.were mainly enriched in the Z1-T group.