Study On The Algicidal Characteristics And Mechanism Of Lysinibacillus Fusiformis DMC-2

When it comes to water ecological environment,cyanobacterial harmful algal blooms(CyanoHABs)often attract people's attention.CyanoHABs have occurred frequently around the world during these decades,mainly because of eutrophication and global warming.Other conditions which include anthropogenic and hydrodynamic,either singly or in concert,can also drive to episodic HAB situations.Massive proliferations of harmful cyanobacteria result in the reduction of biodiversity and oxygen in the water,threatening the safety of aquatic ecosystems seriously,and affecting human health by releasing hepatotoxic microcystins.Biological method refers to the use of biological agents including bacteria,fungi,viruses,protozoa and plants to kill algae or inhibit the growth and reproduction of algae,which have attracted great attention and have been considered as a practical strategy because of their algicidal potential and environmental friendliness.Although the algicidal bacteria has been studied during the decades,an efficient and eco-friendly approach still on the way.We focus on the algicidal bacteria with the ability of degrading microcystins which are rarely studied but closely related to human life.In this study,we screened a strain of Lysinibacillus fusiformis DMC-2 that can reduce the biomass of Microcystis aeruginosa and degrade the microcystins simultaneously,and evaluate its algicidal properties,mechanism and active substances.The analysis and research are expected to provide bacteria resources and theoretical basis for the treatment of Microcystis blooms.The main results are mainly divided into the following points:1.Identification of the algicidal bacteria DMC-2: An algicidal bacteria was isolated from the summer blooms of Dianchi Lake in Yunnan and named as DMC-2;the colony of DMC-2 was off-white,round and smooth on the LB solid medium;The strain DMC-2 is rod-shaped,with round or oval spores in the bacteria,which makes the bacteria expand and appear fusiform when observed under the optical microscope.Based on the results of 16 S rDNA analysis and phenotypic characteristics,DMC-2 was identified as Lysinibacillus,and was closely related to Lysinibacillus fusiformis.2.Algicidal properties of the strain DMC-2: Strain DMC-2 can reduce the biomass of Microcystis aeruginosa and has a high-efficiency algicidal activity.The algicidal ratio of DMC-2 to M.aeruginosa NIES-843 was 86.4% after 6 days of co-cultivation.DMC-2 shows algicidal activity against all tested algae strains,and the algicidal activity against Microcystis aeruginosa was higher.At the same time,it also shows good algicidal activity against harmful cyanobacteria such as filamentous cyanobacteria.The algicidal activity of DMC-2 is not affected by the extracellular polysaccharides produced by algae strains,and it can also achieve a high algicidal effect for eukaryotic algae.DMC-2 kill algae indirectly by releasing the active substances,and it may be secreted and enriched in a large amount in the logarithmic phase of growth.The lower cell density,that is,the lower concentration of the active substances,can produce efficient algicidal effect which is conducive to reducing the impact on aquatic ecology in field applications.3.Algicidal mechanism of the strain DMC-2: DMC-2 makes the algae cells shrink,collapse and rupture from the original full and smooth spherical shape when observed under the scanning electron microscope.The algicidal active substances secreted by DMC-2 affect the expression of some important genes in Microcystis aeruginosa cells.DMC-2 can effectively inhibit the expression of the gene related to microcystin synthesis;the expression level of genes related to the photosynthesis system can be inferred that DMC-2 attacks the photosynthetic system of NIES-843 in a complex way and affects the energy acquisition process,resulting in a decrease in cyanobacteria biomass;at the same time,the corresponding repair mechanism of Microcystis aeruginosa is triggered.DMC-2 can also efficiently degrade MC-LR and has high field application value.4.Algicidal substances of the strain DMC-2: The DMC-2 secreted thermo-insensitive and p H-insensitive algicidal active substances indicating that the substances are not a macromolecular substance such as protein and nucleic acid;and DMC-2 may secret more than one active substance.Cyclohexane extraction has the best activity-oriented results.After a large amount of extraction and enrichment,through preliminary separation by liquid chromatography,activity verification and LC-MS,the inferred chemical formulas of the main components of the three active crude samples are obtained,which laid the experimental foundation for the targeted separation and purification of active substances in the later stage.