| With the development of human society,the influences of agricultural and industrial activities on environment increases dramatically,and excessive nutrients(such as nitrogen,phosphorus,etc.)was released into water bodies,such as lakes,rivers,and bays.The problem of eutrophication of water bodies has intensified and harmful cyanobacteria blooms have become a global issue.Harmful cyanobacteria blooms destroy aquatic ecosystems,endanger human health and cause economic losses,so it is urgent to control cyanobacteria blooms.Algicidal bacteria have attracted a lot of attention because it is of low cost and environmental friendly.Many algicidal bacteria have been isolated from the natural environment.Algicidal bacteria can kill algae cells by direct and indirect ways.The former algicidal bacteria attach the harmful alga directly with their cells,and the latter attach algal cells with active substances secreted extracellularly.Relative to the extensive study of indirect algicidal bacteria,only a few direct algicidal bacteria have been reported.In this thesis,the algicidal chracteristics of an algicidal bacterial strain designated as G9 against Microcystis aeruginosa were studied.The main results are as the follows:1.Identification of the algicidal bacterial strain G9: the colony of G9 was round,with a dry surface,and gray-green spores were grown after prolonged incubation.What's more,the aerial mycelium were white,light yellow pigment produced when grew on Gause's synthetic medium.The spore chains observed under the scanning electron microscope were linear and rod-shaped.According to the 16 S r RNA sequence analysis results and phenotypic characteristics,G9 was identified as Streptomyces and had the closest relationship with Streptomyces globisporus.2.The algicidal characteristics of G9: Streptomyces sp.G9 showed a high algicidal efficiency against M.aeruginosa NIES-843.The algicidal ratio reached99.2% on the 6th day.Meanwhile,G9 had a high algicidal ratio against Microcystis aeruginosa with initial algal cell densities of 6.75×105 cells/m L and 1.35×106 cells/m L.G9 could grow and reproduce with the substances released by lysed algal cells as nutrient sources.Spores were formed in the co-culture system after the inoculated of G9 for 3 days,and the amount of spores increase gradually.What's more,G9 killed tested cyanobacterial strains selectively,and the algicidal effect was not affected by epiphytic bacteria,algal cell morphologies and extracellular secretions.G9 showed almost no algicidal effects on eukaryotic algae;G9 could inhibit the synthesis of MC-LR effectively,the inhibition rates on day 3 and day 12 were 47.9% and 76.2%.G9 degraded microcystin slowly.3.Algicidal mechanism of G9: both the fermented culture and the washed-cells of G9 had a significant algicidal effect on M.aeruginosa,while the cell-free filtrates had no algicidal activity.The cell-free filtrates of G9 did not have algicidal activities even when it grew for longer time,in different mediums and mediums with changed carbon sources,so we concluded that G9 killed algal cells by direct cell attack.In the co-cultivation system,the algal cells were wrapped by hyphae of G9,the algae cells were lysed and lose their integrity,and extrocelluar substances were observable between the algae and bacteria.The periplasmic,cytoplasmic and cytoplasmic membranes of G9 were extracted and the algicidal activities were tested.Only the membrane component showed an algicidal activity against Microcystis aeruginosa.After being heated in water baths with temperatures above 60 ?,the algicidal activity of G9 membrane fraction reduced significantly.It suggests that the algicidal component of the direct algicidal bacteria G9 is located on the membrane and is sensitive to high temperature.4.Effects of environmental conditions on the algicidal activity of G9: G9 and M.aeruginosa were co-cultivated under different illuminations,temperatures,p H values,nitrogen concentrations,and bacterial inoculation volumes,and the results showed that the algicidal activity of G9 was affected by environmental factors.For illuminations,the algicidal effect of G9 was light-dependent,and the algicidal ratio was highest under full-light conditions.For temperatures,in the early stage of co-cultivation,G9 had the highest algicidal ratio at 28?,wheras there was no significant difference in algicidal ratio in the later stage of co-cultivation,all of which exceeded 90%.For p H,G9 had the strongest algicidal activity under alkaline conditions.In different nitrogen concentrations,G9 had the highest algicidal efficiency in mediums with high nitrogen concentrations.G9 could use algal cells to grow and reproduce.So when different inoculum sizes of G9 were inoculated,during the first several days the algicidal ratios were higher with the increase of inoculum sizes,and there were no significant differences in algicidal ratios at the later co-cultivation stage.It seems that the better the conditions for the growth of the algae,the highter the algicidal ratios were,suggesting that G9 mainly attacked the growing algal cells.Finally,the algicidal activity of G9 against M.aeruginosa occurred in natural water was tested under laboratory condition.The result showed that G9 killed M.aeruginosa selectively,and diatom and green alga benifited from the inoculation of G9 and became the dominat alga. |
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