Suppression Performance On Typical Algae Growth Under UV-C Irradiation And Its Mechanism

This thesis explored the suppression effectiveness of typical algae growth under UV-C irradiation. Cyanobacteria Microcystis aeruginosa and three common freshwater green algae Chlorella ellipsoidea, Chlorella vulgaris, and Scenedesmus quadricanda were selected. The membrane integrity of UV-C irradiated algal cells were identified using flow cytometry to determine the UV-C irradiation dose range that can avoid extensive lysis as well as suppress algal growth. The physiological and photosynthetic activities in UV-C irradiated algal cells were investigated to probe into the mechanism of algal suppression effect. The sensitivities of the typical algae to UV-C irradiation were compared based on their responses on growth, membrane integrity, and pigment concentration. The effectiveness and limitation of the integrated engineering processes on algal control in the Lichee lake restoration project at Shenzhen were investigated and UV-C irradiation was proposed for in-situ algal control.The results indicated that UV-C irradiation at 50²00 mJ·cm-2 can effectively suppress M. aeruginosa growth for 5¹5 d in a dose-dependent manner for different initial cell densities settings. Over 80% of the cells exposed to UV-C irradiation at 20 and 50 mJ·cm^-2 remained intact. However, UV-C irradiation at 100 and 200 mJ·cm-2 induced severe cell disintegration in more than 70% of the irradiated cells.The results suggested that the mechanism of growth suppression includes sequenced damage/recovery procedures at cellular, biochemical, and genetical levels. UV-C irradiation suppressed the transcription process of the coding genes, disrupted the absorption and transportation of light energy by reducing aqueous pigments, affected energy transformation and electron transportation by attacking D1 protein and reducing electron transportation rate. The recovery processes of algal cell itself were immediately activated including accelerating D1 protein synthesis, shifting the genetic transcription levels, and increasing heat dissipation. However, these were insufficient to avoid further damage. Such damages on aqueous pigments and D1 protein may be magnified that resulted in further decrease on light absorption and electron transportation. Thus photosynthetic procedures including ATP synthesis and CO2 fixation were disrupted that inducing insufficient supply of both energy and organic materials. Therefore algal metabolic activities might be reduced and led to the growth suppression. The intracellular oxidative stress was enhanced as the increasingly production and accumulation of the reactive oxygen species. Related intracellular oxidative damage might be the sub-results of UV-C irradiation.It was found that neither significant suppression nor disintegration effects on green algae were obvious for UV-C irradiation at 20²00 mJ·cm-2 in this study. The non-toxic C. ellipsoidea, C. vulgaris, and S. quadricanda were less sensitive than M. aeruginosa to UV-C irradiation, which might suggest the potential application of UV-C irradiation on M. aeruginosa bloom control with a predictable low ecological risk.The conventional processes adopted at Lichee Lake were insufficient to control algal growth. The application of UV-C irradiation was proposed for in-situ suppression on algal blooming.