The Study On Inhibition Mechanism Of Cyanobacteria Growth By Irradiation Treatment

The eutrophication of natural water in lakes and drinking water reservoirs has led to an incease in the incidence of blue-green algae. The presence and excessive growth of algae in water poses a significant risk to drinking water treatment. To minimize the threat posed by algae, they have been removed from water by various methods. However, the existing methods are proved to be inefficient or secondary environmental pollution. Thus, new methods are needed to prevent algae from fast and excessive growth in the water sources effectively.As advanced oxidation technologies, gamma-ray irradiation and dielectric barrier discharge have been widely used in environmental protection field. The two technologies have the advantages of easy operation (room temperature and pressure) as well as high availability and no secondary pollution. They are appropriate to remove the environmental pollutants which are difficult to treat by conventional techniques.This thesis explored the suppression effectiveness of typical bloom algae growth under gamma-ray irradiation. Cyanobacteria microcystis aeruginosa and anabaena sp. were selected. The effects of gamma-ray irradiation on chlorophyll, carotenoid, phycocyanin and several antioxidative enzymes (SOD, POD and CAT) were discussed. The changes of surface and cellular ultrastructure in microcystis aeruginosa and anabaena sp. cells befor and after gamma-ray treatment were investigated to probe into the mechanism of algal suppression effect. Meanwhile, the removals of cyanobacteria in natural water by gamma-ray irradiation and dielectric barrier discharge were also discussed. The research results would provide the theoretical basis for the application of this technology. According to the physiological characteristics of cyanobacteria, the inhibition of overwintering cyanobacteria recovery in Lake Taihu by gamma-ray irradiation was also discussed, which provide us a new approach to the control of cyanobacterial bloom. The main results are showed as follows:(1) It was found that the chlorophyll a concentration of microcystis aeruginosa (the initial concentration of chl-a was5.62mg/L) suspensions decreased as the dose increased at five days after gamma irradiation, and the removal efficiency of chlorophyll a was98%when a dose of9kGy was administered. The solution pH influenced the removal of microcystis aeruginosa, with a higher removal efficiency being observed under acidic conditions than in neutral or alkaline conditions. The OH and H2O2derived from the irradiated water play an important role in microcystis aeruginosa removal. Gamma irradiation had an adverse effect on photosynthetic pigments in microcystis aeruginosa cells. Specifically, the levels of Carotenoid and phycocyanin in microcystis aeruginosa cells were reduced considerably in response to increasing doses of radiation. The gradually increased MDA in irradiated microcystis aeruginosa cells following lipid peroxidation indicated oxidative stress in the cells. During oxidative stress, active oxygen species (AOS) trigger the activity of several antioxidative enzymes (SOD, POD and CAT). Relatively low gamma irradiation doses enhanced the activity of antioxidative enzyme. However, high doses of gamma irradiation reduced the activity of antioxidative enzyme in microcystis aeruginosa cells. Based on the SEM and TEM images, the thylakoid in microcystis aeruginosa cell was lysised after gamma-ray irradiation treatment, which disrupted the absorption and transportation of light energy, affected energy transformation and electron transportation, even terminated these process. That induced insufficient supply of both energy and organic materials in microcystis aeruginosa cells. Therefore algal metabolic activities might be reduced and led to the growth suppression.(2) The chlorophyll a concentration of anabaena sp.(the initial chl-a concentration was5.62mg/L) suspensions decreased as the dose increased at five days after gamma irradiation, and the removal efficiency of chlorophyll a was98.7%when a dose of11kGy was selected. Gamma irradiation had an adverse effect on photosynthetic pigment and protein system in anabaena sp. cells. The activity of several antioxidative enzymes (SOD, POD and CAT) increased with relatively low gamma irradiation doses. However, the activity of these enzymes significantly decreased with relatively high gamma irradiation doses. The concentration of MDA increased with the increase of irradiation dose. As shown in the SEM and TEM images, some irradiated anabaena sp. cells atrophied, and the thylakoid lysised. So, the normal physiological function of cell was damaged.(3) The bloom cyanobacteria in Taihu Lake could be effectively removed by gamma-ray irradiation. The removal efficiency of cyanobacteria was95.5%at five days after gamma-ray irradiation when a dose of lOkGy was selected. The contents of photosynthetic pigment and protein considerably decreased as the increase of radiation dose. The low gamma irradiation doses enhanced the activity of SOD and POD in cyanobacterial cells. However, the relatively high gamma irradiation doses reduced the activity of these enzymes. The increase of MDA concentration in irradiated cyanobacterial cells indicated the lipid peroxidation by gamma-ray irradiation.(4) The non-thermal plasma produced by dielectric barrer discharge could remove the bloom cyanobacteria in Taihu Lake. The effects of discharge power, air flow rate, various additives on the removal efficiency of cyanobacteria were discussed. When100W discharge power,1.0L/min air flow rate,18min discharge time,20001x light intensity and4d culture time at25℃were selected, the removal efficiency of bloom cyanobacteria was over87.8%(the initial Chl-a concentration of cyanobacteria was9.58mg/L). The removal efficiency was increased with the increase of discharge power and air flow rate. The contents of Carotenoid and malondialdehyde, the activity of SOD and POD in cyanobacterial cells were changed, and the intracellular materials of cyanobacteria were damaged after treatment.(5) Gamma-ray irradiation significantly inhibited the recovery of overwintering cyanobacteria in Taihu water. The increase of absorbed dose enhanced the inhibition effect. The ratio of chlorophyll a, chlorophyll b and phycocyanin in water with the dose of5kGy were8.66%,16.8%and17.2%to that in contrast sample after40days culture. The recovery of overwintering cyanobacteria in surface sediments was also inhibited by gamma-ray irradiation. When a13kGy dose of irradiation was selected, the ratio of chlorophyll a, chlorophyll b and phycocyanin in culture medium were18.8%,14.8%and20.5%to that in contrast sample after60days culture.