| Nitrogen is one of major pollutants of water pollution, which cause eutrophication of wate, especially the aquaculture water. High nitrogen contamination often result in the disease of cultured fish and shrimp due to low conversion rate and high toxicity of nitrogen contamination. In recent years, many domestic and foreign researches and reports have confirmed the existence of aerobic denitrifying bacteria.This discovery broke through the traditional theory, provided a brank-new idea for the biological nitrogen removal under aerobic conditions. In the present paper, UV-irradiation was introduced to the mutagenesis of an aerobic denitrifying strain, then the denitrification characteristics of the mutant was determined, finally the muatant was introduced to the denitrifying of the culture water.An aerobic denitrifier (strain A762) was mutagenized by UV-irradiation and 8 mutants were selected for their higher ratios of G/C during the preliminary screening. A mutant (B25) with highest rate of TN removal was obtained. The aerobic denitrification characteristics of strains B25 and A762 were investigated. The results showed that: compared with strain A762, strain B25 grew faster and had a higher reduction rate of NO3--N and NO2--N. During the denitrification process of both strains, the pH values in the culture medium gradually increased, while the Oxidation-reduction Potential gradually decreased. The denitrifying capacity of strains B25 and A762 were determined in different nitrite concentration. The experimental results indicate that B25 has more active nitrite reductase and better adaptive growth in media different in nitrite concentration. When the initial NaNO2 concentration was 0.069g/L, NO2--N was reduced completely after 5 days cultivation with strains B25 and A762. However, when the initial NaNO2 concentration were 0.138g/L and 0.207g/L respectively, nitrite nitrogen loss through denitrification in the aquaculture water inoculated with B25 reached 100%,and nitrite nitrogen loss inoculated with A762 reached 72.811% and 33.33% in 5 days, respectively.The effect of environmental factors on growth and nitrogen removal ability of strains A762 and B25 were observed by one-factor and orthogonal experiments,in order to provide references in aquaculture wastewater treatment.The growth and aerobic denitrification performance were much different between A762 and B25 under each environmental factor. The strain B25 has better activity of nitrogen removal ability and better adaptive growth in media of different factor.It was found that aerobic denitrification performance of the strains A762 and B25 were greatly influenced by NaCl concentration, which maybe due to the reason that A762 comes from the sea. Results of orthogonal experiment were that the accumulation of NO2--N by the strain B25 was significantly less than that by the original strain A762 in six of the nine treatment groups.Nitrogen removal ability of strains B25 and A762 were evaluated in the aquaculture water.lt's found that the growth and nitrogen removal ability of strains B25 and A762 depended much on the quantity of tryptone,and nitrogen removal rate of strains B25 and A762 in aquaculture water with 0.25g/L tryptone were evidently removed nitrate and nitrite nitrogen. The strain B25 was more helpful for reducing the pollution of nitrate and nitrite nitrogen in water body, and nitrate and nitrite nitrogen loss through denitrification in the aquaculture water inoculated with B25 reached 21.790% and 48.741% in 4 days, respectively.In conclusion, the method of UV mutagenesis was successfully employed to enhance nitrite and nitrate reductase activitie of aerobic denitrifier demonstrated by this experiment, accordingly improve aerobic denitrification efficiency. The study showed that strain B25 had great potential in the biodenitrification of aquaculture in the future.
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