The Toxic Effects Of Naphthalene On Scenedesmus Obliquus And Microcystis Aeruginosa

In this paper, the international standard algae species, Scenedesmus obliquus and the main species of algae bloom in lake, Microcystis aeruginosa were selected to determine the toxic effects of naphthalene, one of poly cyclic aromatic hydrocarbons and a persistent organic pollutant. The physiological and biochemical response of these two species to different naphthalene concentration levels were measured under monoculture and mixture treatments. The population growth as well as interspecific relationship was explored under different naphthalene stresses when two species grew together. The influences of naphthalene contamination on the primary producers and the community structure in the aquatic ecosystems were discussed. There were several conclusions made as below:(1)The EC50 values of S. obliquus and M. aeruginosa during 96h were 13.34mg / L and 44.26mg / L, repectively, which calculated from the data of the acute toxicity of naphthalene experiments. This result showed that the tolerance of M. aeruginosa to naphthalene was higher than that of S. obliquus.(2) In monoculture condition, the contents of chlorophyll, protein, MDA, SOD of algae were measured after 9 days growing at 0, 0.5, 5, 10 mg / L naphthalene solution. The physiological response of algae to naphthalene stress was first exhibited on the SOD activity, which was significantly induced for S. obliquus at 5 and 10 mg / L naphthalene solution. Contrarily, the photosynthetic pigments content (chlorophyll a, carotenoids) was inhibited by naphthalene. The effects of naphthalene on MDA and protein content were insignificant.(3) In mixture condition, the initial proportion of population density of S. obliquus and M. aeruginosa were set at 1:1, 1:2 and 1:4 under 0, 0.5, 5, 10 mg / L naphthalene solution, respectively. All of the mixtures were cultivated continuously for 9 days.The relationship of S. obliquus and M. aeruginosa was exploitation competition. In the 0 mg / L naphthalene solution, the S. obliquus was the dominant species in all three mixtures with different initial density ratios. According to Lotka-Volterra competition equation, S. obliquus won M. aeruginosa theoretically.Under the naphthalene stresses, the relationship of S. obliquus and M.aeruginosa changed. The inflexion point of population growth curve was advanced for these two species (4d for M. aeruginosa, 1d for S.obliquus). The competitive inhibition parameter of S. obliquus to M. aeruginosa is greater than the latter species to the former.None of the concentration-effect relationship was detected under the naphthalene stress, which might be caused by the decreasing concentration of naphthalene because of its volatilization. Theoretically, S. obliquus or M. aeruginosa both could be the winner in the competition, as the coexistence was unstable.(4) The naphthalene stress changed the number of primary producers and their relationship, which may have negative impact on community structure in aquatic ecosystems.