Toxic Effects And Mechanisms Of Mesotrione On Microcystis Sp. And Scenedesmus Quadricauda

The dispersion of organic xenobiotics such as pesticides and their residues inecosystems is of major public concern. Most of applied pesticides did not reach totheir targets but rather went into adjacent aquatic ecosystems through spray drift,surface runoff, and infiltration. As a consequence, residual pesticides are oftendetected as pollutants in aquatic ecosystems including underground streams, lakes,rivers or even costal sea waters. Herbicides in water are one of the ecologicalenvironment problems and it may have a negative impact on non-target aquaticorganisms. Algae are the major primary producers and a key componentin in aquaticenvironment, which drive the aquatic ecosystem energy and matter cycle and keep thebalance of the structure and function of aquatic ecosystem. The previous studiesshowed that algae are also quite sensitive to herbicides. The herbicides havesignificantly effects on the growth and community structure of algae. However, lessemphasis was placed on toxic effects of mesotrione on algae. In addition, data toillustrate the mechanism of mesotrione on freshwater algae on the relationshipsbetween species of algae, photosynthetic pigment, photosynthetic characteristics andantioxidant system was lake. In this paper, the difference between EC50-nominal(EC50based on nominal concentrations) and EC50-free(EC50based on freely dissolvedconcentrations) values was proposed. This paper was studied that the toxic effects andmechanism of mesotrione on growth, photosynthetic pigment content, photosynthesischaracteristics, antioxidant enzyme activities and antioxidant content of Microcystissp. and Scenedesmus quadricauda, competitive relationship between both algae, andthat bioaccumulation and biodegradation of mesotrione by both algae. All the resultsare summarized as followed:The growth of Microcystis sp. was stimulated under low level of mesotrione, andinhibited under high mesotrione concentrations. However, the growth of Scenedesmusquadricauda was inhibited under low level of mesotrione. The96h EC50-nominalvaluefor mesotrione on Microcystis sp. and S. quadricauda were9.76and7.15mg·L-1,respectively. The96h EC50-freevalue for mesotrione on both algae were6.19and4.41 mg·L-1, respectively. S. quadricauda is more sensitive to mesotrione than Microcystissp.. The toxicity of commercial mesotrione on both algae is higher than analyticalstandards mesotrione.Content of chlorophyll a in Microcystis sp. and S. quadricauda were decreasedunder mesotrione exposure, indicating that photosynthetic light energy capture ofalgae was impaired. Under the stress of mesotrione, the heat dissipation mechanism,energy conversion process and electron transfer process of photosynthesisin in bothalgae were impaired. The initial slope of RLCs of ETR (α), the maximal electrontransport rates in PSII (ETRmax) and the index of light adaptation of PSII (Ik, ETRmax/α)in both algae were decreased. Active oxygen species were generated in elicited cellsof both algae by mesotrione. Anti-stress reaction mechanism in S. quadricauda wasless sensitive than Microcystis sp..A certain proportion of mesotrione left in the medium was accumulated byMicrocystis sp. and S. quadricauda, and significant correlation existed between theaccumulation ability and the tolerance of algae to mesotrione. Compared with S.quadricauda, Microcystis sp. was much more likely to accumulate mesotrione.Although mesotrione was accumulated by both algae, the concomitant degradation ofmesotrione did not occurr during the process of its accumulation. MNBA(4-(methylsulfonyl)-2-nitrobenzoic acid) and AMBA (4-(methylsulfonyl)-2-aminobenzoic acid) was not found in the medium.Mixed cultivation had obvious effects on the population growth of Microcystissp. and S. quadricauda. Specific growth rate of both algae in mixed cultivation waslower than that in pure culture. The competitive relationship between both algae wasaffected by mesotrione. Under the stress of5、10mg·L-1mesotrione, unstable/stablecoexistence states of two species changed to Microcystis sp. displayed strongercompetitive inhibition than S. quadricauda.