The Interaction of Fluroxypyr with the Green Alga Chlamydonomas Reinhardtii
|School||Nanjing Agricultural College|
|Keywords||Fluroxypyr Chlamydomonas reinhardtii Toxic effects Reactive oxygen species Degradation Enrichment|
Fluroxypyr [4-amino-3,5-dichloro-6-fluoro-2-pyridyloxyacetic acid] is a member of the pyridine class of herbicides. Fluroxypyr is an extremely effective post emergence herbicide that controls economically important broadleaf weeds and Alternanthera philoxeroides. Increasing usage of fluroxypyr results in higher risks from alternative products in regards to environmental burden. However, only a few reports have been published on the comparative toxicity of fluroxypyr toward the green algae and fewer reports are involved with biological responses of non target organisms induced by fluroxypyr. The introduction of fluroxypyr through runoff of rain and irrigation water into aquatic environments is likely to have adverse effects on non-target organisms such as algae. Algae are important components of the primary production, which are of initial importance in providing the energy that sustains fish and invertebrates in most aquatic ecosystems. Therefore, the action of herbicides on algae is not only important for the algae themselves, but also for the entire food web. In this study, C. reinhardtii was emploied as a test material because it is a powerful model system in bioassay experiments for investigating the toxicological responses of herbicides to eukaryotic organisms at the molecular level. The aim of the study is to elucidate the effects of excess fluroxypyr on the growth, physiological and biochemical responses of C.reinhardtii cells and to test the potential for C.reinhardtii to bioconcentrate fluroxypyr and trophically transfer the pesticide to invertebrate grazers.The effects of fluroxypyr on algae growth were investigated. Algae were cultivated with fluroxypyr at 0.05-1 mg L-1 for 2 days in dose-dependent experiments and incubated with 0.5 mg L-1 in time-denpendent experiments. Treatments with various levels of fluroxypyr induced only a slight but not significant increase in biomass as empressed by dry weight. The pattern of cell contents showed a typical’low-high-low’change. The contents of chlorophyll progressively increased with fluroxypyr concentrations at 0.05 and 0.25 mg L-1. However, further application of fluroxypyr up to 1 mg L-1 led to the significantly decreased contents A declined toxicity with the exposure time was also observed in this study. Our study revealed that chlorophyll content provided a sensitive and rapid system to determin the response of C.reinhardtii to fluroxypyr.The oxidative stress induced by fluroxypyr and the metabolic adaptation of C. reinhardtii were investigated. Production of reactive oxygen species (ROS) and DNA laddering as one of typical hallmarks of programmed cell death were observed in C.renhardtiiafter exposure to fluroxypyr. The levels of lipid peroxides, expressed as thiobarbituric acid reactive substances (TBARS), showed an increase, indicating oxidative damage to algae. To deal with oxidative damage, activities of antioxidant enzymes in C.reinhardtii substantially changed. Fluroxypyr induced the activities of APX, GST and EST at any level, whereas the activity of SOD was only stimulated at high levels. On the contrary to SOD, the activity of CAT showed a progressively increase at low fluroxypyr concentrations and a decraese at high fluroxypyr concentrations. Analysis of nondenaturing polyacrylamide gel electrophoresis (PAGE) confirmed these results.The bioaccumulation and removal capability of the freshgreen microalgae to fluroxypyr were studied. C.reinhardtii had the ability to accumulate and concentrate fluroxypyr and high removal capability at low fluxopyr concentrations. After 48 h exposure, accumulation of fluroxypyr in C.reinhardtii was greatly enhaced in cells with increases in initial fluroxypyr levels. The significantly larger BCFs were observed at observed at low levels of fluroxypyr. The BCF reached the peak level of 1173 after exposure to fluroxypyr at 0.25 mg L-1. The fluroxypyr biodegradation activity of C.reinhardtii evidently declined with further increases in initial fluroxypyr concentrations. Temporal varation of BCFs at 0.5 mg L-1 fluroxypyr by C.reinhardtii increased with exposure time and reached the maximum value of 271.01 on Day 4 and then declined. The biodegradation percentage of fluroxypyr at 0.5 mg L"1 increased with the expoure time.