Ecological Effects Of Pulsed Arsenic Exposure To Daphnia Magna And Tetrahymena Thermophila

Most aquatic organisms in the natural environment are exposed to unstable concentration of pollutants due to the dilution of rainfall and emission of pollutants,as well as change of water flow velocities.Using a stable concentration in traditional toxicity test to evaluate the toxicity of a contaminant may overestimate the toxic hazard.Use of pulse exposure to measure the biological toxicity of pollutants is more close to the realistic natural environment.In this paper,two typical zooplankton(Daphnia magna and Tetrahymena thermophila)were tested to different concentrations of inorganic arsenic under continuous and pulse exposure scenarios.Survival rate,body length,reproductive number of D.magna,cell concentration of T.thermophila and the arsenic bioaccumulation of two kinds of zooplankton were taken as indicators to study the toxic effect of inorganic arsenic on zooplankton under different exposure modes.Results will provided the toxic effects of inorganic arsenic on zooplankton and the bioaccumulation of arsenic in zooplankton under different exposure scenarios,and also provided scientific data related to pulse exposure and richer methodological basis for the ecological risk assessment of freshwater ecosystems.This study is mainly divided into three parts;1)In this section,we studied the acute toxic effects of inorganic arsenic to D.magna as well as the bioaccumulation of arsenic by D.magna under continuous exposure and pulse exposure scenarios.Under different exposure scenarios,the48h-EC50 of iAs~?was greater than that of iAs~V,indicating that the toxicities to the iAs~?were higher than iAs~V.Secondly,at same concentration of inorganic arsenic,the toxicity of arsenic under continuous exposure was higher than that under pulse exposure.The arsenic bioaccumulation of D.magna with continuous exposure scenarios was significantly higher than that with pulse exposure scenarios.Meawhile,the bioaccumulation of iAs~V was higher than that of iAs~?under continuous exposure scenarios.However,there was no significant difference in the bioaccumulation of the two inorganic arsenic under the pulse exposure scenarios.2)In this section,we studied the chronic toxic effects of inorganic arsenic to D.magna and the bioaccumulation of arsenic by D.magna under pulse exposure conditions.In the chronic toxicity experiment,there was still a large number of deaths,and the mortality increased with the arsenic concentration increasing.Contrary to the acute toxicity experiment results,the death time of D.magna which exposed to iAs~V earlier than the iAs~?,this may be related to the different toxic patterns of inorganic arsenic on acute and chronic toxicity of D.magna.In addition,inorganic arsenic had negative effects on reproductive rate and body length of D.magna in chronic toxicity test.3)In this section,we studied the acute toxic effects of inorganic arsenic to T.thermophila and the bioaccumulation of arsenic in T.thermophila under continuous exposure and pulse exposure scenarios.For the T.thermophila,the toxicity of iAs~?was stronger than that of iAs~V.Comparing the toxic effects of D.magna and T.thermophila to arsenic,it was found that the T.thermophila had stronger tolerance to arsenic.Under the condition of the same concentration of inorganic arsenic,the concentration of T.thermophila cells under continuous exposure was lower than that under pulse exposure.The explanation for this phenomenon was that the pulse interval in pulse exposure provided the recovery time for T.thermophila,or the exposure stage in pulse exposure had a domestication effect on T.thermophila.In addition,the bioaccumulation of arsenic in T.thermophila under continuous exposure mode was significantly higher than that under pulse exposure.The bioaccumulation of iAs~V was higher than that of iAs~?under continuous exposure scenarios.However,there was no significant difference in the enrichment of the two inorganic arsenic under pulse exposure.