Accumulation And Toxicity Effects Of Nano-TiO₂ In Aquatic Food Chain

The environmental bio-safety and potential negative effect of nano-materials have caused wide attention because of its peculiar structure and property.Although the individual plankton is tiny,they are various,widely distributed and at the bottom of the food chain in the freshwater ecosystem so that they play an important role in the structural and functional stability of the ecosystem.However,there were few reports about the inhibited growth of algae in water ecosystem,the changes in the population dynamics and life history traits of predator and the migration and accumulate in the tertiary aquatic food chain.For this purpose,the commonly aquatic plankton in water are taken as experiment subject to explore the toxic effect of nano-TiO₂ in Scenedesmus obliquus and Brachionus calyciflorus and to figure out the migration and accumulate of“S.obliquus—B.calyciflorus—Asplanchna brightwel”in the food chain.The experiment is designed to find out the potential ecological effect of nano-TiO₂ to freshwater phytoplankton and zooplankton so that to evaluate the toxic effect of nano-TiO₂ and provide a theoretical basis for the further application of nanomaterials.The research contents and results were listed as follows:?I?Under three-level illumination intensities?2000 Lux,4060 Lux and 7100 Lux?,the S.obliquus were exposed in different concentrations of nano-TiO₂ to investigate the inhibited growth of it and the effects of illumination intensity on the toxicity of nano-TiO₂.The results showed that the illumination intensity and concentration of nano-TiO₂had a great influence on the cell growth of S.obliquus.The increase of illumination intensity promoted the growth of algal cell density;With the increase of exposure concentration,the algae cell density under three-level illumination intensities was gradually inhibited while the algal cell volume only decreased significantly at the highest concentration?160 mg/L?of nano-TiO₂;With the extension of exposure time and the density of algal cell gradually increased,the ratio growth rate of algal cells cultivated with high illumination intensity under low concentration of nano-TiO₂?0?20mg/L?was the highest,and it cultivated with medium illumination intensity under high concentration?40?160 mg/L?of nano-TiO₂ was the highest.In addition,under the three illumination intensities of high,medium and low,the 72h EC₅₀ value of nano-TiO₂ was28.18 mg/L,43.65 mg/L and 79.43 mg/L while the 48h EC₅₀ value was 53.70 mg/L,57.54 mg/L and 954.99 mg/L.It can be seen that the extending exposure time and increasing illumination intensity can reduce the EC₅₀ value of nano-TiO₂ against S.obliquus and enhance the toxic effect of nano-TiO₂.?II?Acute and chronic toxicological studies were carried out on B.calyciflorus with different concentration of nano-TiO₂ by using population ecology and statistics.The results showed that the 48h LC₅₀ value of nano-TiO₂ was 30.20 mg/L and B.calyciflorus was more sensitive to toxicity of nano-TiO₂ than Daphnia magna.When the concentration of nano-TiO₂ is higher?>1 mg/L?,the maximum and mean population density,the mean population growth rate,the number of mean amictic egg and total production of female were significantly suppressed and the rotifers take less time to reach maximum population density.This illustrates that the intervention of nano-TiO₂has reduced the environmental accommodation and inhibited the population fertility of B.calyciflorus.Besides,the production of resting eggs in the treatment group with nano-TiO₂ concentration of 2.0 mg/L and 2.5 mg/L was significantly higher than that in the control group.When the concentration higher than 1.5 mg/L or even,the mean population mixed rate was also higher and this indicates that the exposure of nano-TiO₂had a conspicuous effect on the sexual production of B.calyciflorus.In addition,the exposure of nano-TiO₂ significantly shortened life expectancy and mean life span.It also reduced net reproductive rate and intrinsic rate of increase but had no obviously effect on generation time.It can be seen that the leakage of nano-TiO₂ in natural water can change the population dynamics of rotifers and other plankton in the freshwater ecosystem and thus affecting the structural and functional stability of the aquatic ecosystem.?III?The three-level food chain of S.obliquus—B.calyciflora—A.brightwelis was simulated in the laboratory to study the trophic transfer and accumulation of nano-TiO₂in the food chain.The results showed that nano-TiO₂ concentration,algae cell exposure time and the interaction between them have a significant effect on the accumulation of nano-TiO₂ by S.obliquus?P<0.01?.In other words,increasing the exposure concentration or prolonging the exposure time of S.obliquus in nano-TiO₂ can significantly promote the accumulation of nano-TiO₂ in algae cells.Secondly,nano-TiO₂ concentration,dietary exposure time and the interaction between them have a significant effect on the accumulation of nano-Ti O₂ by B.calyciflora?P<0.01?.With the delay of dietary exposure time,the accumulation of nano-TiO₂ in rotifers gradually increases and may reach saturation?especially at low concentrations?.Therefore,when the dietary exposure time is prolonged,the accumulation of nano-TiO₂ in B.calyciflorus which feed on algae cells containing different concentrations of nano-TiO₂ may tend to be consistent.Finally,nano-TiO₂ can be trophic transfer to secondary consumers through the food chain and accumulates in the A.brightwel.Furthermore,its biomagnificationfactor is between 0.94±0.19 and 9.62±0.17.It highlights that nano-TiO₂ has biomagnification phenomenon in the aquatic food chain.