The Acclimation Of Earthworms Under The Stress Of Heavy Metal (Cu)

Recently, adverse effect caused by heavy metals in contaminated soils, it gets a wide concern for many years. Earthworms constitute a major part of the animal biomass in terrestrial ecosystems and have a key role in soil maintenance. The use of earthworms to heavy metals from contaminated soil is the current hot research field; earthworms can improve the bioavailability of heavy metals by life activities and the interaction of microbes. The population of earthworm living in the mining area may develop mechanisms, they can tolerate or resist the effects of metal-induced stress. Though utilizing the acclimated earthworms, there were difficulties in collection and multiplication. Therefore, some researchers hoped to domesticate Eisenia foetida to improve the ability of resistance with heavy metals, which had strong abilities of adaptation and reproduction. In this study, we wanted to filter out the suitable conditions (different concentrations and exposed times) to domesticate earthworms, with the effects of the distribution of heavy metal, and the changes in the physiological and biochemical indexes of earthworms; then through the acute toxicity test and comet assay, to compare the different damages to the acclimated earthworms and non pre-exposed earthworms (CK). The results were as follows:The Cu fractions of acid extractable, reducible, oxidizable form were highly increased by acclimated earthworms than CK. the biological availability of heavy metal also increased. The heavy metals were turned to be helpful for the absorption of plants. The earthworms which were domesticated in different concentrations of Cu2+, cultivated in400mg·kg-1for30d, had a certain degree of stress. X100(pre-exposed in100mg·kg-1) had a stronger ability of protein synthesis than CK, and its rate of death was also lower. In the test, the catalase (CAT) of X100was significantly stronger than CK, and then the activity of malondialdehyde (MDA) was lower. But the different of superoxide dismutase(SOD), peroxidase (POD) activities were not significant between acclimated earthworms and CK. X100could resist the stress of heavy metal by regulating the enzymatic activities in the body. In the test of domesticating the earthworms with different time, we found the death rate of the earthworms which were pre-exposed for two weeks, was much lower than other exposed times. Through mortality rates and the changes of the forms of cooper, we found the cooper concentration of100mg·kg-1,2weeks of exposed time, were the optimum conditions for the domestication of earthworms.Eisenia fetida were exposed to a non-lethal copper concentration (100Cu2+mg·kg-1) for two weeks as acclimated earthworms(X), non pre-exposed earthworms as control (CK), and then assessed the toxicological effects of Cu2+on pre-exposed and non pre-exposed earthworms. The results showed that the50%lethal concentrations (LC50) of pre-exposed earthworms and non pre-exposed earthworms were321.83-542.45mg·kg-1and230.83～342.91mg·kg-1on14d, respectively. The survival rate of pre-exposed earthworms increased significantly.This paper studied the stress of copper (400mg·kg-1) on the dynamic DNA damage of pre-exposed earthworms by the comet assay. Our comet assay showed that the date from tail length, tail DNA content and tail moment of earthworms were non-normal distributed. On11d and14d, the level of DNA damage of X was much lower than CK. The results indicated that comet assay was an effective way to assess the genetic damage of earthworms under copper stress. DNA damage in earthworms can be used as biomarker to indicate the effect of heavy metals. The application of acclimated earthworm would supply a new way of phytoremediation of heavy-metal-polluted soils more economically and safely.