Effects Of Cadmium On Food-intake, Growth, Reproduction And Energy Metabolism In The Water Flea, Daphnia Magna (Cladocera)

Cadmium is a toxic heavy metal,which is from the industrial production,artificial phosphate fertilizers and pesticides.Naturally a very large amount of cadmium is released into the environment,about 25,000 tons a year.About half of this cadmium is released into rivers and streams.In recent years,a new type of cadmium-containing material,cadmium-based quantum dots,has been extensively used in the research of life sciences and medicine.As a result,the pollution of cadmium is becoming more and more serious.Daphnia magna,a small aquatic crustacean,is considered a“keystone”species in ecological food webs and is widely distributed in the freshwater environment,and it is sensitivity to biotoxicity of chemicals makes it an ideal species to assess freshwater quality.In this study,we have investigated the effects of cadmium(Cd²⁺)on the food intake,growth and reproductive capacity in dapnia.The experiment setup three different concentrations of Cd²⁺（0.01 mg/L,0.05 mg/L and 0.09 mg/L）and one control group,for 24 h and 48 h.And there were analyzed from the feeding rate and filtration rate,tissues accumulation of Cd²⁺,total antioxidant capacity（T-AOC）,contents of malonyldialdehyde（MDA）,activities of acetylcholine esterase（AChE）,nitric oxide synthase（NOS）activity and contents of nitric oxide（NO）in daphnia when exposed to Cd²⁺.By high performance liquid chromatography（HPLC）,the contents of ATP,ADP and AMP in daphnia were determined.And the ratio of ATP/ADP,ATP/AMP,total adenine nucleotide（TAN）and energy charge（EC）were calculated,Using pH colorimetry,the activity of arginine kinase（AK）in water flea was determined.In addition,a 21-day chronic toxicity test was used.The effects of cadmium stress on the survival rate,the number of molting,reproductive ability,the main reproductive parameters,and the body size of daphnia at the end of the experiment were determined.The experimental results showed that:（1）The feeding and filtration rates of D.magna decreased with increasing Cd²⁺concentrations.Especially,when the water flea was exposed to 0.09 mg/L Cd²⁺for 48h,the feeding rate and filtration rates dropped dramatically（P<0.05,P<0.01）,and the feeding rate was decreased about43.89%compared to the control group.With the increase of Cd²⁺concentration,the accumulation of Cd²⁺in body was significantly higher in the experimental groups than in the control group.The total antioxidant capacity（T-AOC）and content of MDA in the water flea were increased gradually with the concentration level of Cd²⁺.In the group with Cd²⁺concentration of 0.09 mg/L,the levels of T-AOC and MDA increased significantly（P<0.05）compared to the control group.T-AOC was negatively correlated with the feeding rate whose correlation coefficient R² was 0.9521and 0.9389 for 24h and 48h,respectively.The activity of AChE was decreased with increasing Cd²⁺concentration when exposed to 0.05 and 0.09mg/L（P<0.05）.The change trend of NO content was similar to that of NOS,and both increased with the increase of Cd²⁺concentration.At Cd²⁺concentrations of 0.05 mg/L and 0.09 mg/L,the NOS activity was significantly or significantly different from that of the control group（P<0.01,P<0.05）.（2）The survival rate,body length,and moulting times of Daphnia magna were decreasing with the increase of Cd²⁺concentration.With the increase of Cd²⁺concentration,the intrinsic rate of increase（r_m）,the net reproductive rate（R₀）,the cycle growth rate（λ）,and the total reproductive rate（G₀）all showed a decrease;In the early stage of breeding,breeding period and second breeding time are extended.However,there was no significant change in the average time（T）of the generations.（3）With the increase of Cd²⁺concentration,the total ATP content of Daphnia magna showed a decreasing trend.At Cd²⁺concentration of 0.09mg/L,the ATP content was significantly lower than that of the control group（P<0.01）;the ADP content and TAN appeared to rise first and then decreased,and the Cd²⁺concentration was 0.01 mg/L for 24h,and the ADP and TAN contents were extremely high.In the control group（P<0.01）,Cd²⁺treatment for 48h,the content of ADP and TAN was significantly higher than that of the control group（P<0.01）at the concentration of 0.05 mg/L,and the concentration of ADP and TAN were both at the concentration of 0.09 mg/L.Significantly lower than the control group（P<0.01）;AMP content showed an increasing trend,at the concentrations of 0.01 and 0.05 mg/L at 24h after cadmium treatment,AMP content in Daphnia magna was significantly higher than that in the control group（P<0.01）.When cadmium was treated for 48h,the AMP content of each concentration group was significantly higher than that of the control group（P<0.01）;EC,ATP/ADP ratio,ATP/AMP ratio and AK activity showed a decreasing trend.The ATP/ADP ratio and ATP/AMP ratio in each treatment group were significantly or extremely significantly lower than those in the control group（P<0.05 or P<0.01）.Comprehensive research results draw conclusions:The accumulation of cadmium in Daphnia magna not only causes oxidative damage to the body,but also causes abnormalities in the conduction function of the nervous system,resulting in inhibition of feeding capacity of Daphnia magna.At the same time,it also interferes with the energy metabolism in Daphnia magna,which eventually leads to serious effects on the molting,growth and reproduction of Daphnia magna.