The Effects Of Pulsed Cadmium Exposure To Daphnia Magna

Aquatic organisms are always exposed to temporal fluctuations of contaminants. Surfacewater runoff and precipitation-associated hydrologic dilution and dispersion, as well asdegradation activity can generate pulsed exposures. The concentrations which were maximumjust after reaching a water body then change rapidly as a result of varying rates of input anddilution, changes in form, and solubility and degradation.The conventional toxicity testswhich are derived for an exposure scenario of fixed durations and constant contrations cannotaccurately model pulsed exposure. It is important to disscuss and perfect the effects andfoundation of assessment of pulsed exposure. The purpose of this study which exposedD.magna to different concentrations cadmium and the change of individual level(mortality,growth, reproduction, energy reserves, electron transport system, Haemoglobin concentration,etc.) and molecular level(hsp70, MT, SOD, CAT, etc.) were as indicators was used to reportthe physiological and biochemical responses of D.magna exposed to pulsed cadmium. Theresponses of offsprings of D.magna exposed were used to disscuss the shortcoming ofapproaches for laboratory toxicity testing of pulse exposure and implications forenvironmental safety concentration and ecological risk assessment.1. After classical48h acute exposure of traditional toxicology, Daphnia magna continuedto die and the highest mortality was up to five times as much as the end of48h exposure. Theresult implied that cadmium had a latent effect on mortality of D. magna. Latent effectoccurred when accumulated cadmium levels exceed the critical threshold of organism. Theaccumulated cadmium still caused physiological and biochemical responses and expressedtoxicity. Because of latent effect, the mortality caused by traditional LC50concentration wasmuch lager than50%. Classical acute toxicity test would underestimate the effects ofcadmium to D. magna.2. The mortality of single pulse increased with increasing concentration and duration. Thetotal mortality from two cadmium pulses decreased with increasing time interval betweenpulses. This phenomenon can be result of detoxification and elimination during the recoveryperiod. Pulsed exposure increased tolerance of D. magna, produced acclimation andadaptation. The relationship between mechanism established and ahead of duration andconcentration was still needed to explore.3. The exposure of mother could decrease the tolerance of F1generation and the value of48h-EC50decreased with the incresing of exposure concentration of F0. It is not right if weonly considered the responses of exposed organisms themselves. Ignored the effects of pulsed exposure to offspring would underestimate the results of it. Take the effects of pulsedexposure to offspring into account, we may better explain the decline in populations withoutapparent reason.4. After pulsed exposure, the hsp70levels of F0and F1increased with increasing exposedconcentration, but decreased with further increasing of it. The microevolution of hsp70inquantity did not express because of cadmium pulses, the hsp70level of F1was not affected byexposure of F0. But the hsp70level of F1was lower than F0experienced same cadmiumpulses, this may because the exposure of F0influenced the state of F1.5. The pulsed exposure of cadmium could greatly induce MT synthesis in D. magna. Thelevel of MT increased with increasing pulsed cadmium concentration but there was notdose-effect relationship. This may because shorter duration of pulsed exposure limited theinduction of MT or tureover ratios of D. magna increased. Becaused of latent effects, the MTlevel further increased during recovery period. This may be process of increasingdetoxification threshold and was the meaning of recovery period. The MT level induced bysecond cadmium pulse was2times the first one, this may be major physiological basis oftolerance. The MT levels of F1did not affected by F0exposure, but after same pulses, it washigher than that of F0and there was not threshold of it within the range of experiment. Thismay be the reason that the effect of MT was stronger than hsp70in cadmium detoxication.6. Elevated temperature decreased the cadmium tolerance of D. magna, but acclimationand exposure did not decreased the heat shock tolerance. This may because the acclimation ofcadmium induced more higher activity of antioxidant enzyme SOD (superoxide dismutase).The pulsed exposure of cadmium greatly limited the activity of SOD, but the activity of CAT(catalase) increased. The acclimation of temperature and cadmium reduced energy reserves ofD. magna, but there was not synergistic action between them. Pulsed cadmium exposurecould also reduce energy reserves and increase ingestion rates. The activity of ETS (electrontransport system) was not affected by acclimation of temperature and cadmium and pulses ofcadmium. Haemoglobin concentration was not increased by acclimation of temperature andcadmium, but higher concentration of pulsed exposure greatly increased haemoglobinconcentration.