The Transfer Of Cadmium Or Nickel Along The Planktonic Food-chain And Its Physio-biochemical Effect

Heavy metal pollution is a global environmental problem. Thus, environmental monitoring is essential to restore and resolve environmental pollution. Although chemical and physical methods are indispensable to environmental monitoring, they can not provide biological information, hence never satisfying the goal of modern environmental-monitoring. Due to being capable of exhibiting biological information, biomarker playes a very important role in modern environmental-monitoring. However, it is a further challenge to put it into the practice of monitoring cosmically. In the present study, using copepod as the object, we examined the acute toxicity of Cd or Ni to four species of copepods (Tigriopus japonicus, Schmackeria dubia, Calanus sinicus and Apocyclops borneoensis). Meanwhile, the response of the copepods (T. japonicus and S. dubia) to Cd or Ni additions was investigated under laboratory-controlled conditions in the 12 d exposure at the biochemical level (SOD, GPx, GST, AChE, GSH, MDA and GSH/GSSG), so as to discuss metal toxicity and the associated response of copepod, along with sieving out suitable biomarkers to metal pollution. We also investigated the response of MT levels in the copepod T. japonicus to different exposure duration (1, 4, 7 and 12 d) under different metal treatments (Cd or Ni), in order to expatiate the induction mode and mechanism of MT in the copepod under metal stress. Using S. dubia as the object, which had experienced the metal (Cd or Ni) exposure for 12 d, we also studied the influence of different metal treatments on the ultrastructure of muscle and ovary in the copepod, so as to reflect the metal toxicity. Additionally, the influence of metal additions on the reproductive ability of the copepods (T. japonicus, S. dubia and A. borneoensis) was quantified, in order to expound the response of copepod population to metal stress. Finally, We examined the influence of different nitrogen levels and sources on Ni uptake by Prorocentrum donghaiense and its distribution in cellular substructure and biochemical components of algae using iotope approach, to discuss the possible effect mechanism of the ambient nutrient level and source to Ni uptake by algae. Meanwhile, we also investigated the effect of macronutrient (nitrate or phosphate) additions to Ni uptake by phytoplankton (P. donghaiense and Skeletonema costatum) and its subsequent transfer to marine copepods (C. sinicus and Labidocera euchaeta), i.e., to discuss the influence of different nutrient levels on the transfer of Ni along the planktonic food-chain. The results of the present sturdy are described as follows:1. By contrast to Ni, the 24 and 48 h-LC₅₀ values of Cd to the copepods are smaller, hence the copepods showing more susceptivity to Cd than Ni. In the comparison concerning the 24 and 48 h-LC₅₀ values of Ni to different copepods, T. japonicus displayed the biggest value, and C. sinicus showed little difference from S. dubia, both of which showing a smaller value. However, in the Cd comparison, A. borneoensis held the biggest value, T. japonicus stayed at the second position, and C. sinicus had the smallest value. Therefore, the metal toxicity to copepod was correlated with metal classes and organism species.2. Different metal (Cd or Ni) treatments significantly influenced the variant biochemical indexes (SOD, GPx, GST, GSH, MDA and GSH/GSSG), with the change of these indexes showing a relation with metal concentration and duration, implying that these biochemical parameters could be biomarkers of metal pollution, especially for GSH. Regardless of different metal treatments and durations, GSH always firstly responsed, and almost suffered the inhibition-effect, suggesting that this parameter could be a good biomarker in copepod to metal pollution. Meanwhile, both Cd and Ni exhibited the neurotoxicity to copepod, because different metal additions markedly influenced the copepod AChE activity. However, the metals showed their neurotoxcity, more by enhancing the activity of AChE in copepod. Nevertheless, after 12 d exposure of Cd or Ni, at more extent, MDA level in copepods are little correlated with metal treatments. Therefore, it is cautious to choose this parameter as biomarker of oxidative injury, and other biomarkers should be involved, so as to reflect the extent of oxidative injury more comprehensively.3. During the 12 d exposure, heavy metal (Cd or Ni) not only induced the synthesis of MT in the copepod T. japonicus, but also displayed the inhibition-effect, i.e., the change of MT levels in the copepod was correlated with metal concentration and duration. According to the ultrastructure in the copepod S. dubia, heavy metal (Cd and Ni) had already brought the oxidative injury to the animal, due to the muscle being oxidatively injuried and the germ cell suffering from apoptosis.4. Different metal additions influenced the reproductive ability of the copepods, and the effect was associated with metal classes and animal species. Especially, different Cd or Ni significantly inhibited the reproductive capability of T. japonicus, illuminating that the metal effect had reflected at the population level of this copepod.5. Under both high- or low-Ni treatments, different nitrogen levels and sources both significantly influenced Ni uptake by P. donghaiense, with its uptake increasing with an increase of nitrate concentration and being enhanced markedly using urea as the nitrogen source. After being uptaken by P. donghaiens, Ni distribution in cellular substructure was also affected by different nitrogen levels and sources, e.g., Ni content in cell wall and soluble substance decreased, but the content in organelle increased, with an increase of nitrate level, and furthermore urea significantly facilitated the metal distribution in cell wall. Meanwhile, most of Ni distributed in soluble substance (> 70 %). Similarly, the nitrogen concentration and source also affected the Ni distribution in different biochemical components. Ni content in carbohydrate decreased, but the protein-Ni level increased, with an increase of nitrate level. Additionally, urea markedly promoted Ni distribution in proteins, but decreased its content in carbohydrate. However, the lipid-Ni content showed little correlation with different nitrogen levels and sources. Additionally, Ni mainly distributed in proteins (> 50 %).6. Ni uptake by phytoplankton after 24 h of exposure was markedly dependent on nutrient conditions, with a higher nutrient quota facilitating Ni accumulation in the algae. Trophic transfer was quantified by measurements of the Ni assimilation efficiency in C. sinicus and L. euchaeta, feeding on the algae under different nutrient treatments. Ni assimilation efficiency generally increased with an increase of nutrient concentration in the algae. However, ambient nutritional conditions had little effect on the physiological turnover rate constant of Ni by copepods, which was more affected by the physiological status of copepod. For example, feeding on the diatom-Ni under P treatments, in comparison to L. euchaeta, the physiological turnover rate constant of Ni in C. sinicus increased approximately 2 times. A significant positive correlation was found between the Ni assimilation efficiencies of the copepods and the % intracellular Ni in the algal cells, testifying the hypothesis "copepod only assimilates metal in the 'cytoplasmic' pool. Thus, nutrient-enrichment may lead to an increase in Ni uptake and transfer in the marine plankton.