The Effects Of Heavy Metals On Phytoplankton Growth And Community Structure In Taihu Basin

In recent years, heavy metal pollution has raised widespread concern because it has become a major factor affecting water quality. Heavy metal pollution not only affect the growth of phytoplankton, but also has an impact on the ecological function of phytoplankton with the increase of the heavy metal in surface water. Therefore, content of the heavy metals in the surface water and phytoplankton community composition in the Taihu Basin have been surveyed. At the same time, we have studied the inhibitory effect of different heavy metals on phytoplankton growth, and the inhibitory mechanism of Cu on Anabaena flos-aquae via the physiological, biochemical detection and gene expression analysis. From these efforts, it has provided a theoretical basis for the scientific evalutation of ecological risk of heavy metals. The main research results are shown as follows:1. A total of 148 species of phytoplankton from 71 genera, seven phylum have been identified in the Taihu Basin. The structure of phytoplankton community showed a temporal change. The phytoplankton biomass in the wet season was highest at 67.29 mg-L-1, while the biomass in the dry season was lowest with only 11.14 mg·L-1. The diversity of phytoplankton in the flat season was higher than that in the wet season, and in the dry season, the phytoplankton diversity was the lowest. Heavy metals in the Taihu Basin showed a spatial and temporal difference. The order of the content of six heavy metals was Mn>Zn>Cu>Cr>Pb>Cd. In the wet season, the contents of Zn, Mn, Cd and Pb was higher than the other two seasons, with 18.11μgL-1,30.01μgL-1,0.019 ug·L-1 and 0.099μgL-1, respectively. In the flat season, the contents of Cr and Cu was higher than the other two seasons, with 1.26 μg·L-1 and 4.27 μgL-1, respectively. And the heavy metal content at each point was significantly different. Canonical correspondence analysis and Pearson correlation analysis showed that phytoplankton community in the dry season was obviously affected by Cu, Zn, COD and DOC. In the flat season, it was significantly associated with TP and TN. In the wet season it was affected by temperature, TN, turbidity and Zn significantly. Phytoplankton community was affected by nutrients and temperature largly. However, in the dry season and wet season, heavy metals, such as Cu and Zn, could affect phytoplankton community structure.2. The inhibition on six kinds of phytoplankton (Anabaena flos-aquae, Microcystis aeruginosa, Synechococcus, Selenastrum dibraianum, Scenedesmus obliquus, Chlorella vulgaris) showed dose-dependent and time-dependent behaviors by six heavy metals (zinc(Zn), cadmium(Cd), copper(Cu), chromium(Cr), manganese(Mn) and lead(Pb)). The inhibitory effect on Cyanophyta was significantly greater than that on Chlorophyta. The higher the concentration was, the stronger the toxic effects. Overall, the heavy metal toxicity followed the order of Cu≈=;Zn;≈Cr>Cd>Pb>Mn, while there was a big difference between different heavy metal toxicity to the same species of phytoplankton. The specific performance was shown as follows:(1) The heavy metal toxicity to Microcystis aeruginosa was in the order of Cr>Zn≈Cd≈Cu>pb>Mn. (2) To Anabaena flos-aquae, the heavy metal toxicity was in the order of Cu>Cr>Zn≈Cd>Pb>Mn. (3) To Synechococcus, the order of heavy metal toxicity was Cu≈Cr≈Cd≈Zn>Pb>Mn. (4) To Chlorella vulgaris, the order was Cu^Cd>Cr>Zn>Pb>Mn. (5) To Scenedesmus obliquus, the order was Cu>Cd≈ Zn>Cr>Pb>Mn. (6) To Selenastrum dibraianum, the order was Cr>Zn>Cu>Cd>Pb>Mn.3. The growth of Anabaena flos-aquae was inhibited under copper stress compared with the control group. As the stress concentration increased, the degree of inhibition enhanced. Under copper treatment, the survival ratio of Anabaena flos-aquae was significantly decreased, and cells grew slowly. The expression of cell division gene ftsH, and atpB gene was significantly decreased showed by quantitative PCR. Photosynthetic activity was inhibited, and photosynthesis-related genes, psaC and psbB, was inhibited, while the expression of nblA gene, which is related to the degradation of phycobilisome was increased. The level of reactive oxygen species was significantly increased. The membrane structure of Anabaena flos-aquae was changed after high concentration of copper exposed. Meanwhile the fragmentation of DNA occurred, and the genetic material was damaged.