The interactions between phytoplankton and pollutants in eutropic freshwater : sufferer and vector

I Abstract The present thesis focused on two aspects of the interactions between phytoplankton and chemical pollution in aquatic systems. On the one hand, phytoplankton suffers from a diverse array of pollutants in aquatic systems. In this thesis, the growth inhibition of three groups of biocides towards two groups of phytoplankton was systematically investigated. The three cyanobacteria were generally more tolerant than the three green algae in response to the five herbicides and two fungicides (p<0.05). In contrast, the sensitivity of the two groups of phytoplankton to antibiotics was more complex. Significant differences were observed for clarithromycin, thiamphenicol, tetracycline and trimethoprim (p<0.001), while no significant difference for sulfamethoxazole was seen. Meanwhile, a non-targeted metabolomic approach was employed to gain more insight into the different responses of cyanobacteria and green algae under herbicide exposures (i.e., to atrazine and diuron). Among the major metabolic changes observed in response to herbicide exposure was the alteration of amino acids metabolism, such as phenylalanine, tyrosine, methionine, asparagine, glycine and serine. In comparison to Microcystis, the green algae were more sensitive to the herbicides as the cells resorted to amino acid catalysis pathways to obtain energy when the photosynthesis is constrained. A rapid response in central carbon metabolism was observed in green algae but not in Microcystis. In general, metabolites in the TCA cycle, CBB cycle, and pentose phosphate pathway were up-regulated while the levels of glycolysis pathway metabolites were decreased. Moreover, the accumulation of glycolate and glycoxylate in the photorespiration pathway were increased in the Microcystis but decreased in the green algae in response to high exposures of the herbicides. On the other hand, chemicals-tolerant phytoplankton taxa might act as vectors of the pollutants, especially in eutrophic freshwaters where a high biomass of phytoplankton is often observed. In this thesis, firstly endocrine disruptive compounds (EDCs) of varied mechanisms of action were chemically and bioanalytically investigated in different matrices from Taihu Lake (China). Results from cell-based bioassays suggested the occurrence of estrogenic, anti-estrogenic, anti-androgenic, and anti-glucocorticogenic activities, while no androgenic and glucocorticogenic activities were observed in the samples. Results from the chemical analysis suggested that EDCs were widely distributed in water, sediment and cyanobacterial bloom samples. In order to further evaluate the role of cyanobacterial blooms in the distribution of EDCs in eutrophic freshwaters, endocrine disruptive potentials in phytoplankton samples were evaluated throughout a year-long surveillance. Results of the Spearman correlation analysis concluded that the concentrations of ten EDCs were negatively correlated with cyanobacteria biomass, suggesting the potential occurrence of biomass dilution effects of EDCs due to the huge biomass of cyanobacteria during bloom season. The present study provides complementary information about the potential endocrine disruptive risks of cyanobacteria blooms in eutrophic freshwaters. Moreover, the vector effects of cyanobacterial bloom on inorganic pollutants were also evaluated. Besides the wide detection of several metals in cyanobacteiral blooms, biomass dilution effects were also found. Overall, this thesis constitutes the basis for a comprehensive understanding of the interactions between chemical pollution and phytoplankton for the management of eutrophic freshwaters.