Zinc tolerance of freshwater diatoms isolated from sites with zinc pollution; and pH effect on zinc toxicity

Sammanfattning: The aim with this study was to observe the zinc (Zn), tolerance for different strains isolated from sites with Zn pollution and to observe if lower pH decreases the Zn toxicity. Another aim was to see if malformations could be an indicator for Zn pollution. Zn is found naturally in the bedrock, sediment and waterbodies. Mining is one activity which had led to high values of Zn > 60 mg Zn l⁻¹ in some areas in Sweden. Today diatoms are used as indicators for organic pollution, the amount of nutrients and also for the pH condition. Due to problem with high values of metals, the diatom indicator has been developed further to also involve detection of metal pollution. Zn can affect diatoms growth rate, photosynthesis, cell division as well as their silica transport and thereby leading to malformed diatoms. In this study Zn toxicity was observed as decrease in number of cells, degree of malformations and as a decrease in the photosystem II efficiency. The hypotheses for the study are the following: the tested strains isolated from polluted sites would have higher tolerance to Zn than diatoms isolated from non- polluted sites, Zn would be less toxic in pH 5 compared to pH 7 and malformations could indicate Zn pollution. The observed results confirmed the hypothesis regarding the tested strains having higher tolerance towards Zn than diatoms from non- polluted sites, by the approximated EC ₅₀ values of 0,91; 1,35 and 2,27 mg Zn l⁻¹ respectively. However, the observed results for the hypothesis regarding the lower pH decreasing the Zn toxicity, were contradictory. The hypothesis was verified by higher EC ₅₀ values for the cell growth and fluorescence in pH 5 than in pH 7 for the short term experiments. Whereas the hypothesis was rejected by a higher degree of malformation in pH 5 than in pH 7 and by a higher cell growth in pH 7 than in pH 5 in the long term experiment. The different results regarding the pH effect on Zn toxicity might be explained by the different exposure times, probably the low pH is causing more stress to the cells pre-grown in pH 7 than it relieves the Zn toxicity. The long term experiment displayed significant more malformations in the higher Zn concentrations in comparison to the controls under both pH 5 and 7, and verified that malformations could detect high Zn concentrations and indicate Zn pollution.