ABSTRACT
In this study, selenite was found to be acutely toxic to the fresh water algae Anabaena constricta and Chlorella ellipsoidea. However, the latter was more tolerant. Severe reduction in chlorophyll a content was significant reaching 65.2%, 61.4% for Anabaena and Chlorella respectively at 30mg L[-1] Se after 24 hrs incubation. The EC50 values were: 138 mg SeL-1, 141 mg Se L[-1] at one day; 120 mg Se L[-1], 126 mg Se L[-1] at 3 days; 91 mg Se L[-1], 100 mg Se L[-1] at 5 days and 67 mg Se L[-1], 70 mg Se L[-1] at 7 days for Anabaena and Chlorella, respectively. Addition of the clay minerals kaolinite and bentonite decreased selenite toxicity considerably, bentonite was more effective. The EC50 values for combined effect of selenium and either kaolinite or bentonite in reducing growth after 7 days incubation were 76,83 mg Se L[-1] for Anabaena and 73, 81 mg Se L[-1] for Chlorella. Phosphorus uptake by Chlorella was enhanced and went parallel with a concomitant elevation in pH values of the medium with increasing selenite concentrations, and observation which was controversely to that exhibited in Anabaena cultures. Acid and alkalize phosphatase activity decreased in both organisms with increasing Se concentrations