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Heavy Metal Tolerance, Biosorption And Bioaccumulation By Some Microalgae (Egyptian Isolates)
Three fresh water microalgal isolates [Phormidium ambiguum (Cyanobacterium), Pseudochlorococcum typicum and Scenedesmus quadricauda var. quadrispina (Chlorophyta)] were tested for tolerance and removal of mercury (Hg2+), lead (Pb2+) and cadmium (Cd2+) in aqueous solutions as a single metal species at conc. 5-100 mg / L under controlled laboratory conditions. The obtained results showed that Hg2+ was the most toxic of the three metal ions to the test algae even at low concentration (<20 mg/L).While lower concentration of Pb2+ and Cd2+ (5–20 mg / L) induced insignificant changes in the algal growth (chlorophyll a and protein), elevated concentrations (40-100 mg / L) were inhibitory to the growth.. The results also revealed that Ph. ambiguum was the most sensitive alga to the three metal ions even at lower concentrations (5 and 10 mg / L). P. typicum and S. quadricauda were more tolerant to high metal concentrations up to 100 mg / L. The bioremoval of heavy metal ions (Hg2+, Pb2+ and Cd2+) by P. typicum from aqueous solution showed that the highest percentage of metal bioremoval occurred in the first 30 minutes of contact recording 97% (Hg2+), 86% (Cd2+) and 70% (Pb2+). Transmission electron microscopy (TEM) was used to study the interaction between heavy metal ions and P. typicum cells. At ultrastructural level, an electron dense layers were detected on the algal cell surfaces when exposed to Cd , Hg, and Pb. At the same time, dark spherical electron dense bodies were accumulated in the vacuoles of the algal cells exposed to Pb. Excessive accumulation of starch around the pyrenoids as well as deteriorations of the algal cell organells exposed to the three metal ions were also observed .
New Egyptian Journal of Microbiology Vol. 17 (2) 2007: pp. 76-88