Bioremediation of mercury by some micro-algal isolates

Mercury is both persistent and toxic in terrestrial and aquatic ecosystems and can be accumulated in excess of permissible levels, leading to soil, surface, and groundwater contamination. This study was conducted to evaluate the potential of five microalgal isolates, namely, Synechoccocus aquatilis R.S. 03, Synechoccocus elongatus Lin 20M, Nostoc commune R.S. 02, Anabaena sp. Pal 05, and the unidentified isolate designated as Makalawang 03 for the bioremediation of mercury. The isolates were harvested at mid-log phase and then cultured in both the unmodified BG-11 medium, which served as the control and the modified BG-11 medium with 0.10 ppm HgCl2 for 28 days. Growth based on cell density and chlorophyll a was determined spectrophotometrically at 730 nm and 663 nm, respectively, at days 1, 5, 7, 11, 14, 18, 20, 22, 25 and 28. Heavy metal analysis was accomplished by exposing the isolates to 0.10 ppm HgCl2 for 48h, followed by harvesting if the cells and subjecting the supernatant to Atomic Absorption Spectrophotometry (AAS). Biomass-free modified medium served as control in this analysis. The difference between the metal concentration in the biomass-free modified medium and the remaining concentration in the supernatants was assumed to be taken up by the microalgae, which indicated the metal removal potential of each isolate. For all the five isolates, no significant difference was found between the growth rates of the cells cultured in the control and in the modififed medium, indicating that the cells are able to survive for mercury present. In the heavy metal analysis, the concentrations of mercury in the modified media where the microalgae were inoculated were lower than that in the biomass-free modified medium, indicating that they were able to remove some of the mercury from the solution. The amount of mercury taken up by the cells in the cultures of S. aquatilis R.S. 03, S. elongatus Lin 20M, Anabaena sp. Pal 05, and isolate Makalawang 03 were significantly different from those of the medium containing N. commune R.S. 02. S. elongatus Lin 20 M and Anabaena sp. Pal 05 removed equal amounts of mercury. S. aquatilis R.S. 03 showed the greatest potential because this isolate had higher growth rates in the modified medium based on cell density and chlorophyll a measurements and removed more mercury from the solution than the cells of the other isolates.