Mercury reductase activity and Hg2+ lowering capacity of a Mercury Resistant Bacteria (MRB) (Bacillus sp. S1) which was isolated from Kalimas River of Surabaya, Indonesia were studied. The activity was determined by Mercury Reductase Assay System (MRAS) in a solution mixture contained 50 mM PBS (pH ± 7.0), 0.5 mM EDTA, 200 µM MgSO4, 0.1% (v/v) ß-merchaptoethanol, 200 µM NADH2 and 25 mg/L HgCl2 and one volume of crude extract incubated at room temperature for various interval period of time. Mercury reductase activity was measured spectrophotometrically at 340 nm. One unit of reductase activity was defined as one molar of oxidized NADH2 produced per total cell per minute. Results of study showed that, the isolate could resist the concentration of HgCl2 up to 11 mg/L. At 30 minute incubation period at room temperature, the highest mercury reductase activity and the Hg2+ lowering capacity was found to be 0.006 unit/109cell and
1.48 mg/L/109cell/minute, respectively with the reduction efficiency of Hg2+ to Hg0 of 0.18% per minute. Therefore, it can be concluded that the Bacillus sp. S1 isolate could be assumed to be exellent mercury bioremediation agent since it was found to be highly mercury resistant and very efficient to reduce cationic mercury (Hg2+) to elemental mercury (Hg0).

Keywords: Kalimas-Surabaya, Bacillus sp. S1, Mercury resistant, Mercury reductase

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