Hexavalent chromium reduction by novel chromate resistant alkaliphilic <i>Bacillus</i> sp. strain KSUCr9a

Abstract

Alkaliphilic bacterial strain termed KSUCr9a was isolated from soil and water samples collected from various soda lakes located in northern Egypt. KSUCr9a was tolerance up to 75 mM Cr (VI), with minimum inhibition concentration (MIC) value of 80 mM, in alkaline medium (pH 10.5) containing 10% NaCl. Analysis of 16S rDNA of strain KSUCr9a identified this bacterial strain as Bacillus sp., with sequence similarity of 99%, and was referred to as Bacillus sp. strain KSUCr9a. In addition to its tolerance to Cr(VI), Bacillus sp. KSUCr9a showed high resistance to other heavy metals including Cd²⁺ (50 mM), Mo²⁺ (75 mM), Mn²⁺ (100 mM), Cu²⁺ (2 mM), Ni²⁺ (100 mM), Pb²⁺ (75 mM), Co²⁺ (5 mM) and Zn²⁺ (2 mM). Bacillus sp. KSUCr9a demonstrated good chromate bio-reduction ability, as it could rapidly reduce up to 100 μM within 24 h. In addition, at initial Cr(VI) concentration of 200 μM, complete chromate reduction was achieved within 48 h. Furthermore, at initial Cr(VI) concentration of 300, 400 and 500 μM, 92.8, 75.5 and 39.8% of chromate reduction was achieved within 72 h. Bacillus sp. KSUCr9a was able to reduce Cr(VI) in a wide range of NaCl (0 to 20%), indicating the halotolerance nature of this alkaliphilic bacterial strain. Addition of glucose as an electron donor to the culture medium led to significant increase of both growth and chromate reduction by Bacillus sp. KSUCr9a. Maximum Cr(VI) reduction was exhibited in alkaline medium (pH 9) containing 0.8% glucose at incubation temperature of 35°C and under static culture condition. Under optimum Cr (VI) bioreduction conditions, 169.2 μM of Cr(VI) was completely reduced within 24 h, indicating a good ability of Bacillus sp. KSUCr9a of Cr(VI) detoxification under alkaline condition. Furthermore, Cr(VI)-reduction by Bacillus sp. KSUCr9a was slightly induced in the presence of other heavy metals, such as Mn²⁺, Co²⁺, Mo²⁺ and Cu²⁺ at concentration of 50 mg/L along with Cr(VI) in the culture medium. Moreover, Bacillus sp. KSUCr9a showed the ability of repeated bioreduction of chromate without any addition of exogenous nutrients, indicating its possible application in chromate detoxification.

Key words: Chromate reduction, bioremediation, heavy metals, Bacillus sp., soda lakes.