Isolation and characterization of novel potent Cr(VI) reducing alkaliphilic Amphibacillus sp. KSUCr3 from hypersaline soda lakes

Abstract

A strain KSUCr3 with extremely high Cr(VI)-reducing ability under alkaline conditions was isolated from hypersaline soda lakes and identified as Amphibacillus sp. on the basis of 16S rRNA gene sequence analysis. The results showed that Amphibacillus sp. strain KSUCr3 was tolerant to very high Cr(VI) concentration (75 mM) in addition to high tolerance to other heavy metals including Ni²⁺ (100 mM), Mo²⁺ (75 mM), Co²⁺ (5 mM), Mn²⁺ (100 mM), Zn²⁺ (2 mM), Cu²⁺ (2 mM) and Pb (75 mM). Strain KSUCr3 was shown to be of a high efficiency in detoxifying chromate, as it could rapidly reduce 5 mM of Cr(VI) to a non detectable level over 24 hrs. In addition, strain KSUCr3 could reduce Cr(VI) efficiently over a wide range of initial Cr(VI) concentrations (1-10 mM) in alkaline medium under aerobic conditions without significant effect on the bacterial growth. Addition of glucose, NaCl and Na₂CO₃ to the culture medium caused a dramatic increase in Cr(VI)-reduction by Amphibacillus sp. strain KSUCr3. The maximum chromate removal was exhibited in alkaline medium containing 1.5% Na₂CO₃, 0.8% glucose, and 1.2% NaCl, at incubation temperature of 40ºC and shaking of 100 rpm. Under optimum Cr(VI) reduction conditions, Cr(VI) reduction rate reached 237 µMh¹ which is one of the highest Cr(VI) reduction rate, under alkaline conditions and high salt concentration, compared to other microorganisms that has been reported so far. Furthermore, the presence of other metals, such as Ni²⁺, Co²⁺, Cu²⁺ and Mn²⁺ slightly stimulated Cr(VI)-reduction ability by the strain KSUCr3. The isolate, Amphibacillus sp. strain KSUCr3, exhibited an ability to repeatedly reduce hexavalent chromium without any amendment of nutrients, suggesting its potential application in continuous bioremediation of Cr(VI). The results also revealed the possible isolation of potent heavy metals resistant bacteria from extreme environment such as hypersaline soda lakes.