Optimization of cultural conditions of Arthrobacter sp. Sphe3 for growth-associated chromate(VI) reduction in free and immobilized cell systems

Abstract

The current study aimed to characterize Arthrobacter sp. Sphe3 ability to reduce Cr(VI) in suspended cell cultures as well as in immobilized form using Ca-alginate beads. Adaptation studies in the presence of 5mgL−1 Cr(VI) showed a significant increase in specific growth rate from 0.25 to 0.3h−1 and bioremoval percentage from 64% to 94% (p<0.05), whereas Arthrobacter sp. Sphe3 could tolerate up to 50mgL−1 Cr(VI). Optimization of culture conditions resulted in complete reduction of 45mgL−1 Cr(VI) at 30°C, pH 8 and 10gL−1 of glucose. High glucose concentrations helped at reducing (80±2.4)% of initial 100mgL−1 Cr(VI), whereas the bacterial strain could tolerate 850mgL−1 Cr(VI). Cr(III) formation was first evidenced by the appearance of a green insoluble precipitate in the medium. Cell biomass was successfully immobilized in Ca-alginate beads that were evaluated for their stability. Cell release was sharply decreased when 4% Na-alginate was used under non-shaking conditions. Biotransformation efficiency was enhanced when 25–50mgcellsmL−1 Na-alginate from the exponential growth phase were collected and co-encapsulated with either 1% glucose and 0.5% (NH4)2SO4, or 1% LB medium. Immobilized biocatalyst could be reused up to 6 continuous cycles in the presence of 10mgL−1 Cr(VI), but its performance was lowered at higher metal concentrations comparing with free cells that significantly maintained their reducing ability up to 300mgL−1 Cr(VI).