Growth-dependent Cr(VI) reduction by Alteromonas sp. under haloalkaline conditions: toxicity, removal mechanism and effect of heavy metals

Abstract

Abstract Bacterial reduction of hexavalent chromium (Cr(VI)) to trivalent chromium (Cr(III)) is a sustainable bioremediation approach. However, Cr(VI) by bacteria is severely impeded by Cr(VI) toxicity and complex environmental conditions like salt, alkaline pH and heavy metals. Hence, there is a need for Cr(VI) reducing bacteria to thrive as well as to metabolize under complex conditions. This study investigated Cr(VI) reduction, toxicity and removal mechanisms under complex conditions using an Alteromonas sp. isolated from the aerobic granular sludge cultivated from seawater-borne microorganisms. Rapid and complete removal of 100 mg/L Cr(VI) was achieved within 24 h under haloalkaline conditions (salinity: 3.5 to 7.5; pH 8 to 11). This strain exhibited high tolerance to heavy metals under haloalkaline conditions and reduced 100 mg/l Cr(VI) within 24 h in the presence of 100 mg/L As(V), 100 mg/L Pb(II), 50 mg/L Cu(II) or 5 mg/L Cd(II). The toxicity of Cr(VI) on the bacterial cells was evident by the increased reactive oxygen species levels and inhibition of esterase activity. Regardless of Cr(VI) toxicity, the cells grew and efficiently reduced Cr(VI) to Cr(III). The bacterial Cr(VI) reduction was strongly dependent on the growth, necessitating actively growing cells and growth medium. While, resting cells and spent medium barely contributed to Cr(VI) reduction. The biochemical assays revealed efficient Cr(VI) reduction using a cytosolic protein fraction from Alteromonas sp. and an exogenous reducing agent (e.g., NADPH). This study demonstrates an efficient Cr(VI) reduction system for potential Cr(VI) bioremediation applications under complex conditions including extreme haloalkaline conditions and toxic heavy metals.