Comparative Study On The Process And Mechanism Of Vanadium(?) Reduction By Different Gram Bacteria

Vanadium is a transition metal element widely existing in the earth's crust.With the development of modern industry,the phenomenon of vanadium pollution in the environment is becoming more and more serious,in which groundwater pollution has been widely concerned.Although the prospect of bioremediation of vanadium-contaminated environment has been widely recognized,the reported functional species are very limited,most of which are gram-negative bacteria.However,little is known about the bioremediation of vanadium by gram-positive bacteria that exist widely in the environment.Typical Gram-postive bacteria Bacillus subtilis and Gram-negative Thauera humireducens Sg Z-1 could effectively reduce V(?).When initial V(?)concentration was 50 mg/L,the reduction efficiency of V(?)were 84.33±1.57%and 91.43±1.80%by Bacillus subtilis and Thauera humireducens Sg Z-1 after 10 days.The substrate utilization and growth process of V(?)removal by Bacillus subtilis and Thauera humireducens Sg Z-1 accords with the classical Monod equation.The reduction of V(?)with ethanol as carbon source followed the pseudo-first-order kinetic equation,and the kinetic rate constants were 0.21 d^-1 and 0.16 d^-1,respectively.V(?)was bio-reduced to insoluble vanadium(?),which was mainly distributed in cells.Membrane fractions,cytoplasmic fractions and cell free extracts could effectively reduce V(?).Intracellular reduction was dominant in gram-positive bacteria Bacillus subtilis and extracellular reduction was dominant in gram-negative bacteria Thauera humireducens Sg Z-1.Intracellular V(?)reduction were catalyzed by NADH and nitrite reductase(NIR)encoded by nir K gene,and cytochrome c and extracellular polymeric substances(EPS)played a role in extracellular reduction.Thauera humireducens Sg Z-1 secreted riboflavins and quinones into solution to accelerate the extracellular reduction of V(?).Oxidative stress induced by reactive oxygen species(ROS)from dissimilatory V(?)reduction was alleviated through strengthened superoxide dismutase(SOD)and catalase(CAT)activities.EPS with chemically reactive hydroxyl(-OH)and carboxyl(-COO^-)groups also contributed to V(?)binding and reduction as well as ROS scavenging.This study can deepen the understanding of gram-positive bacteria and gram-negative for V(?)bio-detoxification and offer microbial resources for bioremediation of V(?)-polluted environment.