Isolation And Identification Of A Sulphate Reducing Bacteria And The Mechanism Research Of Its Removal Of Cadmium

With the development of industry,a large amount of waste water was discharged into the environment,which contains toxic heavy metals and sulfate with extremely high concentration.This waste water brings pollution to the environment and endangers the health of human beings,among which cadmium pollution is the most severe one.Compared with the traditional physical and chemistry methods,biological process is cheaper and more environmental friendly,whose effect can last for a longer period as well.Moreover,sulfate reducing bacteria(SRB)could produce hydrogen sulfide,which could form cadmium sulfide sediment with cadmium ion in the polluted water.Compared to the adsorption process,precipitate way is more quickly and its production is more stable.A cadmium-resistant bacterium was isolated from activated sludge and identified as Desulfovibrio sp.821 on the basis of phylogenesis.It showed optimum growth at 28?and pH 7.0.Desulfovibrio sp.821 was supposed to remove 87.7%and 95%Cd2+from aqueous medium of an initial cadmium concentraition of 5mg/L after 24 and 120 h,respectively.In an initial cadmium concentraition of 10mg/L,strain 821 was able to remove 47.2%and 70%Cd2+.Fourier transform infrared spectroscopy(FTIR)and atomic force microscope(AFM)analysis confirmed the biosorption of Cd2+and the formation of cadmium sulfide on the cell surface.The FTIR analysis also implied that there might be some polymer groups rich in carboxyl and hydroxyl,like polysaccharide,aliphatic acid and proteins involved in the Cd2+biosorption process.Besides,cadmium distribution results displayed a difference in the presence of Cd2+,which suggested that there might be an alteration of metabolism mechanism when the heavy metal pressure changed.Real-time relative quantitative PCR showed an alteration in the expression levels of czcA and a key regulatory gene in the dissimilation sulfate reduction process,dsrAB in the presence of Cd2+.CzcA gene,a component of czcCBA operon,was amplified from genomic DNA of strain 821,which suggested the chromosomal-borne Cd2+resistance.This special bacterium could provide an eco-friendly candidate for cadmium bioremediation process.However,most SRB are anaerobic bacterium.They usually form a mutualistic relationship with methanogens,which makes it hard to separate a single strain.Moreover,the valence of sulfur is variable and sulfur-containing compounds are much more complicated in metabolism,which restrict the deeper understanding of the sulfate reducing process.Thus,we choose a common and important metal,cadmium,as our study object.We believe that a better understanding of the mechanism of cadmium removal will have an important impact on the bioremediation application of SRB.