Reduction of chromate (CrO 2 4 ) by an enrichment consortium and an isolate of marine sulfate-reducing bacteria K.H. Cheung a , Ji-Dong Gu a,b, * a Department of Ecology & Biodiversity, Laboratory of Environmental Toxicology, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, PR China b The Swire Institute of Marine Science, The University of Hong Kong, Shek O, Cape dÕAguilar, Hong Kong SAR, PR China Abstract An enrichment consortium and an isolate (isolate TKW) of sulfate-reducing bacteria (SRB) have been obtained from metal-contaminated marine sediments of Tokwawan, Hong Kong SAR. These bacteria are capable of reducing highly toxic and soluble hexavalent chromium (Cr 6þ ) enzymatically into less toxic and insoluble trivalent chromium (Cr 3þ )underanaerobicconditions.Theenrichmentconsortiumalmostcompletely(98.5%)reduced0.6mMCr 6þ in168 handtherateofreductionwas0.5g(Cr 6þ )g(protein) 1 h 1 .Incomparison,withCr 6þ asthesoleelectronacceptor(as asurrogateforSO 2 4 ),isolateTKWreduced94.5%oftheinitiallyaddedCr 6þ (0.36mM)in288h,withtherateof0.26g (Cr 6þ )g(protein) 1 h 1 .Adsorptionbythesebacteriawasnotthemajormechanismcontributingtothetransformation orremovalofCr 6þ .ThebiomassandCr 3þ in the cultures increased simultaneously with the reduction of Cr 6þ .These indigenous SRB might have potential application in bioremediation of metal contaminated sediments. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: Hexavalentchromium;Metalreduction;Soleelectronacceptor;Marineenrichmentconsortium;Sulfate-reducingbacteria; Bioremediation 1. Introduction Chromium (Cr) is an essential trace metal for living organisms; however, its high toxicity, mutagenicity and carcinogenicity render it hazardous at very low con- centration (Venitt and Levy, 1974; EPA, 1998; McLean et al., 2000; Cheung and Gu, 2002). Medical research has further confirmed that hexavalent chromium (Cr 6þ ) exposureisassociatedwithincreasedhumanlungcancer risk (Gibb et al., 2000). Accordingly, chromium and its compoundsareplacedontheprioritylistoftoxicchem- icalsofmanycountriesincludingUSA,UKandCanada (Hedgecott, 1994). As the application of chromium is extensiveinvariousindustrieslikechrome-plating,wood preservation and alloy (especially important for the resistance of corrosion in stainless steel) formation, chromium-associated pollution is of increasing concern nowadays(Shakoorietal.,2000;GuandCheung,2001; Ryan et al., 2002). Apart from traditional physico- chemicaltreatments,anumberofbiologicalassaysusing microorganisms have been studied and developed to remedy chromium-contaminated water. The two major processesbeinginvestigatedareadsorptionofmetalson biological materials (i.e. biosorption) including cells of microorganisms and plants, and dissimilatory reduction of metal ions from higher valent state to lower one (i.e. biotransformation) through enzymatic reaction or indi- rectly with metabolite produced (Lovley, 1993; Wang andShen,1995;TobinandRoux,1998;Leeetal.,2000; Chemosphere 52 (2003) 1523–1529 www.elsevier.com/locate/chemosphere * Corresponding author. Tel.: +852-2299-0605; fax: +852- 2517-6082. E-mail address: jdgu@hkucc.hku.hk (J.-D. Gu). 0045-6535/03/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0045-6535(03)00491-0