ORIGINAL ARTICLE Hexavalent chromium reduction by bacterial consortia and pure strains from an alkaline industrial effluent H.A. Pin ˜o ´ n-Castillo 1 , E.M.S. Brito 2 , M. Gon ˜ i-Urriza 3 , R. Guyoneaud 3 , R. Duran 3 , G.V. Nevarez-Moorillon 4 , J.F. Gutie ´ rrez-Corona 1 , C.A. Caretta 5 and G.E. Reyna-Lo ´ pez 1 1 Departamento de Biologı ´a, Divisio ´ n de Ciencias Naturales y Exactas, Universidad de Guanajuato, Guanajuato, Gto., Mexico 2 Departamento de Ingenierı´a Civil, Divisio ´n de Ingenierı´as, Universidad de Guanajuato, Guanajuato, Gto., Mexico 3 Equipe Environnement et Microbiologie, IPREM UMR CNRS5254, Ba ˆ timent IBEAS, Universite ´ de Pau et des Pays de l’Adour, Pau Cedex, France 4 Facultad de Ciencias Quı´micas, Universidad Auto ´ noma de Chihuahua. Chihuahua, Chih., Mexico 5 Departamento de Astronomı´a, Divisio ´ n de Ciencias Naturales y Exactas, Universidad de Guanajuato. Guanajuato, Gto., Mexico Introduction Chromium is a metal existing under several oxidation states, ranging from )2 to +6. Among them, Cr(VI) and Cr(III) are of major environmental significance because of their persistence and stability. Cr(VI) is a strong oxi- dizing agent, commonly present as hydrochromate (HCrO 4 ) ), chromate (CrO 4 ) ) or dichromate (Cr 2 O 7 ) oxyanions, depending on pH (United States, Environ- mental Protection Agency, US EPA 1998). Because of its high solubility in water (Losi et al. 1994; Barceloux 1999), Cr(VI) is highly toxic and has been listed as one of the most dangerous substances by the US EPA (1998). Cr(VI) compounds are mutagenic, carcinogenic (Losi et al. 1994) and inhibit enzymes and nucleic acid synthesis (Gunarat- nam and Grant 2008). By contrast, Cr(III) is less toxic and much less mobile. It forms stable complexes with organic ligands (Zayed and Terry 2003) and precipitates at physiological pH as hydroxide [Cr(OH) 3 ] or hydrated oxide (Cr 2 O.H 2 O) (Ehrlich 2002). In contaminated soils and industrial wastes, chromium availability is influenced by many processes such as organic and inorganic complexes formation, oxidation ⁄ reduction, precipitation ⁄ dissolution or adsorption ⁄ desorption. These Keywords adsorption, bacterial communities, chromium (VI), reduction, t-RFLP. Correspondence Georgina E. Reyna-Lo ´ pez, Departamento de Biologia, Divisio ´ n de Ciencias Naturales y Exactas, Universidad de Guanajuato. Cjon. de jalisco s ⁄ n, Col. valenciana, 36000 Guanajuato, Gto., Mexico. E-mail: reinalg@quijote.ugto.mx 2010 ⁄ 0663: received 22 April 2010, revised 5 August 2010 and accepted 23 August 2010 doi:10.1111/j.1365-2672.2010.04849.x Abstract Aims: To characterize the bacterial consortia and isolates selected for their role in hexavalent chromium removal by adsorption and reduction. Methods and Results: Bacterial consortia from industrial wastes revealed signi- ficant Cr(VI) removal after 15 days when incubated in medium M9 at pH 6Æ5 and 8Æ0. The results suggested chromium reduction. The bacterial consortia diversity (T-RFLP based on 16S rRNA gene) indicated a highest number of operational taxonomic units in an alkaline carbonate medium mimicking in situ conditions. However, incubations under such conditions revealed low Cr(VI) removal. Genomic libraries were obtained for the consortia exhibiting optimal Cr(VI) removal (M9 medium at pH 6Æ5 and 8Æ0). They revealed the dominance of 16S rRNA gene sequences related to the genera Pseudomonas ⁄ Stenotrophomonas or Enterobacter ⁄ Halomonas, respectively. Isolates related to Pseudomonas fluorescens and Enterobacter aerogenes were efficient in Cr(VI) reduction and adsorption to the biomass. Conclusions: Cr(VI) reduction was better at neutral pH rather than under in situ conditions (alkaline pH with carbonate). Isolated strains exhibited signifi- cant capacity for Cr(VI) reduction and adsorption. Significance and Impact of Study: Bacterial communities from chromium- contaminated industrial wastes as well as isolates were able to remove Cr(VI). The results suggest a good potential for bioremediation of industrial wastes when optimal conditions are applied. Journal of Applied Microbiology ISSN 1364-5072 Journal of Applied Microbiology 109, 2173–2182 ª 2010 The Society for Applied Microbiology No claim to Mexican Government works 2173