Industrial residues with high concentrations of hexavalent chromium [Cr(VI)], characterized by an alkaline pH (between 9 and 13) and high salinity (around 100 psu), were used as a source for extremophilic chromium-resistant and -reducing microorganisms. An investigation of biodiversity through MiSeq showed the presence of 20 bacterial classes, with Bacilli (47%), Negativicutes (15%), Bacteriodia (8%), Gammaproteobacteria (7%) and Clostridia (5%) being the most abundant. The bioprospection allowed the cultivation of 87 heterotrophic bacterial colonies and 17 bacterial isolates at the end of the isolation, and screening procedures were obtained. The isolates were related to Cellulosimicrobium aquatile, C. funkei, Acinetobacter radioresistens, Staphylococcus equorum, S. epidermis, Brachybacterium paraconglometratum, Glutamicibacter creatinolyticus, Pseudomonas songnenensis, Microbacterium algeriense and Pantoea eucalypti, most of them being resistant to Cr(VI). Resistances of up to 400 ${\mathrm{m}\mathrm{g}.\mathrm{L}}^{-1}$ of chromate were obtained for four related strains (QReMLB55A, QRePRA55, QReMLB33A and QReMLB44C). The C. aquatile strain QReMLB55A and the P. songnenensis strain QReMLB33A were exposed to ${\mathrm{K}}_2{\mathrm{C}\mathrm{r}}_2{\mathrm{O}}_7$ (200 ${\mathrm{m}\mathrm{g}.\mathrm{L}}^{-1}$) under optimal conditions, diminishing 94% and 24% of the Cr(VI) in 6 days, respectively. These strains exhibited a high potential for chromium remediation biotechnologies.

Two bacterial consortia (C55 and C33), obtained from an industrial residue contaminated with hexavalent chromium (Cr(VI)), were used to study the behavior of their mixture for depleting this ion in liquid media. In the absence of Cr(VI), C55 showed a greater growth rate than C33, while the latter exhibited biofilm formation. In the presence of this ion, C55 showed resistance up to 800 mg·L ⁻¹ and an ability to diminish up to 400 mg·L ⁻¹ of the Cr(VI) from the medium, while for C33, these concentrations were 400 and 200 mg·L ⁻¹ , respectively. Bacterial synergism between these consortia was evaluated using different compound ratios (C55:C33 ratios of 1:1, 1:2, and 2:1), growing at 50, 100, and 200 mg·L ⁻¹ Cr(VI). The best half‐lives of Cr(VI) decrease were 16, 31, and 98 h, respectively, for the 1:1 mixture. The ability of C33 and the mixed consortia to form biofilms was verified. MiSeq sequencing revealed 4 major populations for C55 (in a total of 14) and 3 for C33 (8), most of which were common. After an isolation process, 2 bacterial strains were obtained from C55 and 4 from C33. Three of these strains (QRePLB33E, similar to Oceanobacillus profundus ; QRePLB33G, to Shouchella clausii ; and QRePLB55C, to Cellulosimicrobium funkei ) showed resistance to Cr(VI) and the ability to remove 100% of it at least up to 300 mg·L ⁻¹ . Thus, synergism between different bacterial consortia obtained from the same site is possible and can improve, by complementing their capacities, both the growth rate and the ability to diminish the xenobiotic from the medium.

In search of a Cr(VI) removal bioprocess, within a circular economy perspective, we combined two autochthonous bacterial consortia obtained from industrial residues and an agricultural waste (rice husk of Oryza sativa). After characterizing the rice husk (rh) properties, the bacterial growth and biofilm formation, we studied the taxonomical profile of the bacterial consortia (C55, C33, and C21, the last is a combination 2:1 of the previous ones). By using SEM analysis, we confirmed the presence of a smooth layer of exopolysaccharides (EPS) over the rice husk surface. Total bacterial biodiversity, assessed through MiSeq analysis, showed that the addition of Cr(VI) stimulated the growth of two main populations ( Cellulosimicrobium and Siminovitchia ) while inhibiting Peptoniphilus , Cutibacterium and Rhodococcus . We also evaluated chromium depletion, in batch experiments, with particular emphasis on its sorption to the agroresidue and its biotransformation by the biomass. When exposed to 200 mg.L ^-1 Cr(VI), the system containing both rh and the bacterial consortium completely removed Cr(VI) from the effluent with half-times between 53 and 67 h. The combination of Cr(VI) biosorption by rh and its biotransformation to Cr(III) by bacterial consortia confirmed an effective synergy capable of removing 92% of Cr(VI). This work supports the potential of an innovative bioprocess based on bacterial biofilms grown on rh for the mitigation of Cr(VI) contamination.

Réduction du chrome par des bactéries aérobies: La contamination par les métaux lourds dans les environnements industriels constitue un problème écologique et de santé publique majeur, nécessitant des solutions durables pour en atténuer l'impact. Parmi ces métaux, le chrome hexavalent Cr(VI) est particulièrement préoccupant en raison de sa haute toxicité, de sa persistance dans l'environnement et de sa capacité à s'accumuler dans les sols et les eaux, affectant ainsi la biodiversité et posant des risques sérieux pour la santé humaine. La rémediation du Cr(VI) représente un défi important en raison de sa stabilité et de sa mobilité dans l'environnement. Dans ce contexte, les bactéries résistantes au Cr(VI) offrent une alternative prometteuse pour la biorestauration des environnements contaminés, en complément des méthodes conventionnelles de traitement des déchets industriels. Ces microorganismes, capables de tolérer et de transformer le Cr(VI) en une forme moins toxique Cr(III), jouent un rôle clé dans la décontamination.Cette étude s'est concentrée sur le site de Química Central de México (QCM), situé à San Francisco del Rincón, dans l'État de Guanajuato, au Mexique. Ce site, historiquement contaminé par des activités industrielles, a servi de modèle pour étudier les mécanismes de résistance au Cr(VI) chez les bactéries et évaluer leur potentiel de dégradation et de traitement était d'analyser, d'un point de vue écologique, génomique et physiologique, les bacteries de ce contaminant. L'objectif principal de cette recherche était d'analyser écologiquement, génomiquement et physiologiquement les bactéries résistantes au Cr(VI) isolées de ce site, afin de comprendre leur adaptation à des environnements contaminés et d'explorer leur application potentielle dans des processus de biorestauration.