Isolation and Characterization of Hexavalent Chromium-Reducing Rhizospheric Bacteria From a Wetland

Scirpus americanus Pers. occurs naturally in "San Germán," a pond that serves as a receptor of industrial wastewater in Guanajuato, México. This plant accumulates metals mainly in the root: concentrations (mg/kg) of Cr, As, Cd and Se were 970, 49, 41, and 85 respectively. Analysis of rhizosphere samples indicated bacterial population of 10(8) cfu g(-1) in media with 0.2 mM Cr(VI) and 10 mM sodium gluconate. Thirteen isolates were obtained and phylogenetic analyses (16S rRNA) indicated they corresponded to genera of Agrobacterium, Arthrobacter, Microbacterium, Curtobacterium, Rhodococcus, Xanthomonas and Pseudomonas. Cr(VI) reduction was evaluated using the diphenyl carbazide method. The isolates accomplished 5-40% (20 microM) of reduction in assays of resting cell and tolerated 0.5-5.0 mM Cr(VI). Eight strains used nitrate and thirteen used iron and chromium as electron acceptors to grow under anaerobic conditions. Cr(VI) reduction by five strains occurred at pH values (7-9) and NaCl concentrations (0.5-1.0 M) in basal medium. A mixed culture of strains (S17 and S28) reached a chromium removal of 100% at 0.2 mM Cr(VI) initial concentration. Aerobically, this consortium was capable of 93.8% Cr(VI) reduction of 81 microg L(-1) Cr(VI) of the industrial effluent, indicating their possible use in environmental cleanup.