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

Abstract

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 10⁸ 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 μM) 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 μgL⁻¹ Cr(VI) of the industrial effluent, indicating their possible use in environmental cleanup.

KEYWORDS:

• Bioremediation
• bacterial consortia
• Scirpus americanus Pers

ACKNOWLEDGMENTS

This work was supported by National Council of Science and Technology (Conacyt). We thank Dr. Gene Bartholomew for valuable help in reviewing the manuscript. We extend our thanks to the anonymous reviewers for their positive and relevant comments that improved the manuscript.

Notes

¹Bioaccumulation factor is metal concentration in plant at harvest/concentration of metal in sediment.

²Each value represents the average of three replicates ± standard deviation.

³,*Different letters within a column indicate significant statistical difference by Tukey's test (p ≤ 0.05).

¹Cr(VI) Minimum Inhibitory Concentration for isolates incubated in Tris minimal medium for 3–5 d at 28°C (see Methods).

²Cr(VI) removal percentage is the difference between inoculated, live or dead cultures and sterile controls; each value represents the average of three replicates ± standard deviation.

^3,4Cultures were incubated in phosphate buffer supplemented with 20 μM Cr(VI) at 28°C for 1 h under aerobic conditions (see Methods).

¹Cultures were incubated aerobically in modified Tris basal media supplemented with 0.2 mM Cr(VI) at 28°C for 48 h (see Methods).

²Cr(VI) removal percentage is the difference between inoculated cultures and sterile controls.

³Each value represents the average of three replicates; different letters within a row indicate significant statistical difference by Tukey's test (p ≤ 0.05).

¹The treatments were inoculated with 7.5 × 10⁴ cell mL⁻¹ then were incubated aerobically in modified Tris basal media at 28°C for 48 h (see Methods).

²Each value represents the average of three replicates; different letters within a row indicate significant statistical difference by Tukey's test (p ≤ 0.05).

³Not detected.

¹Cultures were incubated aerobically at 28°C for 48 h and Cr(VI) removal was measured at the end of the experiment.

²Each value represents the average of three replicates.