A rhamnolipid biosurfactant reduces cadmium toxicity during naphthalene biodegradation

Abstract

A model cocontaminated system was developed to determine whether a metal-complexing biosurfactant, rhamnolipid, could reduce metal toxicity to allow enhanced organic biodegradation by a Burkholderia sp. isolated from soil. Rhamnolipid eliminated cadmium toxicity when added at a 10-fold greater concentration than cadmium (890 microM), reduced toxicity when added at an equimolar concentration (89 microM), and had no effect at a 10-fold smaller concentration (8.9 microM). The mechanism by which rhamnolipid reduces metal toxicity may involve a combination of rhamnolipid complexation of cadmium and rhamnolipid interaction with the cell surface to alter cadmium uptake.

FIG. 1

Effect of cadmium concentration on the growth of a Burkholderia sp. on naphthalene. Each point represents the mean protein concentration for triplicate flasks. Error bars represent standard deviations.

FIG. 2

Effect of the rhamnolipid (Rhl) concentration on the growth of a Burkholderia sp. on naphthalene in the presence of 89 μM cadmium. Each point represents the mean protein concentration for triplicate flasks. Error bars represent standard deviations.

FIG. 3

Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of concentrated (10×) supernatants of suspensions of a Burkholderia sp. The gel was stained using a silver-staining procedure for LPS. Lanes: 1, buffer; 2, 5 μg of BSA; 3, 4485.6 μg (890 μM) of rhamnolipid; 4, supernatant of the Burkholderia sp. treated only with MSM; 5, supernatant of the Burkholderia sp. treated with 8.9 μM rhamnolipid; 6, supernatant of the Burkholderia sp. treated with 89 μM rhamnolipid; 7, supernatant of the Burkholderia sp. treated with 890 μM rhamnolipid; 8, 1 μg of P. aeruginosa serotype 10 LPS; 9, 10 μg of P. aeruginosa serotype 10 LPS. The gel was imaged and the band density (integrated density value) was determined using the SpotDenso function of AlphaImager (Alpha Innotech, San Leandro, Calif.).