Rhamnolipid biosurfactants decrease the toxicity of chlorinated phenols to Pseudomonas putida DOT-T1E

To investigate the effect of a mixture of rhamnolipid R1 and R2 biosurfactants produced by a Pseudomonas aeruginosa strain on the toxicity of phenol and chlorophenols to Pseudomonas putida DOT-T1E.

Toxicity was quantified by the effective concentration 50% (EC50), that is the concentration that causes a 50% inhibition of bacterial growth. The presence of 300 mg l(-1) rhamnolipids, that is at about twice their critical micelle concentration (CMC), increased the EC50 of phenol, 4-chlorophenol, 2,4-dichlorophenol and 2,4,5-trichlorophenol by about 12, 19, 32 and 40%, respectively, and consequently reduced the bioavailability and the freely dissolved concentration of the toxic phenolic compounds. The reduction was related to the phenols' octanol-water partition coefficients (K(ow)).

The reduction in toxicity of the phenols can be explained by a combination of toxin accumulation in biosurfactant micelles and hydrophobic interactions of the phenols with rhamnolipid-based dissolved organic carbon.

Results provide evidence that next to the effect of the micelle formation also hydrophobic interactions with rhamnolipid-based dissolved organic carbon affects the bioavailability of the phenols. Quantifying the effect of biosurfactants on the toxicity of hydrophobic compounds such as phenols thus appears to be a useful approach to assess their bioavailable equilibrium concentration.