Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/28/2000
Publication Date: 7/1/2001
Interpretive Summary: Organic chemicals such as pesticides are often degraded slowly in soils, because they are relatively insoluble in water. Surfactants and detergents are sometimes used to help get more of the pesticide dissolved into the water phase so microbes can degrade them. However, synthetic detergents and surfactants are often toxic to microbes and can be pollutants themselves, so their use is limited. Rhamnolipids are natural surfactants produced by a number of bacterial species. Because they are natural products they may be more benign to biodegrading bacteria and be more biodegradable themselves, and, therefore, may be better candidates for dissolving pesticides for biodegradation. In this study, we compared the effect of a rhamnolipid mixture produced by Pseudomonas aeruginosa with that of the commonly used synthetic surfactant Triton X-100 on the biodegradation of the pesticides atrazine, trifluralin, and coumaphos in aqueous suspension, and in soil slurries. Both surfactants slightly enhanced the degradation of trifluralin in aqueous culture but had little effect on its degradation in soil slurries. Both surfactants had a negative effect on atrazine degradation in aqueous culture, and no effect on its degradation in soil slurries. The presence of Triton X-100 strongly inhibited coumaphos degradation in aqueous culture and in soil slurries. Rhamnolipids improved both the rate and extent of coumaphos degradation in aqueous culture but improved only the extent of its degradation in soil slurries. This research shows that the biologically-produced rhamnolipid surfactant may be useful in situations where synthetic surfactants fail,and will be valuable to scientists and engineers who try to develop methods for the use of surfactants in the clean-up of contaminated soils.
Technical Abstract: The effect of surfactants on the biodegradation of trifluralin and atrazine (by Streptomyces PS1/5), and coumaphos (by degrading consortia from a contaminated cattle dip) in liquid cultures and soil slurries was tested at different concentrations of a rhamnolipid mixture (Rh-mix) and Triton X-100 (TX-100). The extent of trifluralin biodegradation in liquid culture was improved up to 40% at high concentrations of both surfactants. The extent of atrazine degradation dropped from 34% to 18% in the presence of either surfactant. Coumaphos biodegradation improved slightly from 71% to 91%, at high Rh-mix dosages above 3000 uM, however, its mineralization was readily inhibited by TX-100 at amounts above its critical micelle concentration (CMC). In soil slurries, the extent of both trifluralin and atrazine biodegradation were higher in Hagerstown A (HTA) soil than in Hagerstown B (HTB) soil, and were not significantly affected by the presence of either surfactant. Trifluralin biodegradation onset was retarded at increasing concentrations of surfactants. In absence of surfactant up to 98% of coumaphos in both slurries was transformed. At increasing dosages of Rh-mix, the onset of coumaphos biodegradation was retarded, but the removal efficiency of the pesticide increased to practically 100%. Rh-mix and TX-100 depletion was observed during Streptomyces PS1/5 microbial growth in liquid cultures. Rh-mix concentration also decreased during coumaphos biodegradation, while TX-100 concentration was not affected. These results suggest that the onset and extent of biodegradation of hydrophobic organic compounds (HOC) might be affected by co-metabolic activity of the degraders against surfactants added for the purpose of increasing the target compound water solubility (bioavailability).