When biosurfactants compete with pollutants: Rhamnolipid production and moisture-dependent performance in...

Rhamnolipids have long been studied for their potential in bioremediating hydrocarbon-contaminated soils. While some studies report enhanced hydrocarbon biodegradation in their presence, others observe limited or no effect. To clarify the factors behind these discrepancies and assess rhamnolipid effectiveness, we carried out experiments with 20 strains of Pseudomonas aeruginosa, the best-studied rhamnolipid producers. All strains showed significantly higher rhamnolipid production when cultivated with a hydrophilic carbon source compared to hydrophobic diesel oil. The synthesis rate was also higher under nitrogen-limited conditions. These findings challenge the assumption that rhamnolipids are primarily produced to enhance hydrocarbon uptake. To investigate their efficiency, we exogenously added rhamnolipids to sand microcosms. They only accelerated hydrocarbon degradation under low moisture conditions (PWP). When sufficient water was available, rhamnolipids became less effective as they were consumed as accessible carbon by the microbiota. This was supported by community analysis, with the greatest rhamnolipid-driven shift observed under PWP. These findings underscore the complexity of rhamnolipid behavior in soils and importance of water availability when evaluating their role in hydrocarbon degradation. Their persistence and functionality under dry conditions suggest particular value in drought-prone environments. This work highlights the need to better understand when and where rhamnolipids are the most effective.