Effect of rhamnolipid solubilization on hexadecane bioavailability: enhancement or reduction?

[Display omitted] •Rhamnolipid-solubilization increases availability of hexadecane for P. aeruginosa.•Rhamnolipid-solubilization reduces bioavailability of hexadecane for P. putida.•Blocking effect of surfactants accounts for reduction in hexadecane bioavailability.•Bacterial compatibility is important to overcome the blocking effect. In this study, liquid culture systems containing rhamnolipid-solubilized, separate-phase, and multi-state hexadecane as the carbon source were employed for examining the effect of rhamnolipid solubilization on the bioavailability of hexadecane. Experimental results showed that the uptake of rhamnolipid-solubilized hexadecane by Pseudomonas aeruginosa ATCC 9027, a rhamnolipid producing strain, was enhanced compared to the uptake of mass hexadecane as a separate phase, indicating rhamnolipid solubilization increased the bioavailability of hexadecane for this bacterium. For Pseudomonas putida CICC 20575 which does not produce but degrade rhamnolipid, the uptake of either rhamnolipid-solubilized hexadecane or multi-state hexadecane was inhibited. The reduction of bioavailability was assumed to be the consequence of the blocking effect caused by the partition of rhamnolipid molecules at the hexadecane-water interface. The results show that how rhamnolipid solubilization changes the bioavailability of hexadecane depends on the bacterial compatibility to rhamnolipid. The study adds insight into the knowledge of biosurfactant-associated bioavailability of hydrophobic organic compounds (HOCs), and is of importance for application of biosurfactants in bioremediation of HOCs.