ELECTRON-MICROSCOPY OF RHAMNOLIPID (BIOSURFACTANT) MORPHOLOGY - EFFECTS OF PH, CADMIUM, AND OCTADECANE

A rhamnolipid biosurfactant produced by Pseudomonas aeruginosa ATCC 90 27 is reported to increase the aqueous dispersion and biodegradation o f petroleum hydrocarbons and to complex heavy metals, These reports in dicate the potential for application of rhamnolipids in remediation of contaminated sites, Effective use of rhamnolipids will require unders tanding of rhamnolipid morphology and the effects of pH and organic an d inorganic contaminants on that morphology. We used cryo-transmission electron microscopy to investigate the morphology of vitrified, froze n hydrated suspensions of rhamnolipid over a pH range of 5.5 to 8.0, a nd to determine the effect of a model alkane, octadecane, and a model heavy metal, cadmium, on rhamnolipid morphology, Micrographs clearly s howed that rhamnolipid morphology was a function of pH, changing from lamellar, to vesicular, to micellar as pH increased. The effect of cad mium and octadecane on rhamnolipid morphology was determined at pH 6.8 and 7.0, where maximum cadmium complexation and maximum octadecane di spersion occurs. Cadmium seemed to stabilize rhamnolipid vesicle struc tures as shown by an increase in vesicle number and a decrease in vesi cle diameter, In contrast, octadecane favored the micellar structure a s shown by the complete absence of vesicles. (C) 1995 Academic Press, Inc.