Evidence from liposome encapsulation for transport-limited microbial metabolism of solid alkanes

The recalcitrance of xenobiotics may be caused by an absence of transforming enzymes or by their inability to enter microbial cells. A nondestructive method for differentiating between these two possibilities is described. The solid n-alkanes octadecane (C18) and hexatriacontane (C36) were encapsulated into phosphatidylcholine bilayers (liposomes). The uptake and metabolism rates of encapsulated and unencapsulated substrates were then compared. During 1 h at 25 degree C, a Pseudomonas isolate took up 1.3% of radiolabeled and unencapsulated C18 (solid state) versus 23.5% of labeled and encapsulated C18. Growth at 25 degree C occurred with an apparent ks of 2453 ± 148 mg/liter. Liposome encapsulation decreased this Ks to 60 ± 12 mg/liter. At 34 degree C, growth on C18 (liquid state) occurred with an apparent Ks of 819 ± 83 mg/liter and on the readily available carbon source succinate, Ks values were 80 ± 10 and 13 ± 7 mg/liter at 25 to 34 degree C, respectively. At 25 degree C, the isolate grew on C36 with an apparent Ks of 2,698 ± 831 mg/liter. Liposome encapsulation decreased the Ks more than 60-fold to 41 ± 7 mg/liter, resulting in the complete utilization of 400 mg of C36 per liter in 16 h. Since controls excluded the metabolic utilization of phosphatidylcholine, the results clearly identify transport limitation as the cause for C36 recalcitrance