Utilization of the C9 aromatic hydrocarbon n-propylbenzene by Sphingobium barthaii KK22 and coexistence of multiple biodegradation pathways.

n-Propylbenzene is an environmental pollutant belonging to a class of heavily used nonpolar alkylated aromatic solvents referred to as the C9 aromatics. Although n-propylbenzene is detected in different environmental matrices and displays toxicity, its bacterial biodegradation has been little explored. Consequently, few transformation products have been identified, and comprehensive biodegradation pathways were not constructed. Understanding n-propylbenzene biotransformation shall be useful to predict its fate and transport in the environment. Therefore, n-propylbenzene biotransformation by soil bacterium, Sphingobium barthaii KK22, was examined by liquid chromatography electrospray ionization tandem mass spectrometry (LC/ESI-MS/MS) through product ion scan collision induced dissociation (CID) analyses. Targeted CID of unknown biotransformation products resulted in the proposal of structures for at least 18 compounds and based upon these results, metabolites were organized into biotransformation pathways which revealed multiple routes to the TCA cycle. Decarboxylation of the n-propylbenzene alkyl side chain was proposed as a key part of the biodegradation process-so-called alkyl chain shortening. At the same time, the aromatic ring of n-propylbenzene was vulnerable to dioxygenation no matter the alkyl chain length or degree of alkyl chain oxidation resulting in numerous 3-, 2- and 1-carbon chain length compounds and their aromatic ring-opened counterparts. Quantitative analyses by LC and growth monitoring by absorbance confirmed that this bacterium eliminated 100 mg/L n-propylbenzene from culture media and that it utilized n-propylbenzene as a carbon source. In the natural environment, catabolically versatile soil sphingomonads such as S. barthaii may be contributors to the biodegradation of alkylated aromatic nonpolar pollutants such as n-propylbenzene.