Variation of bacterial community and alkane monooxygenase gene abundance in diesel n-alkane contaminated subsurface environment under seasonal water table fluctuation

n-Alkanes, the main component of diesel fuel, are common light non-aqueous phase liquids (LNAPLs) that threaten ecological security. The subsurface from vadose zone, through fluctuating zone, to saturated zone, is a critical multi-interface earth layer which significantly affects the biodegradation processes of n-alkanes. A pilot-scale diesel contaminated aquifer column experiment has been undertaken to investigate the variations of bacterial community and alkane monooxygenase (alkB) gene abundance in these zones due to water-table fluctuations. The n-alkanes formed a layer immediately above the water table, and when this was raised, they were carried upwards through the fluctuating zone into the vadose zone. Water content and n-alkanes component C10-C12 are main factors influencing bacterial community variation in the vadose zone, while C10-C12 is a key driving factor shaping bacterial community in the fluctuating zone. The most abundant bacterial phyla at all three zones were Proteobacteria, Firmicutes and Actinobacteria, but moisture-niche selection determined their relative abundance. The intermittent wetting cycle resulted in higher abundance of Proteobacteria, and lower abundance of Actinobacteria in the vadose and fluctuating zones in comparison to the control column with a static water-table. The abundances of the alkB gene variants were relatively uniform in different zones, probably because the bacterial populations harboring alkB gene are habituated to biogenic n-alkanes rather than responding to diesel fuel contamination. The variation in the bacterial populations with height due to moisture-niche selection had very little effect on the alkB gene abundance, possibly because numerous species in both phyla (Proteobacteria and Actinobacteria) carry an alkB gene variant. Nevertheless, the drop in the water table caused a short-term spike in alkB gene abundance in the saturated zone, which is most likely associated with transport of solutes or colloids from the fluctuating zone to bacteria species in the saturated zone, so a fluctuating water table could potentially increase n-alkane biodegradation function.