The abundance of nahAc genes correlates with the 14C-naphthalene mineralization potential in petroleum hydrocarbon-contaminated oxic soil layers

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Abstract
In this study, we evaluated whether the abundance of the functional gene nahAc reflects aerobic naphthalene degradation potential in subsurface and surface samples taken from three petroleum hydrocarbon contaminated sites in southern Finland. The type of the contamination at the sites varied from lightweight diesel oil to high molecular weight residuals of crude oil. Samples were collected from both oxic and anoxic soil layers. The naphthalene dioxygenase gene nahAc was quantified using a replicate limiting dilution-polymerase chain reaction (RLD-PCR) method with a degenerate primer pair. In the non-contaminated samples nahAc genes were not detected. In the petroleum hydrocarbon-contaminated oxic soil samples nahAc gene abundance [range 3 × 101–9 × 104 copies (g dry wt soil)−1] was correlated (Kendall non-parametric correlation r2 = 0.459, p < 0.01) with the aerobic 14C-naphthalene mineralization potential (range 1 × 10−5–0.1 d−1) measured in microcosms at in situ temperatures (8 °C for subsurface and 20 °C for surface soil samples). In these samples nahAc gene abundance was also correlated with total microbial cell counts (r2 = 0.471, p < 0.01), respiration rate (r2 = 0.401, p < 0.01) and organic matter content (r2 = 0.341, p < 0.05). NahAc genes were amplified from anoxic soil layers indicating that, although involved in aerobic biodegradation of naphthalene, these genes or related sequences were also present in the anoxic subsurface. In the samples taken from the anoxic layers, the aerobic 14C-naphthalene mineralization rates were not correlated with nahAc gene abundance. In conclusion, current sequence information provides the basis for a robust tool to estimate the naphthalene degradation potential at oxic zones of different petroleum hydrocarbon-contaminated sites undergoing in situ bioremediation.