Abstract
Bioremediation offers a sustainable approach for removal of polycyclic aromatic hydrocarbons (PAHs) from the environment; however, information regarding the microbial communities involved remains limited. In this study, microbial community dynamics and the abundance of the key gene (PAH-RHDα) encoding a ring hydroxylating dioxygenase involved in PAH degradation were examined during degradation of phenanthrene in a podzolic soil from the site of a former timber treatment facility. The 10,000-fold greater abundance of this gene associated with Gram-positive bacteria found in phenanthrene-amended soil compared to unamended soil indicated the likely role of Gram-positive bacteria in PAH degradation. In contrast, the abundance of the Gram-negative PAHs-RHDα gene was very low throughout the experiment. While phenanthrene induced increases in the abundance of a small number of OTUs from the Actinomycetales and Sphingomonadale, most of the remainder of the community remained stable. A single unclassified OTU from the Micrococcaceae family increased ~20-fold in relative abundance, reaching 32% of the total sequences in amended microcosms on day 7 of the experiment. The relative abundance of this same OTU increased 4.5-fold in unamended soils, and a similar pattern was observed for the second most abundant PAH-responsive OTU, classified into the Sphingomonas genus. Furthermore, the relative abundance of both of these OTUs decreased substantially between days 7 and 17 in the phenanthrene-amended and control microcosms. This suggests that their opportunistic phenotype, in addition to likely PAH-degrading ability, was determinant in the vigorous growth of dominant PAH-responsive OTUs following phenanthrene amendment. This study provides new information on the temporal response of soil microbial communities to the presence and degradation of a significant environmental pollutant, and as such has the potential to inform the design of PAH bioremediation protocols.
Highlights
Soil pollution is a global problem; there are about 342,000 contaminated sites in Europe alone, with this number set to increase to over 500,000 by 2025 (Anon, 2012)
Pairwise comparisons on bacterial community structures in phenanthrene amended soil revealed that this polycyclic aromatic hydrocarbons (PAHs) had no significant effect on bacterial community structures on days 0 and 2, but bacterial community structures on day 7 of phenanthrene amended soil were significantly different from those on day 17
The presence of phenanthrene led to significant shifts in the structure of soil bacterial communities, and this change was dominated by an increase in the relative abundance of a small number of opportunistic taxa
Summary
Soil pollution is a global problem; there are about 342,000 contaminated sites in Europe alone, with this number set to increase to over 500,000 by 2025 (Anon, 2012). Polycyclic aromatic hydrocarbons (PAHs) are a class of toxic chemicals, composed of at least two benzene rings arranged in various conformations (Wickliffe et al, 2014). PAHs are released to the environment upon incomplete combustion of fossil fuels and are ubiquitous in nature, with soil as their major sink (Collins et al, 2013). Due to their carcinogenic and mutagenic potential, 16 of these compounds are listed as priority pollutants by the United States Environmental Protection Agency (Wang et al, 2010). Culture independent analyses have revealed that representatives of Proteobacteria, Actinobacteria, and Firmicutes are the bacteria most likely to increase during biodegradation of PAHs and petroleum hydrocarbons (Fuentes et al, 2014)
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