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Unlabelled: Environmental contamination by polycyclic aromatic hydrocarbons (PAHs) typically occurs as mixtures of compounds. In this study, the response of indigenous soil bacterial and fungal communities to mixtures containing phenanthrene, fluoranthene and benzo(a)pyrene in various combinations was examined using molecular fingerprinting techniques and quantification of a key PAH degradative gene. Results were compared to a parallel study by Sawulski et al. (2014) which examined the effect of these PAHs on soil microbial communities when added as single contaminants. The rate of degradation of individual PAHs varied depending on whether the PAH was present as a single contaminant or in a mixture; phenanthrene was degraded most rapidly when present as a sole contaminant, fluoranthene was removed faster in the presence of the lower molecular weight phenanthrene and the rate of benzo(a)pyrene degradation was reduced in the presence of the 4-ring PAH, fluoranthene. Bacterial and fungal assemblages differed significantly between treatments regardless of which PAH was added to soil. Although less abundant than the Gram-negative PAH-RHDα gene, the gene associated with Gram-positive bacteria responded to a greater extent to the presence of PAHs, either as single compounds or as mixtures and this increase was significantly correlated with PAH degradation., Significance and Impact of the Study: Contaminated sites generally contain complex mixtures of pollutants. Development of effective bioremediation strategies for contaminated soils requires knowledge of the response of soil microbial communities to such mixtures. This study provides information on the degradation of different mixtures of three priority pollutants in soil with a history of polycyclic aromatic hydrocarbon contamination and examines the response of soil bacterial and fungal communities to the presence of these pollutants as sole contaminants or as part of a mixture. This is one of few studies to-date to compare the effects of single compounds and pollutant mixtures on more than one soil microbial community., (© 2015 The Society for Applied Microbiology.)

Development of successful bioremediation strategies for environments contaminated with recalcitrant pollutants requires in-depth knowledge of the microorganisms and microbial processes involved in degradation. The response of soil microbial communities to three polycyclic aromatic hydrocarbons, phenanthrene (3-ring), fluoranthene (4-ring) and benzo(a)pyrene (5-ring), was examined. Profiles of bacterial, archaeal and fungal communities were generated using molecular fingerprinting techniques (TRFLP, ARISA) and multivariate statistical tools were employed to interpret the effect of PAHs on community dynamics and composition. The extent and rate of PAH removal was directly related to the chemical structure, with the 5-ring PAH benzo(a)pyrene degraded more slowly than phenathrene or fluoranthene. Bacterial, archaeal and fungal communities were all significantly affected by PAH amendment, time and their interaction. Based on analysis of clone libraries, Actinobacteria appeared to dominate in fluoranthene amended soil, although they also represented a significant portion of the diversity in phenanthrene amended and unamended soils. In addition there appeared to be more γ-Proteobacteria and less Bacteroidetes in soil amended with either PAH compared to the control. The soil bacterial community clearly possessed the potential to degrade PAHs as evidenced by the abundance of PAH ring hydroxylating (PAH-RHDα) genes from both gram negative (GN) and gram positive (GP) bacteria in PAH-amended and control soils. Although the dioxygenase gene from GP bacteria was less abundant in soil than the gene associated with GN bacteria, significant (p < 0.001) increases in the abundance of the GP PAH-RHDα gene were observed during phenanthrene and fluoranthene degradation, whereas there was no significant difference in the abundance of the GN PAH-RHDα gene during the course of the experiment. Few studies to-date have examined the effect of pollutants on more than one microbial community in soil. The current study provides information on the response of soil bacterial, archaeal and fungal communities during the degradation of three priority pollutants and contributes to a knowledge base that can inform the development of effective bioremediation strategies for contaminated sites.