Bacterial Community Dynamics and Polycyclic Aromatic Hydrocarbon Degradation during Bioremediation of Heavily Creosote-Contaminated Soil

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Abstract
Bacterial community dynamics and biodegradation processes were examined in a highly creosote-contaminated soil undergoing a range of laboratory-based bioremediation treatments. The dynamics of the eubacterial community, the number of heterotrophs and polycyclic aromatic hydrocarbon (PAH) degraders, and the total petroleum hydrocarbon (TPH) and PAH concentrations were monitored during the bioremediation process. TPH and PAHs were significantly degraded in all treatments (72 to 79% and 83 to 87%, respectively), and the biodegradation values were higher when nutrients were not added, especially for benzo( a )anthracene and chrysene. The moisture content and aeration were determined to be the key factors associated with PAH bioremediation. Neither biosurfactant addition, bioaugmentation, nor ferric octate addition led to differences in PAH or TPH biodegradation compared to biodegradation with nutrient treatment. All treatments resulted in a high first-order degradation rate during the first 45 days, which was markedly reduced after 90 days. A sharp increase in the size of the heterotrophic and PAH-degrading microbial populations was observed, which coincided with the highest rates of TPH and PAH biodegradation. At the end of the incubation period, PAH degraders were more prevalent in samples to which nutrients had not been added. Denaturing gradient gel electrophoresis analysis and principal-component analysis confirmed that there was a remarkable shift in the composition of the bacterial community due to both the biodegradation process and the addition of nutrients. At early stages of biodegradation, the α- Proteobacteria group (genera Sphingomonas and Azospirillum ) was the dominant group in all treatments. At later stages, the γ- Proteobacteria group (genus Xanthomonas ), the α- Proteobacteria group (genus Sphingomonas ), and the Cytophaga-Flexibacter-Bacteroides group ( Bacteroidetes ) were the dominant groups in the nonnutrient treatment, while the γ- Proteobacteria group (genus Xathomonas ), the β- Proteobacteria group (genera Alcaligenes and Achromobacter ), and the α- Proteobacteria group (genus Sphingomonas ) were the dominant groups in the nutrient treatment. This study shows that specific bacterial phylotypes are associated both with different phases of PAH degradation and with nutrient addition in a preadapted PAH-contaminated soil. Our findings also suggest that there are complex interactions between bacterial species and medium conditions that influence the biodegradation capacity of the microbial communities involved in bioremediation processes.

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