A Comparison of the Microbial Community and Functional Genes Present in Free-Living and Soil Particle-Attached Bacteria from an Aerobic Bioslurry Reactor Treating High-Molecular-Weight PAHs

Abstract

High-molecular-weight (HMW) polycyclic aromatic hydrocarbons (PAHs) contaminate a wide range of ecosystems, including soils, groundwater, rivers and harbor sediments. The effective removal of HMW PAHs is a difficult challenge if a rapid remediation time and low economic cost are required. Bioremediation provides a cheap and eco-friendly cleanup strategy for the removal of HMW PAHs. Previous studies have focused on removal efficiency during PAHs bioremediation. In such studies, only limited research has targeted the bacterial communities and functional genes present in such bioremediation systems, specifically those of free-living (aqueous) bacteria and soil particle-attached bacteria present. In this study, a high-level of HMW PAH (1992 mg/kg pyrene) was bioremediated in an aerobic bioslurry reactor (ABR) for 42 days. The results showed a pseudo first order constant rate for pyrene biodegradation of 0.0696 day−1. The microbial communities forming free-living bacteria and soil-attached bacteria in the ABR were found to be different. An analysis of the aqueous samples identified free-living Mycobacterium spp., Pseudomonas putida, Rhodanobacter spp. and Burkholderia spp.; these organisms would seem to be involved in pyrene biodegradation. Various biointermediates, including phenanthrene, catechol, dibenzothiophene, 4,4′-bipyrimidine and cyclopentaphenanthrene, were identified and measured in the aqueous samples. When a similar approach was taken with the soil particle samples, most of the attached bacterial species did not seem to be involved in pyrene biodegradation. Furthermore, community level physiological profiling resulted in significantly different results for the aqueous and soil particle samples. Nevertheless, these two bacterial populations both showed positive signals for the presence of various dioxygenases, including PAHs-RHDα dioxygenases, riesk iron-sulfur motif dioxygenases and catechol 2,3-dioxygenases. The present findings provide a foundation that should help environmental engineers when designing future HMW PAH bioremediation systems that use the ABR approach.