Effect of biostimulation on the distribution and composition of the microbial community of a polycyclic aromatic hydrocarbon-contaminated landfill soil during bioremediation

Abstract

Dumping wastes generated from the oil industry to the local waste disposal facilities remains the most common means of oil waste management. In industrial landfills, the most serious ecological problem is the contamination of soil and local groundwater by landfill leachate. Polycyclic aromatic hydrocarbons (PAHs) represent a hazardous group of harmful chemical compounds that threaten human and ecosystem health. This study focuses on a PAH-contaminated soil from a landfill site in New South Wales, Australia. Soil was exposed to two different bioremediation treatments, natural attenuation and biostimulation using pea straw. The bacterial community composition and diversity of the PAH-contaminated soil were also examined using high throughput Illumina MiSeq sequencing. The results revealed that PAHs were degraded naturally by indigenous microorganisms; however, the addition of plant residues led to enhanced degradation (66.6%) at the beginning of the treatment, although in all treatments a degradation plateau occurred between 43 and 102 days of incubation during bioremediation. Quantitative PCR analysis showed an increase in the number of 16S rRNA and ITS gene copies as well as genes associated with Gram-positive PAH-degrading bacteria in the soil amended with pea straw. Next generation sequencing results and diversity indices revealed that a high proportion of the sequences from all soil samples belonged to the Proteobacteria phyla; gammaprotobacteria was the dominant class in all the investigated samples. Proteobacteria, Bacteroidetes, Firmicutes and Cyanobacteria were the most dominant in the natural attenuation and pea straw treated soil bacterial communities. The largest shift in bacterial communities was in the pea straw amended soils with increased abundances by Day 42 of Pseudomonas, Pseudoxanthomonas, Lewinella and Parvibaculum; these organisms are all known for their abilities to degrade petroleum hydrocarbons. However by Day 102, these organisms were decreased or not detected in both NA and PS treated soil samples at the end of incubation which may explain the degradation plateau. These findings suggest that Gram-negative PAH-degrading bacteria, mainly Pseudomonas sp., may be the key contributor to PAH-degradation in the landfill-polluted soil.