Microbial communities in bioremediation of soils contaminated by polycyclic aromatic hydrocarbons

Polycyclic aromatic hydrocarbons (PAHs) are produced by incomplete combustion of organic matter and hydrocarbon fuels, but also by natural processes. PAHs are released to the environment causing the contamination of soil with subsequent risks for humans and ecosystems. Soil PAH degradation depends on biotic and abiotic factors and the composition and structure of microbial community also play a fundamental role. Synergy between microbial groups can result in a complete PAH mineralization. Enhancement of this process is the basis of bioremediation of PAH contaminated soils. Indeed, soil bioremediation can be accelerated by the addition of fungal species, able to metabolize recalcitrant organic compounds, and of compost, rich in microbial species. In this frame, the aim of this research was to study the role of microorganisms in soil PAH degradation and to assess the structural and functional changes in microbial community along the time. A mesocosm study was set-up by incubating an industrial soil spiked with benzo[a]pyrene and anthracene (150 mg/Kg, each) in three different treatments: 1) addition of compost; 2) addition of four fungal species; 3) no addition. The mesocosms were incubated in controlled conditions of temperature and humidity along 273 days, during which PAH concentrations, biomass and structure of microbial community, through PLFA profile, were monitored. The native soil microbial community was able to reduce anthracene (96%) and benzo[a]pyrene (52%), although both the two PAHs were degraded quickly in treated soil respect to untreated. The addition of compost improved mainly benzo[a]pyrene degradation, increasing it by 8%. Along the time, Gram-positive bacteria and actinomycetes remained constant, whereas Gram-negative bacteria and fungus decreased. The metabolic activity of Gram-negative bacteria was predominant compared to the other microbial groups. Soil compost addition effectively improves bioremediation, increasing PAH degradation.