Bioaugmentation of polycyclic aromatic hydrocarbons-contaminated soil with Pleurotus ostreatus and Sphingobium sp. NS7 and their synergistic effect

Abstract:

[Background] Fungi and bacteria are considered to play a synergistic role in the bioremediation of polycyclic aromatic hydrocarbons (PAHs)-contaminated soil. However, there are limited studies on their synergistic degradation in soil. [Objective] This paper aims to investigate the different PAHs-degrading abilities of Pleurotus ostreatus and Sphingobium sp. NS7 and reveal their synergistic role in the biodegradation of anthracene and benzo[a]anthracene in soil. [Methods] The PAH degradation by P. ostreatus and Sphingobium sp. NS7 was performed in liquid culture. Soil microcosms were set up with radiolabeled anthracene and benzo[a]anthracene to investigate the fate of PAHs after bioaugmentation. [Results] NS7 can efficiently degrade low-molecular-weight (LMW) PAHs, as it removed almost all the anthracene when anthracene was the only carbon source. In the case of compound contamination, the bacterium removed >90% of phenanthrene, anthracene, fluoranthene, and pyrene while degraded benzo[a]pyrene with a low rate (9.76%). By contrast, P. ostreatus demonstrated relatively high removal rate (21.18%) of benzo[a]pyrene. The degradation rate of LMW PAHs by P. ostreatus was much lower than that by NS7. In the natural soil, the mineralization rate of anthracene and benzo[a]anthracene was 18.61% and 4.28%, respectively. P. ostreatus promoted benzo[a]anthracene mineralization in natural soil (increased by 2.24 folds) but NS7 failed to significantly improve anthracene mineralization. Thus, the competition of indigenous microbial communities might affect the survival of exogenous microorganisms. A sterile soil system excluding the microbial competition was used to investigate the helper role of fungal hyphae in the synergistic degradation. With the help of fungal hyphae, the mineralization of anthracene was enhanced by NS7 (mineralization rate up from 1.75% to 5.91%), but no promotion effect was observed in benzo[a]anthracene mineralization. Therefore, the reason for the enhancement of benzo[a]anthracene mineralization in natural soil by P. ostreatus might be that the fungal hyphae promoted the migration of indigenous PAHs-degrading bacteria in soil matrix and thus enhanced the contact of them with the contaminant. [Conclusion] Bacteria can efficiently degrade LMW PAHs while fungi showed high degradation rate of high-molecular-weight PAHs like benzo[a]pyrene. Enhanced PAH mineralization in natural soil was observed after P. ostreatus inoculation, and the mechanism might be that the fungal hyphae promoted migration of indigenous bacteria in the soil matrix. This study deepens our understanding of the synergistic degradation of PAHs in soil by fungus and bacterium and lays a theoretical basis for soil bioremediation based on the synergistic effect of fungi and bacteria.