Characteristics Of PAHs Contamination And The Microbial Responses In Soil Around Steel Plants

Polycyclic Aromatic Hydrocarbons(PAHs)are a group of typical persistent organic pollutants widely present in the natural environment and potentially harmful to human health and the ecological environment.Exogenous PAHs pollution can also impact microbial ecology in the environment and change the community structure and diversity of microorganisms.One of the most effective ways to remediate PAHs pollution is to screen PAHs degrading bacteria from the polluted environment to obtain high efficiency degrading bacteria for ecological restoration.Therefore,it is important to understand the PAHs pollution characteristics and the associated microbial information in the environment.Contaminated soil samples were collected around the typical steel plants in six provinces in China to measure their PAHs concentrations and relevant physical and chemical characteristics.And then six samples with the highest PAHs pollution levels and ecological risks in each provinve were selected to explore the bacterial community structure in the contaminated soils via the PacBio full-length 16S r RNA gene amplicon sequencing and qPCR.At the same time,we also screened the PAHs degrading bacteria and studied their degradation efficiency.The main findings are as follows:(1)Polycyclic Aromatic Hydrocarbons were detected in multiple samples in Jinan,Shandong Province;Hangzhou,Zhejiang Province;Anshan,Liaoning Province;Taiyuan,Shanxi Province;Panzhihua,Sichuan Province;and Kunming,Yunnan Province,with the concentration range of 1.12 to 264.20 mg/kg.The average of PAHs concentrations are in the order:Jinan(3.86±3.16 mg/kg)<Hangzhou(4.13±4.28 mg/kg)<Anshan(10.39±2.89 mg/kg)<Taiyuan(11.67±4.45 mg/kg)<Panzhihua(19.07±5.34mg/kg)<Kunming(130.24±106.70 mg/kg).Stepwise regression analyses showed that total organic carbon was the main controlling factor of PAHs residue in the test soils.The Nemerow index evaluation method indicated that soils from Jinan,Hangzhou and Anshan were mostly clean or lightly contaminated by PAHs.But soil samples from Taiyuan,Panzhihua and Kunming were moderately or heavily contaminated by PAHs.It was found that the TEQ_{Ba P} values in multiple soil samples exceeded the prescribed risk value(0.55 mg/kg)of the benzo[a]pyrene toxicity equivalent ecological risk assessment,indicating that PAHs pollution constitutes a potential ecological risk.(2)The soil samples with the highest pollution level and ecological risk at each location were sequenced using the Pacbio full-length 16S r RNA gene amplicon sequencing.Some typical PAH-degrading bacteria such as the genera of Lysobacter,Marinobacter,Kocuria,Pseudarthrobacter and Bacillus megaterium,Lysobacter yangpyeongensi,were found in all the test soil samples,while other PAHs-degrading bacteria were only found in the soil from Kunming that contains PAHs with higher molecular weight.Combined with qPCR analyses,it was found that the relative abundance of some PAHs-degrading bacteria such as Lysobacter and Haliangium was low,but the absolute abundance was high in the heavily PAHs polluted soils.The bacterial community diversity and richness were the lowest in the soil from Taiyuan,and the highest in the soil from Hangzhou.The soil microbial co-generation network analyses showed that Ellin6067?Kocuria,Planomicrobium,RB41,Salinimicrobium,Marinobacter,Pseudoxanthomonas,etc,were the key PAHs degrading bacteria in the soil bacterial community.(3)16 different strains of phenanthrene/pyrene degrading bacteria were screened and isolated from the sequenced soil samples from Kunming,Yunnan Prvince;Panzhihua,Sichuan Province;and Anshan,Liaoning Province,but no such degrading bacteria were screened in soil samples from Jinan,Shandong Province;Hangzhou,Zhejiang Province,and Taiyuan,Shanxi Province.The degradation rates of phenanthrene and pyrene by the 16 isolated strains within 7 days were 2.29%-95.96%and 1.68%-24.26%at the initial concentrations of 100 mg/L and 50 mg/L,respectively.Based on the phylogenetic tree of 16S r RNA gene,the main degrading bacteria were the genera of Rhodococcus,Pseudarthrobacter,Pseudoxanthomonas,Mycolicibacterium,Microbacterium,Nocardioides,Streptomyces,Variovorax,Ensifer and Paenarthrobacter.Except for Mycolicibacterium and Paenarthrobacter,all other bacteria genera were detected in the corresponding test soils by the PacBio sequencing.At the species level,the isolated strains WR3(Ensifer Adhaerens)and WR22(Mycolicibacterium Gilvum)were sequenced in the bacterial community.It was found that the 16 isolated PAHs-degrading bacteria contained multiple functional genes such as RHD?-GP,nid A,nid B,nar Aa,ndo,and pdo.