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Considerable efforts that isolate and characterize degrading bacteria for polycyclic aromatic hydrocarbons (PAHs) have focused on contaminated environments so far. Here we isolated three distinctive pyrene (PYR)-degrading bacteria from a paddy soil that was not contaminated with PAHs. These included a novel Bacillus sp. PyB-9 and efficient degraders, Shigella sp. PyB-6 and Agromyces sp. PyB-10. All three strains could utilize naphthalene, phenanthrene, anthracene, fluoranthene and PYR as sole carbon sources, and degraded PYR in a range of temperatures (27–37 °C) and pH (5–8). Strains PyB-6 and PyB-10 almost completely degraded 50 mg L−1 PYR within 15 days, and 75.5% and 98.9% of 100 mg L−1 PYR in 27 days, respectively. The kinetics of PYR biodegradation was well represented by the Gompertz model. Ten and twelve PYR metabolites were identified in PYR degradation process by strains PyB-6 and PyB-10, respectively. Chemical analyses demonstrated that the degradation mechanisms of PYR were the same for strains PyB-6 and PyB-10 with initial dioxygenation mainly on C-4,5 positions of PYR. The degradation of 4,5-phenanthrenedicarboxylic acid was branched to 4-phenanthrenecarboxylic acid pathway and 5-hydroxy-4-phenanthrenecarboxylic acid pathway, both of which played important roles in PYR degradation by strains PyB-6 and PyB-10. To our knowledge, Shigella sp. and Agromyces sp. were found for the first time to possess the capability for PAHs degradation. These findings contributed to upgrading the bank of microbial resource and knowledge on PAH biodegradation. [ABSTRACT FROM AUTHOR]

Purpose: Soil organic matter (SOM) in different density fractions has different characteristics and turnover rates. The environmental fates and risks of polycyclic aromatic hydrocarbons (PAHs) associated with different soil fractions are inevitably different during the transformation of SOM. Understanding the distribution and toxic equivalence of PAHs in soil density fractions helps in elucidating the SOM characteristics and position roles in environmental risks of PAHs in contaminated soils. Materials and methods: Four long-term field contaminated soils were physically separated into free light fractions (F-LF), occluded light fractions (O-LF), and heavy fractions (HF) by using density liquids (1.60 g cm−3) after sonication, and the distribution and toxic equivalence of PAHs in the three fractions were studied and assessed. Furthermore, the roles of organic carbon (OC) content, black carbon (BC) content, and physical morphological characteristics of different soil fractions in PAHs distribution and environmental risks were discussed. Results and discussion: The contents of total PAHs in F-LF and O-LF were in the range of 95.13–361.71 mg kg−1 and 226.32–1159.90 mg kg−1, respectively, and both of them were far higher than those of in HF, which ranged from 4.80 to 8.94 mg kg−1. Although the F-LF and O-LF accounted for only 0.57–11.96% and 0.22–1.07% of the studied soils, they were associated with 9.52–47.88% and 11.43–27.47% of soil total PAHs. The contents of total PAHs in different soil fractions exhibited an insignificant correlation with their OC content but had a significant correlation with BC content (**P < 0.01). The calculated average values of total TEQBaP (toxic equivalence of benzo[a]pyrene) in soil fractions followed the order of O-LF > F-LF ≫ HF. Conclusions: It is noteworthy that the enrichment capacity and high toxicity of PAHs in O-LF related to high BC content. We suggested that organic matter losing or soil physical structure destruction could lead to the exposure of PAHs in O-LF and the increase of environmental risks of PAHs in soil. [ABSTRACT FROM AUTHOR]

Co-contamination of polycyclic aromatic hydrocarbons (PAHs) and heavy metals in soil is very common worldwide. In this study, free (FE) and immobilized (IE) Escherichia sp. on biochar solidified with alginate were used to reveal the effect of cadmium (Cd) on pyrene biodegradation in solution, spiked soil, and field co-contaminated soils. The results showed that when there was no coexisting Cd, the average removal rates of pyrene in the FE and IE treatments were 99.3% and 92.9%, respectively, in solution within 28 days, and 90.2% and 77.4%, respectively, in spiked soil incubated for 90 days. When the concentration of coexisting Cd was 1 mg L−1 and 50 mg L−1 in solution, negligible biodegradation of pyrene was observed for both FE and IE treatments. In spiked soils, the removal rate of pyrene for FE (88.7%) was significantly higher than that for IE (52.9%) when the content of coexisting Cd was 1 mg kg−1. However, there was negligible biodegradation of pyrene in both FE and IE treatments when the content of coexisting Cd was 50 mg kg−1. In field co-contaminated soil, the biodegradation of PAHs was negligible for all treatments. In comparison to FE, IE did not show superiority in bioremediating co-contaminated soil in terms of PAH removal in this study. • Free/immobilized Escherichia sp. in solution without Cd highly degraded pyrene. • Pyrene degradation blocked by Cd at 1 mg L−1 for free/immobilized cells in solution • Pyrene degradation inhibited by Cd at 1 mg kg−1 for immobilized cell in spiked soil • Degradation of pyrene was higher overall by free cells than by immobilized cells. • Free/immobilized cells did not enhance PAH dissipation in field co-contaminated soil. [ABSTRACT FROM AUTHOR]