Benzo(a)pyrene degradation and microbial community responses in composted soil

Effects of different organic substrate compositions on the decontamination of aged PAH-polluted soils through outdoor co-composting

The effects of different organic substrate compositions on the efficiency of outdoor co-composting as a bioremediation technology for decontaminating soil polluted by polycyclic aromatic hydrocarbons (PAHs) were investigated. Four different substrate mixtures and two different aged PAH-contaminated soils were used in a semi-pilot-scale experiment that lasted nearly 700 days. The two soils (A and B) differed concerning both the initial concentrations of the Ʃ16 US EPA PAHs (5926 vs. 369 mg kg-1, respectively) and the type of predominant PAH group by molecular weight. The experiments revealed that while the composition of the organic substrate had an impact on the rate of PAH degradation, it did not significantly influence the final extent of PAH degradation. Notably, the organic substrate consisting of green waste and wood chips (GW) was found to facilitate the most rapid rate of PAH degradation (first-order rate constant k = 0.033 ± 0.000 d-1 with soil A over the initial 42 days of the experiment and k = 0.036 ± 0.000 d-1 with soil B over the initial 56 days). Despite the differences in organic substrate compositions and types of soil being treated, PAH degradation levels exceeded at least 95% in all the treatments after more than 680 days of co-composting. Regardless of the composition, the removal of low- and medium- molecular-weight (2-4 rings) PAHs was nearly complete by the end of the experiment. Furthermore, high-molecular-weight PAHs (5 rings and more) were significantly degraded during co-composting, with reductions ranging from 54% to 79% in soil A and from 59% to 68% in soil B. All composts were dominated by Proteobacteria, Firmicutes, and Actinobacteria, with significant differences in abundance between soils. Genera with PAH degradation potentials were detected in all samples. The results of a battery of toxicity tests showed that there was almost no toxicity associated with the final composts.

Persistent polycyclic aromatic hydrocarbons removal from sewage sludge-amended soil through phytoremediation combined with solid-state ligninolytic fungal cultures

Polycyclic aromatic hydrocarbons (PAHs) are widely present in the environment, causing increasing concern because of their impact on soil health, food safety and potential health risks. Four bioremediation strategies were examined to assess the dissipation of PAHs in agricultural soil amended with sewage sludge over a period of 120 days: soil-sludge natural attenuation (SS); phytoremediation using maize (Zea mays L.) (PSS); mycoremediation (MR) separately using three white-rot fungi (Pleurotus ostreatus, Phanerochaete chrysosporium and Irpex lacteus); and plant-assisted mycoremediation (PMR) using a combination of maize and fungi. In the time frame of the experiment, mycoremediation using P. chrysosporium (MR-PH) exhibited a significantly higher (P < 0.05) degradation of total PAHs compared to the SS and PSS treatments, achieving a degradation rate of 52 %. Both the SS and PSS treatments demonstrated a lower degradation rate of total PAHs, with removal rates of 18 % and 32 %, respectively. The PMR treatments showed the highest removal rates of total PAHs at the end of the study, with degradation rates of 48-60 %. In the shoots of maize, only low- and medium-molecular-weight PAHs were found in both the PSS and PMR treatments. The calculated translocation and bioconversion factors always showed values < 1. The analysed enzymatic activities were higher in the PMR treatments compared to other treatments, which can be positively related to the higher degradation of PAHs in the soil.

Adaptation strategies and functional transitions of microbial community in pyrene-contaminated soils promoted by lead with Pseudomonas veronii and its extracellular polymeric substances

Pyrene was designated as a remediation target in this study, and low contamination of lead (Pb) was set to induce heavy metal stress. Pseudomonas veronii and its extracellular polymeric substances (EPSs) were chosen for biofortification, with the aim of elucidating the structural, metabolic, and functional responses of soil microbial communities. Community analysis of soil microorganisms using high-throughput sequencing showed that the co-addition of P. veronii and EPSs resulted in an increase in relative abundance of phyla associated with pyrene degradation, and formed a symbiotic system dominated by Firmicutes and Proteobacteria, which involved in pyrene metabolism. Co-occurrence network analysis revealed that the module containing P. veronii was the only one exhibiting a positive correlation between bacterial abundance and pyrene removal, indicating the potential of bioaugmentation in enriching functional taxa. Biofortification also enhanced the abundance of functional gene linked to EPS production (biofilm formation-Pseudomonas aeruginosa) and pyrene degradation. Furthermore, 17 potential functional bacteria were screened out using random forest algorithm. Lead contamination further promoted the growth of Proteobacteria, intensified cooperative associations among bacteria, and increased the abundance of bacteria with positive correlation with pyrene degradation. The results offer novel perspectives on alterations in microbial communities resulting from the synergistic impact of heavy metal stress and biofortification.

Enhancing resistance and cell survival in Acipenser ruthenus liver, gill, and kidney cells: The potential of heat shock protein inducers against PAH-benzo[a]pyrene stress

The Caspian Sea has faced many environmental challenges, such as oil pollution. Heat shock proteins (HSPs) play a critical role in stress conditions and physiological changes caused by disease or injury. By evaluating the effects of various HSP inducers (HSPi), including Pro-Tex® (NOP: 800 mM), amygdalin (AMG: 80 mM), and a novel synthetic compound derived from pirano piranazole (SZ: 80 µm) on isolated cells from Sterlet Sturgeon (Acipenser ruthenus) treated with 75% IC50 PAH-benzo[a]pyrene (BaP; B75). This study examines whether there is a correlation between exposure to the BaP pollutant and HSPs in fish. In vitro, after culturing cells from the liver, kidney, and gills, they were treated with HSPi compounds in the presence and absence of BaP. Western blotting was used to assess HSP27, HSP70, and HSP90 expression patterns. A variety of enzyme activities were measured before (without treatment) and after treatment with HSPis and HSPi + B75, including cytochrome P450 (CYP450) activity, specific enzyme activity for acetylcholinesterase (AChE), antioxidant capacity, liver indicator enzymes, cortisol levels, and immunity parameters. When compared to the control group, cells treated with B75 showed the lowest AChE enzyme activity (p < 0.0001). CYP450 activity was highest in group B75, while HSPi caused the opposite effect (p < 0.0001). HSPi + B75 increased HSP levels and antioxidant parameters while decreasing cortisol and liver indicator enzymes (p < 0.0001). HSPi may be a powerful and reliable method for enhancing the resistance of A. ruthenus to BaP stresses before exposure. Treating cells with HSP-inducing compounds, such as NOP, AMG, and SZ, can assist them in managing stress and increase HSP (27, 70, and 90) protein expression. Furthermore, the study findings suggest that HSPis can also mitigate the adverse effects of stress, ultimately increasing cell survival and resistance.

Experimentally and spectroscopically evidenced mechanistic study of butyl peroxyacid oxidative degradation of benzo[a]pyrene in soil

Benzo[a]pyrene (BaP) is a major indicator of soil contamination and categorized as a highly persistent, carcinogenic, and mutagenic polycyclic aromatic hydrocarbon. An advanced peroxyacid oxidation process was developed to reduce soil pollution caused by BaP originating from creosote spills from railroad sleepers. The pH, organic matter, particle size distribution of soil, and concentrations of BaP and heavy metals (Cd, Cu, Zn, Pb, and As) in the BaP-contaminated soils were estimated. A batch experiment was conducted to determine the effects of organic acid type, soil particle size, stirring speed, and reaction time on the peroxyacid oxidation of BaP in the soil samples. Additionally, the effect of the organic acid concentration on the peroxyacid degradation of BaP was investigated using an oxidizing agent in spiked soil with and without hydrogen peroxide. The results of the oxidation process indicated that BaP and heavy metal residuals were below acceptable Korean standards. A significant difference in the oxidative degradation of BaP was observed between the spiked and natural soil samples. The formation of a peroxyacid intermediate was primarily responsible for the enhanced BaP oxidation. Further, butyric acid could be reused thrice without losing the efficacy (<90%). The systematic peroxyacid oxidative degradation mechanism of BaP was also discussed. A qualitative analysis of the by-products of the BaP reaction was conducted, and their corresponding toxicities were determined for possible field applications. The findings conclude that the developed peroxyacid oxidation method has potential applications in the treatment of BaP-contaminated soils.

Structural, metabolic, and functional characteristics of soil microbial communities in response to benzo[a]pyrene stress

Understanding the characteristics of soil microbes responding to benzo[a]pyrene (BaP) helps to deepen the knowledge of the risks of BaP to soil ecosystem. In this study, the structural, metabolic, and functional responses of soil microbial communities to BaP (8.11 mg kg^-1) were investigated. Analysis of microbial community structure based on 16 S rRNA and ITS gene sequencing indicated that BaP addition enriched microbes associated with aromatic compound degradation (Sphingomonas, Bacilli, Fusarium) and oligotrophs (Blastocatellaceae, Rokubacteriales), but inhibited Cyanobacteria involved in nitrogen-fixing process. Network analysis showed that the bacterial community enhanced intraspecific cooperation, while fungal community mainly altered the keystone taxa under BaP stress. Biolog EcoPlate assay demonstrated that microbial metabolism of carbon sources, especially nitrogen-containing sources, was stimulated by BaP addition. Functional analysis based on enzyme activity tests, functional gene quantification, and function annotation showed that nitrogen-cycling processes, especially nitrogen fixation, were significantly inhibited. These results suggest that BaP-tolerant microbes may establish cooperative relationships and compete for resources and ecological niches with sensitive microbes, especially those associated with nitrogen cycling, ultimately leading to enhanced carbon source utilization and restricted nitrogen cycling. This study clearly elucidates the adaptation strategies and functional shifts of soil microbial communities to BaP contamination.

Effects of Corn Straw and Citric Acid on Removal of PAHs in Contaminated Soil Related to Changing of Bacterial Community and Functional Gene Expression

Root exudates can stimulate microbial degradation in rhizosphere, but it is unclear whether the rhizodegradation of polycyclic aromatic hydrocarbons (PAHs) occurs in corn straw-amended soil. Either citric acid or corn straw was added into PAHs-contaminated soil to investigate their effect on the removal of PAHs. Either corn straw (Y) or combined application of corn straw and citric acid (YN100) significantly (p < 0.05) enhanced the removal of soil PAHs by 8.43% and 18.62%, respectively. Both Y and YN100 treatments obviously increased the abundance of PAHs degraders and the potential hosts of PAH-ring hydroxylating dioxygenase (PAH-RHDα) genes. Interestingly, the copies of PAH-RHDα Gram-negative bacteria genes under YN100 treatment was significantly (p < 0.05) higher than those under Y treatment. The present results indicated that combined application of corn straw and citric acid could efficiently enhance the removal of PAHs in soil, mainly via increasing the relative abundances of PAH-degrading bacteria and the expression of PAH-RHDα genes in contaminated soil.

Impacts of bio-stimulants on pyrene degradation, prokaryotic community compositions, and functions

Bio-stimulation of the indigenous microbial community is considered as an effective strategy for the bioremediation of polluted environments. This examination explored the near effects of various bio-stimulants on pyrene degradation, prokaryotic community compositions, and functions using 16S rRNA amplicon sequencing and qPCR. At first, the results displayed significant differences (p < 0.05) between the prokaryotic community structures of the control group, PYR (contains pyrene only), and bio-stimulants amended groups. Among the bio-stimulants, biochar, oxalic acid, salicylate, NPK, and ammonium sulfate augmented the pyrene degradation potential of microbial communities. Moreover, the higher abundance of genera, such as Flavobacterium, Hydrogenophaga, Mycobacterium, Rhodococcus, Flavihumibacter, Pseudomonas, Novosphingobium, etc., across the treatments indicated that these genera play a vital role in pyrene metabolism. Based on the higher abundance of GP-RHD and nidA genes, we speculated that Gram-positive prokaryotic communities are more competent in pyrene dissipation than Gram-negative. Furthermore, the marked abundance of nifH, and pqqC genes in the NPK and SA treatments, respectively, suggested that different bio-stimulants might enrich certain bacterial assemblages. Besides, the significant distinctions (p < 0.05) between the bacterial consortia of HA (humic acid) and SA (sodium acetate) groups from NPK, OX (oxalic acid), UR (urea), NH4, and SC (salicylate) groups also suggested that different bio-stimulants might induce distinct ecological impacts influencing the succession of prokaryotic communities in distinct directions. This work provides new insight into the bacterial degradation of pyrene using the bio-stimulation technique. It suggests that it is equally important to investigate the community structure and functions along with studying their impacts on degradation when devising a bio-stimulation technology.

Effect of biochar-immobilized Sphingomonas sp. PJ2 on bioremediation of PAHs and bacterial community composition in saline soil

This study aimed to investigate the bioremediation efficiency and bacterial regulation mechanism of biochar-immobilized bacterium (BM) in polycyclic aromatic hydrocarbon (PAH)-contaminated saline soil by conducting pot experiments. In BM treatment, PAH-degrading strain Sphingomonas sp. PJ2 was inoculated into biochar produced at 400 °C and 600 °C using the pine needles (BM400 and BM600). The removal rates of PAHs, soil physicochemical properties, abundance of PAH-ring hydroxylating dioxygenase (PAH-RHD), and bacterial community composition were determined. After 60 days of bioremediation, BM treatment significantly (P < 0.05) increased the removal rate of PAHs compared with biochar and PJ2 alone (15.94% and 37.3%, respectively). BM treatment clearly improved the physicochemical properties of saline soil. Moreover, the amount of Gram-positive PAH degraders increased in BM-treated soils compared with other treatments, and their gene abundance had a strong positive correlation with the removal rates of PAHs in soils (r = 0.896; P < 0.01). Furthermore, BM treatment increased the abundance of Sphingomonas genus, indicating that the strain PJ2 could survive and colonize in PAH-contaminated saline soil under the protection of biochar. This study provided an effective and green approach for the remediation and improvement of the PAH-contaminated saline soil.

Insights into Bacterial Community Involved in Bioremediation of Aged Oil-Contaminated Soil in Arid Environment

Soil contamination by hydrocarbons due to oil spills has become a global concern and it has more implications in oil producing regions. Biostimulation is considered as one of the promising remediation techniques that can be adopted to enhance the rate of degradation of crude oil. The soil microbial consortia play a critical role in governing the biodegradation of total petroleum hydrocarbons (TPHs), in particular polycyclic aromatic hydrocarbons (PAHs). In this study, the degradation pattern of TPHs and PAHs of Kuwait soil biopiles was measured at three-month intervals. Then, the microbial consortium associated with oil degradation at each interval was revealed through 16S rRNA based next generation sequencing. Rapid degradation of TPHs and most of the PAHs was noticed at the first 3 months of biostimulation with a degradation rate of pyrene significantly higher compared to other PAHs counterparts. The taxonomic profiling of individual stages of remediation revealed that, biostimulation of the investigated soil favored the growth of Proteobacteria, Alphaprotobacteria, Chloroflexi, Chlorobi, and Acidobacteria groups. These findings provide a key step towards the restoration of oil-contaminated lands in the arid environment.

Assessment of Biological Contribution to Natural Recovery of Anthropized Freshwater Sediments From Argentina: Autochthonous Microbiome Structure and Functional Prediction

Monitored natural recovery (MNR) is an in situ technique of conventional remediation for the treatment of contaminated sediments that relies on natural processes to reduce the bioavailability or toxicity of contaminants. Metabarcoding and bioinformatics approaches to infer functional prediction were applied in bottom sediments of a tributary drainage channel of Río de La Plata estuary, in order to assess the biological contribution to MNR. Hydrocarbon concentration in water samples and surface sediments was below the detection limit. Surface sediments were represented with high available phosphorous, alkaline pH, and the bacterial classes Anaerolineae, Planctomycetia, and Deltaproteobacteria. The functional prediction in surface sediments showed an increase of metabolic activity, carbon fixation, methanogenesis, and synergistic relationships between Archaeas, Syntrophobacterales, and Desulfobacterales. The prediction in non-surface sediments suggested the capacity to respond to different kinds of environmental stresses (oxidative, osmotic, heat, acid pH, and heavy metals), predicted mostly in Lactobacillales order, and the capacity of Alphaproteobacteria, Betaproteobacteria, Gammaproteobacteria, and Actinomyces classes to degrade xenobiotic compounds. Canonical correspondence analysis (CCA) suggests that depth, phosphate content, redox potential, and pH were the variables that structured the bacterial community and not the hydrocarbons. The characterization of sediments by metabarcoding and functional prediction approaches, allowed to assess how the microbial activity would contribute to the recovery of the site.

Bioremediation of diesel and gasoline-contaminated soil by co-vermicomposting amended with activated sludge: Diesel and gasoline degradation and kinetics

Present study aims to examine the efficiency of co-vermicomposting amended with activated sludge and E. fetida earthworm for bioremediation of diesel and gasoline from contaminated soil. The diesel and gasoline removal efficiency and degradation rates coefficients were estimated with gas chromatography (GC) analysis and first-order kinetics. The removal of gasoline and diesel in different co-vermicomposting processes with and without E. fetida ranged between 65-100% and 24.94-63.93%, respectively within 90- day experiment. Removal of gasoline and diesel increased in soil with addition of earthworm (E. fetida); higher degradation rate coefficients (k) were observed for co-vermicomposting with earthworm compared with co-vermicomposting processes. The highest k (0.014) for diesel degradation was estimated for microcosm reactor 4 (R4), where high numbers of E. fetida accelerate the less biodegradable organic contaminant from the soil matrices. The reasonable survival rates of earthworms in exposure to high concentration of petroleum-derivatives contaminated soils indicated increased activity of ligninolytic diesel-degrading earthworms and microorganisms. Therefore, co-vermicomposting amended with activated sludge is suggested as feasible and promising technologies for bioremediation of high content of organic contaminants from the soil matrices.

Bioremediation of PAH-Contaminated Soils: Process Enhancement through Composting/Compost

Bioremediation of contaminated soils has gained increasing interest in recent years as a low-cost and environmentally friendly technology to clean soils polluted with anthropogenic contaminants. However, some organic pollutants in soil have a low biodegradability or are not bioavailable, which hampers the use of bioremediation for their removal. This is the case of polycyclic aromatic hydrocarbons (PAHs), which normally are stable and hydrophobic chemical structures. In this review, several approaches for the decontamination of PAH-polluted soil are presented and discussed in detail. The use of compost as biostimulation- and bioaugmentation-coupled technologies are described in detail, and some parameters, such as the stability of compost, deserve special attention to obtain better results. Composting as an ex situ technology, with the use of some specific products like surfactants, is also discussed. In summary, the use of compost and composting are promising technologies (in all the approaches presented) for the bioremediation of PAH-contaminated soils.

The distinct response of phenanthrene enriched bacterial consortia to different PAHs and their degradation potential: a mangrove sediment microcosm study

Understanding the microbial community succession to polycyclic aromatic hydrocarbons (PAHs) and identification of important degrading microbial groups are crucial for the designing of appropriate bioremediation strategies. In the present study, two distinct phenanthrene enriched bacterial consortia were treated against high molecular weight (Pyrene, Benzo (a) pyrene and Benzo (a) fluoranthene) and the response was studied in term of taxonomic variations by using High Throughput Illumina sequencing and qPCR analysis. Overall, the type of PAHs significantly affected the composition and the relative abundance of bacterial communities while no obvious difference was detected between bacterial communities of benzo (a) pyrene and benzo (a) fluoranthene treatments. Genera, Novosphingobium, Pseudomonas, Flavobacterium, Mycobacterium, Hoeflae, and Algoriphagus dominated all PAHs treatment groups indicating that they could be the key PAHs degrading phylotypes. Due to the higher abundance of gram-negative PAH-ring hydroxylating dioxygenase gene than that of gram-positive bacteria in all treated groups, we speculated that gram-negative bacteria may contribute more in the PAH degradation. The studied sediments harbored rich PAHs degrading bacterial assemblages involved in both low and high molecular weight PAHs and these findings provided new insight into the perspective of microbial PAHs bioremediation in the mangrove ecosystem.

Dissipation of polycyclic aromatic hydrocarbons (PAHs) in soil amended with sewage sludge and sludge compost

In this study, greenhouse experiments were conducted under the condition of different amendment ratios and planting tall fescue (Festuca arundinacea). The amendment ratios of sewage sludge or sludge compost to soil were of 0, 10, 25, and 50% (w/w). The removal rates of PAH, catalase, and dehydrogenase activities of amended soil and accumulation of PAHs by vegetation were detected to investigate the differences of PAH dissipation in sludge-amended and compost-amended soils. The initial PAH concentrations in three amended soils increased with the more addition of sludge or compost. After 126-day experiment, maximum PAH removal rates were observed in sludge-amended and compost-amended soils with PAH concentration of about 200 μg kg^-1. And the removal of PAHs showed better efficiencies in compost soil rather than in sludge soil. The more catalase activity and dehydrogenase activity of soil were obtained, respectively, in sludge soil and compost soil. The results indicated that the mechanism of PAH dissipation in two types of amended soils were different. The abundant amount of microorganism dominated PAH dissipation in sludge soils, and PAHs dissipated mainly caused of intense activity of microorganism in compost soils. In addition, PAH accumulation in tall fescue suggested that the transference approach of PAHs was from soil to the roots, and then accumulated in the shoots of tall fescue. It was prone to store up more PAHs in vegetation in the condition of high molar weight of PAHs, more biomass of vegetation, and heavy PAH concentration in soil.

Effects of benzo [a] pyrene (BaP) on the composting and microbial community of sewage sludge

Benzo [a] pyrene (BaP), the most ubiquitous polycyclic aromatic hydrocarbons (PAHs) found in sludge, can impact the composting processes of sewage sludge as well as the quality of compost produced. In the present study, we investigated the effects of BaP at various concentrations on physicochemical characteristics, heavy metal passivation, and microbial community during the composting processes. The removal efficiency of BaP at 5 and 20 mg kg^-1 after composting was 51.1% and 74.2%, respectively. In comparison with the control, the content of residual Cu, Pb, Cr and Ni in 5 mg kg^-1 BaP contained system declined dramatically on the second day of composting, while such content in 20 mg kg^-1 BaP system significantly decreased on the 8th day. Regardless of the presence of BaP in the sludge, composting process had a positive passivation effect on Cu, Pb, Cr and Ni. A stronger inhibitory effect of BaP at higher concentration was observed on microorganism, which reduced microbial abundance and species in the composting, and influenced microbial diversity. Besides, microbial communities in BaP-containing composting would improve the transformation of silicates and minerals, increase the concentration of humus and extend the passivation time of heavy metals. As these results verified, composting process could remove BaP from the sludge effectively, and BaP had a significant impact on heavy metal passivation and abundance and composition of microbial community during the composting process.

Fate of di (2‑ethylhexyl) phthalate in different soils and associated bacterial community changes

Di (2‑ethylhexyl) phthalate (DEHP) is a ubiquitous organic pollutant, which has caused considerable pollution in arable soils. In this study, the relationship between DEHP degradation potential and soil properties in 12 agricultural soils (S1-S12) was examined in a microcosm based experiment. Six of these soils were then selected to monitor patterns in bacterial community responses. It was found that DEHP degradation was positively correlated with bacterial counts in the original soils, suggesting a key role for bacteria in degradation. However, DEHP metabolism did not always lead to complete degradation. Its monoester metabolite, mono (2‑ethylhexyl) phthalate (MEHP), was present at appreciable levels in the two acidic soils (S1 and S2) during the incubation period of 35 days. Based on high-throughput sequencing data, we observed a greater impact of DEHP contamination on bacterial community structure in acidic soils than in the other soils. Nocardioides, Ramlibacter and unclassified Sphingomonadaceae were enriched in the two near-neutral soils where degradation was highest (S4 and S7), suggesting that these organisms might be efficient degraders. The relative abundance of Tumibacillus was greatly reduced in 50% of the six soils examined, demonstrating a high sensitivity to DEHP contamination. Furthermore, putative organic-matter decomposing bacteria (including Tumebacillus and other bacteria taxa such as members from Micromonosporaceae) were greatly reduced in the two acidic soils (S1 and S2), possibly due to the accumulation of MEHP. These results suggest a crucial role of soil acidity in determining the fate and impact of DEHP in soil ecosystems, which deserves further investigation. This work contributes to a better understanding of the environmental behavior of DEHP in soil and should facilitate the development of appropriate remediation technologies.

The influence of heavy metals, polyaromatic hydrocarbons, and polychlorinated biphenyls pollution on the development of antibiotic resistance in soils

The minireview is devoted to the analysis of the influence of soil pollution with heavy metals, polyaromatic hydrocarbons (PAHs), and the polychlorinated biphenyls (PCBs) on the distribution of antibiotics resistance genes (ARGs) in soil microbiomes. It is shown that the best understanding of ARGs distribution process requires studying the influence of pollutants on this process in natural microbiocenoses. Heavy metals promote co-selection of genes determining resistance to them together with ARGs in the same mobile elements of a bacterial genome, but the majority of studies focus on agricultural soils enriched with ARGs originating from manure. Studying nonagricultural soils would clear mechanisms of ARGs transfer in natural and anthropogenically transformed environments and highlight the role of antibiotic-producing bacteria. PAHs make a considerable shift in soil microbiomes leading to an increase in the number of Actinobacteria which are the source of antibiotics formation and bear multiple ARGs. The soils polluted with PAHs can be a selective medium for bacteria resistant to antibiotics, and the level of ARGs expression is much higher. PCBs are accumulated in soils and significantly alter the specific structure of soil microbiocenoses. In such soils, representatives of the genera Acinetobacter, Pseudomonas, and Alcanivorax dominate, and the ability to degrade PCBs is connected to horizontal gene transfer (HGT) and high level of genomic plasticity. The attention is also focused on the need to study the properties of the soil having an impact on the bioavailability of pollutants and, as a result, on resistome of soil microorganisms.

The potential impact on the biodegradation of organic pollutants from composting technology for soil remediation

Large numbers of organic pollutants (OPs), such as polycyclic aromatic hydrocarbons, pesticides and petroleum, are discharged into soil, posing a huge threat to natural environment. Traditional chemical and physical remediation technologies are either incompetent or expensive, and may cause secondary pollution. The technology of soil composting or use of compost as soil amendment can utilize quantities of active microbes to degrade OPs with the help of available nutrients in the compost matrix. It is highly cost-effective for soil remediation. On the one hand, compost incorporated into contaminated soil is capable of increasing the organic matter content, which improves the soil environment and stimulates the metabolically activity of microbial community. On the other hand, the organic matter in composts would increase the adsorption of OPs and affect their bioavailability, leading to decreased fraction available for microorganism-mediated degradation. Some advanced instrumental analytical approaches developed in recent years may be adopted to expound this process. Therefore, the study on bioavailability of OPs in soil is extremely important for the application of composting technology. This work will discuss the changes of physical and chemical properties of contaminated soils and the bioavailability of OPs by the adsorption of composting matrix. The characteristics of OPs, types and compositions of compost amendments, soil/compost ratio and compost distribution influence the bioavailability of OPs. In addition, the impact of composting factors (composting temperature, co-substrates and exogenous microorganisms) on the removal and bioavailability of OPs is also studied.

Impact of nitrogen-polycyclic aromatic hydrocarbons on phenanthrene and benzo[a]pyrene mineralisation in soil

When aromatic hydrocarbons are present in contaminated soils, they often occur in mixtures. The impact of four different (3-ring) nitrogen-containing polycyclic aromatic hydrocarbons (N-PAHs) on ^12/14C-phenanthrene and ^12/14C-benzo[a]pyrene (B[a]P) mineralisation in soil was investigated over a 90 d incubation period. The results revealed that both ^12/14C-phenanthrene and ^12/14C-benzo[a]pyrene showed no significant mineralisation in soils amended with 10mgkg ^-1 and 100mgkg ^-1 N-PAHs (p>0.05). However, increases in lag-phases and decreases in the rates and extents of mineralisation were observed, over time. Among the N-PAHs, benzo[h]quinoline impacted ¹⁴C-phenanthrene mineralisation with extended and diauxic lag phases. Furthermore,^12/14C-B[a]P and ¹⁴C-benzo[a]pyrene-nitrogen-containing polycyclic aromatic hydrocarbons (¹⁴C-B[a]P-N-PAHs) amended soils showed extensive lag phases (> 21 d); with some ¹⁴C-B[a]P-N-PAH mineralisation recording <1% in both concentrations (10mgkg ^-1 and 100mgkg ^-1), over time. This study suggests that the presence of N-PAHs in contaminated soil may impact the microbial degradation of polycyclic aromatic hydrocarbons (PAHs) and the impact was most likely the result of limited success in achieving absolute biodegradation of some PAHs in soil.

Naphthalene biodegradation under oxygen-limiting conditions: community dynamics and the relevance of biofilm-forming capacity

Toxic polycyclic aromatic hydrocarbons (PAHs) are frequently released into the environment from anthropogenic sources. PAH remediation strategies focus on biological processes mediated by bacteria. The availability of oxygen in polluted environments is often limited or absent, and only bacteria able to thrive in these conditions can be considered for bioremediation strategies. To identify bacterial strains able to degrade PAHs under oxygen‐limiting conditions, we set up enrichment cultures from samples of an oil‐polluted aquifer, using either anoxic or microaerophilic condition and with PAHs as the sole carbon source. Despite the presence of a significant community of nitrate‐reducing bacteria, the initial community, which was dominated by Betaproteobacteria, was incapable of PAH degradation under strict anoxic conditions, although a clear shift in the structure of the community towards an increase in the Alphaproteobacteria (Sphingomonadaceae), Actinobacteria and an uncultured group of Acidobacteria was observed in the enrichments. In contrast, growth under microaerophilic conditions with naphthalene as the carbon source evidenced the development of a biofilm structure around the naphthalene crystal. The enrichment process selected two co‐dominant groups which finally reached 97% of the bacterial communities: Variovorax spp. (54%, Betaproteobacteria) and Starkeya spp. (43%, Xanthobacteraceae). The two dominant populations were able to grow with naphthalene, although only Starkeya was able to reproduce the biofilm structure around the naphthalene crystal. The pathway for naphthalene degradation was identified, which included as essential steps dioxygenases with high affinity for oxygen, showing 99% identity with Xanthobacter polyaromaticivorans dbd cluster for PAH degradation. Our results suggest that the biofilm formation capacity of Starkeya provided a structure to allocate its cells at an appropriate distance from the toxic carbon source.