Improving Polycyclic Aromatic Hydrocarbon Biodegradation in Contaminated Soil Through Low-Level Surfactant Addition After Conventional Bioremediation

Metagenomic insights into the microbial cooperative networks of a benz(a)anthracene-7,12-dione degrading community from a creosote-contaminated soil

Genotoxicity of PAH-contaminated soils can eventually increase after bioremediation due to the formation and accumulation of polar transformation products, mainly oxygenated PAHs (oxy-PAHs). Biodegradation of oxy-PAHs has been described in soils, but information on the microorganisms and mechanisms involved is still scarce. Benz(a)anthracene-7,12-dione (BaAQ), a transformation product from benz(a)anthracene frequently detected in soils, presents higher genotoxic potential than its parent PAH. Here, using sand-in-liquid microcosms we identified a specialized BaAQ-degrading subpopulation in a PAH-contaminated soil. A BaAQ-degrading microbial consortium was obtained by enrichment in sand-in-liquid cultures with BaAQ as sole carbon source, and its metagenomic analysis identified members of Sphingobium, Stenotrophomonas, Pusillimonas, Olivibacter, Pseudomonas, Achromobacter, and Hyphomicrobiales as major components. The integration of data from metabolomic and metagenomic functional gene analyses of the consortium revealed that the BaAQ metabolic pathway was initiated by Baeyer-Villiger monooxygenases (BVMOs). The presence of plasmid pANTQ-1 in the metagenomic sequences, identified in a previous multi-omic characterization of a 9,10-anthraquinone-degrading isolate recovered from the same soil, suggested the occurrence of a horizontal gene transfer event. Further metagenomic analysis of the BaAQ-degrading consortium also provided insights into the potential roles and interactions within the consortium members. Several potential auxotrophies were detected, indicating that relevant nutritional interdependencies and syntrophic associations were taking place within the community members, not only to provide suitable carbon and energy sources, but also to supply essential nutrients and cofactors. Our work confirms the essential role that BVMO may play as a detoxification mechanism to mitigate the risk posed by oxy-PAH formation during bioremediation of contaminated soils.

Metagenomic analysis of ready biodegradability tests to ascertain the relationship between microbiota and the biodegradability of test chemicals

Ready biodegradability tests conducted in accordance with the Organisation for Economic Co-operation and Development guidelines (test 301C or test 301F) are performed using activated sludge (AS) prepared by the Chemicals Evaluation and Research Institute (AS-CERI) or that taken from a sewage treatment plant (AS-STP). It had been reported that AS-CERI had lower activity than AS-STP in biodegrading test chemicals, and that biodegradation was accelerated by increasing the volume of the test medium. However, these phenomena have not been clarified from the perspective of the microbiota. In this study, using metagenomic analysis, we first showed that the microbiota of AS-CERI was biased in its distribution of phyla, less diverse, and had greater lot-to-lot variability than that of AS-STP. Second, after cultivation for a long period of time, the microbiota of AS-STP and AS-CERI became more similar to each other in terms of community structure. Third, determining degraders of test substances when each substance was actively biodegraded was found to be an effective approach. Finally, we clarified experimentally that a large volume of test medium increased the number of species that could degrade test substances in the condition where the initial concentrations of each substance and AS-STP were kept constant.

Comparing biochar and hydrochar for reducing the risk of organic contaminants in polluted river sediments used for growing energy crops

In Europe alone, >200 million m³ of river sediments are dredged each year, part of which are contaminated to such an extent that they have to be landfilled. This study compares the use of biochar and hydrochar for the remediation of sediment contaminated with pentachlorobenzene, hexachlorobenzene, lindane, trifluralin, alachlor, simazine, and atrazine with the motivation to make sediments contaminated by such priority substances usable as arable land for growing energy crops. Biochar and hydrochar originating from Miscanthus giganteus and Beta vulgaris shreds were compared for their potential to reduce contaminant associated risk in sediments. Specifically, by investigating the effects of sorbent amendment rate (1, 5, and 10 %) and incubation time (14, 30, and 180 d) on contaminant bioaccessibility, toxicity to the bacteria Vibrio fischeri, as well as toxicity and plant uptake in Zea mays. Biochar reduced contaminant bioaccessibility up to five times more than hydrochar. The bioaccessibility of contaminants decreased up to sevenfold with increasing incubation time, indicating that the performance of carbonaceous sorbents may be underestimated in short-term lab experiments. Biochar reduced contaminants toxicity to Vibrio fischeri, whereas hydrochar was itself toxic to the bacteria. Toxicity to Zea mays was determined by contaminant bioaccessibility but also sorbent feedstock with cellulose rich Beta vulgaris based sorbents exhibiting toxic effects. The plant uptake of all contaminants decreased after sorbent amendment.

Recent Advances in the Study of the Remediation of Polycyclic Aromatic Compound (PAC)-Contaminated Soils: Transformation Products, Toxicity, and Bioavailability Analyses

Polycyclic aromatic compounds (PACs) encompass a diverse group of compounds, often found in historically contaminated sites. Different experimental techniques have been used to remediate PACs-contaminated soils. This brief review surveyed over 270 studies concerning remediation of PACs-contaminated soils and found that, while these studies often measured the concentration of 16 parent polycyclic aromatic hydrocarbons (PAHs) pre- and post-remediation, only a fraction of the studies included the measurement of PAC-transformation products (PAC-TPs) and other PACs (n = 33). Only a few studies also incorporated genotoxicity/toxicity/mutagenicity analysis pre- and post-remediation (n = 5). Another aspect that these studies often neglected to include was bioavailability, as none of the studies that included measurement of PAH-TPs and PACs included bioavailability investigation. Based on the literature analysis, future remediation studies need to consider chemical analysis of PAH-TPs and PACs, genotoxicity/toxicity/mutagenicity, and bioavailability analyses pre- and post-remediation. These assessments will help address numerous concerns including, among others, the presence, properties, and toxicity of PACs and PAH-TPs, risk assessment of soil post-remediation, and the bioavailability of PAH-TPs. Other supplementary techniques that help assist these analyses and recommendations for future analyses are also discussed.

Spatial analysis of a hydrocarbon waste-remediating landfarm demonstrates influence of management practices on bacterial and fungal community structure

Cultivation of dedicated soil plots called 'landfarms' is an effective technology for bioremediation of hydrocarbon waste generated by various industrial practices. To understand the influence of soil conditions on landfarm microbial communities, analysis of bacterial and fungal community structure using next-generation sequencing at different sections and depths was performed across a hydrocarbon-waste landfarm in Regina, Saskatchewan, Canada. While a core set of hydrocarbon-associated bacterial and fungal taxa are present throughout the landfarm, unique bacterial and fungal operational taxonomic units are differentially abundant at sections within the landfarm, which correlate with differences in soil physiochemical properties and management practices. Increased frequency of waste application resulted in strong positive correlations between bacterial community assemblages and elevated amounts of oil, grease and F3 - F4 hydrocarbon fractions. In areas of standing water and lower application of hydrocarbon, microbial community structure correlated with soil pH, trace nutrients and metals. Overall, diversity and structure of bacterial communities remain relatively stable across the landfarm, while in contrast, fungal community structure appears more responsive to soil oxygen conditions. Results are consistent with the hypothesis that years of bioremediation activity have shaped microbial communities; however, several management practices can be undertaken to increase efficiency of remediation, including the removal of standing water and soil tilling across the landfarm.

Beyond the obvious: Environmental health implications of polar polycyclic aromatic hydrocarbons

The genotoxic, mutagenic and carcinogenic effects of polar polycyclic aromatic hydrocarbons (polar PAHs) are believed to surpass those of their parent PAHs; however, their environmental and human health implications have been largely unexplored. Oxygenated PAHs (oxy-PAHs) is a critical class of polar PAHs associated with carcinogenic effects without enzymatic activation. They also cause an upsurge in reactive oxygen species (ROS) in living cells. This results in oxidative stress and other consequences, such as abnormal gene expressions, altered protein activities, mutagenesis, and carcinogenesis. Similarly, some nitrated PAHs (N-PAHs) are probable human carcinogens as classified by the International Agency for Research on Cancer (IARC). Heterocyclic PAHs (polar PAHs containing nitrogen, sulphur and oxygen atoms within the aromatic rings) have been shown to be potent endocrine disruptors, primarily through their estrogenic activities. Despite the high toxicity and enhanced environmental mobility of many polar PAHs, they have attracted only a little attention in risk assessment of contaminated sites. This may lead to underestimation of potential risks, and remediation end points. In this review, the toxicity of polar PAHs and their associated mechanisms of action, including their role in mutagenic, carcinogenic, developmental and teratogenic effects are critically discussed. This review suggests that polar PAHs could have serious toxicological effects on human health and should be considered during risk assessment of PAH-contaminated sites. The implications of not doing so were argued and critical knowledge gaps and future research requirements discussed.

Distribution and contamination status of phthalic acid esters in the sediments of a tropical monsoonal estuary, Cochin - India

Phthalic acid esters (PAEs) are a group of endocrine-disrupting chemicals listed as priority pollutants by United States Environmental Protection Agency (USEPA, 2009). This study provides baseline information on seasonal distribution and contamination status of six phthalic acid esters (∑₆PAEs) in sediments of a tropical estuary (Cochin-India). In general, the sediments accumulated more PAEs during the post monsoon (mean 2325 ngg^-1; between 1402 and 3121 ngg^-1) and monsoon (mean 1372 ngg^-1; between 331 and 4015 ngg^-1) periods indicating land run off as the major transport pathway. Moderate run off and comparatively high residence time lead to effective sorption and settling of PAEs in the surface sediments during post monsoon season. Despite a high discharge of PAEs in to the water column, their deposition on to the sediments occurs at a lower rate during monsoon than that post monsoon season. PAEs were (mean 810 ngg^-1; between 44 and 1722 ngg^-1) lowest in pre monsoon season. The pre monsoon season is characterized by a minimal runoff consequent to the trapping of these organic pollutants in the river catchment area. The mid and high molecular PAEs (DEHP-Di ethylhexyl phthalate, BBP-Benzyl butyl phthalate and DnBP-Di-n-butyl phthalate) were the dominant congeners relative to the low molecular weight congeners (DMP-Dimethyl phthalate and DEP-Diethyl phthalate). DEHP and BBP levels exceeded permissible risk levels indicating a serious ecological hazard to the estuarine ecosystem.

Implications of Bioremediation of Polycyclic Aromatic Hydrocarbon-Contaminated Soils for Human Health and Cancer Risk

Bioremediation uses soil microorganisms to degrade polycyclic aromatic hydrocarbons (PAHs) into less toxic compounds and can be performed in situ, without the need for expensive infrastructure or amendments. This review provides insights into the cancer risks associated with PAH-contaminated soils and places bioremediation outcomes in a context relevant to human health. We evaluated which bioremediation strategies were most effective for degrading PAHs and estimated the cancer risks associated with PAH-contaminated soils. Cancer risk was statistically reduced in 89% of treated soils following bioremediation, with a mean degradation of 44% across the B2 group PAHs. However, all 180 treated soils had postbioremediation cancer risk values that exceeded the U.S. Environmental Protection Agency (USEPA) health-based acceptable risk level (by at least a factor of 2), with 32% of treated soils exceeding recommended levels by greater than 2 orders of magnitude. Composting treatments were most effective at biodegrading PAHs in soils (70% average reduction compared with 28-53% for the other treatment types), which was likely due to the combined influence of the rich source of nutrients and microflora introduced with organic compost amendments. Ultimately, bioremediation strategies, in the studies reviewed, were unable to successfully remove carcinogenic PAHs from contaminated soils to concentrations below the target cancer risk levels recommended by the USEPA.