Effects of short and long-term thermal exposure on microbial compositions in soils contaminated with mixed benzene and benzo[a]pyrene: A short communication

Integrated thermal and biostimulant enhanced bioremediation of PAHs and arsenic co-contaminated soil by an indigenous microbial consortium: Performance, mechanisms and limitations

Polycyclic aromatic hydrocarbons (PAHs) and arsenic (As) co-contaminated soil has emerged as a significant environmental concern. In this study, the performance and underlying mechanisms of integrated thermal and biostimulant enhanced bioremediation (ITBEB) of PAHs and As co-contaminated soil by an indigenous microbial consortium were explored. The results demonstrated that thermal treatment with temperatures elevated from 25 °C to 55 °C, in combination with biostimulants of rhamnolipids or yeast extract, improved the bioremediation efficiency of both PAHs and As. The highest bioremediation efficiency, 69.03 % for total PAHs and 17.69 % for As, were achieved at 55 °C with the addition of yeast extract, compared to 39.85 % and 8.69 %, respectively, at 25 °C without biostimulants. During the ITBEB process, the indigenous microbial consortium was capable of simultaneously degrading PAHs and transforming inorganic As into less toxic volatile forms via methylation. Notably, the abundance of functional bacteria, such as Bacillus, increased substantially from 8.29 % initially to 63.71 % at 55 °C with yeast extract, contributing significantly to the enhanced bioremediation efficiency. Furthermore, the highest abundances of functional genes PAH-RHDα and arsM were observed at 35 °C with biostimulants, whereas their expression declined at higher temperatures of 45 °C and 55 °C. This observation, combined with the results of correlation analyses, indicated that the effectiveness of bioremediation was strongly influenced by the presence of functional bacteria and their associated genes, as well as environmental factors such as iron content, pH, and soil organic matter. Overall, this study provides valuable insights into the synergistic effects of thermal treatment and biostimulants in the bioremediation of PAHs and As co-contaminated soils, offering a foundation for the development of more effective and sustainable remediation strategies.

The mechanism of benzene degradation in groundwater by indigenous microbial degradation from the perspectives of isotopes and microorganisms in cold regions of China

Groundwater benzene contamination is widespread, threatening ecosystems and human health. However, the biodegradation mechanisms of benzene under microaerobic conditions in cold regions remain poorly understood. This study selected benzene from groundwater in a chemical industrial park in the northeast China, conducting microcosm dynamic experiments to monitor microbial growth, benzene concentration, and carbon isotope changes, using an isotope fractionation model to elucidate microbial degradation patterns. High-throughput sequencing was also employed to explore microbial community dynamics and degradation pathways. The results indicated that indigenous microorganisms exhibited strong tolerance to benzene concentrations of 10, 20, 30 and 50 mg/L, with degradation efficiencies of 63.66 %, 68.26 %, 69.59 % and 67.23 %. The stable carbon isotopes of benzene shifted towards more positive values, increasing by approximately 3 ‰. The enrichment factor (εc) under microaerobic conditions ranged from -4.5 ‰ to -1.1 ‰. Proteobacteria was the dominant phylum (89.84 %), with Pseudomonas, Acinetobacter, Hydrogenophaga, and Variovorax as key degrading genera. Their abundance first increased and then decreased. Compared to the uncontaminated samples, the abundance increased by 3.1-5.7 times. The key functional genes for benzene degradation include M00548 (dmpK, dmpL, dmpM, dmpN, dmpO, dmpP) and M00547 (todC1, todC2, todB, todA, todD). With the increase in benzene concentration, the functional genes M00548 and M00547 exhibited increases in abundance by factors of 4.407-7.109 and 1.277-6.823, respectively. This elucidates the underlying mechanism behind the changes in benzene degradation efficiency and rate as a function of concentration. The findings provide foundational information to promote the development of more effective bioremediation strategies.

Effects of nitrile compounds on the structure and function of soil microbial communities as revealed by metagenomes

The proliferation of nitrile mixtures has significantly exacerbated environmental pollution. This study employed metagenomic analysis to investigate the short-term effects of nitrile mixtures on soil microbial communities and their metabolic functions. It also examined the responses of indigenous microorganisms and their functional metabolic genes across various land use types to different nitrile stressors. The nitrile compound treatments in this study resulted in an increase in the abundance of Proteobacteria, Actinobacteria, and Firmicutes, while simultaneously reducing overall microbial diversity. The key genes involved in the denitrification process, namely, nirK, nosZ, and hao, were down-regulated, and NO3--N, NO2--N, and NH4+-N concentrations decreased by 7.7%-12.3%, 11.1%-21.3%, and 11.3%-30.9%, respectively. Notably, pond sludge samples exhibited a significant increase in the abundance of nitrogen fixation-related genes nifH, vnfK, vnfH, and vnfG following exposure to nitrile compounds. Furthermore, the fumarase gene fumD, which is responsible for catalyzing fumaric acid into malic acid in the tricarboxylic acid cycle, showed a substantial increase of 7.2-10.6-fold upon nitrile addition. Enzyme genes associated with the catechol pathway, including benB-xylY, dmpB, dmpC, dmpH, and mhpD, displayed increased abundance, whereas genes related to the benzoyl-coenzyme A pathway, such as bcrA, dch, had, oah, and gcdA, were notably reduced. In summary, complex nitrile compounds were found to significantly reduce the species diversity of soil microorganisms. Nitrile-tolerant microorganisms demonstrated the ability to degrade and adapt to nitrile pollutants by enhancing functional enzymes involved in the catechol pathway and fenugreek conversion pathway. This study offers insights into the specific responses of microorganisms to compound nitrile contamination, as well as valuable information for screening nitrile-degrading microorganisms and identifying nitrile metabolic enzymes.

Microplastics and PAHs mixed contamination: An in-depth review on the sources, co-occurrence, and fate in marine ecosystems

Microplastics (MPs) and polycyclic aromatic hydrocarbons (PAHs) are toxic contaminants that have been found in marine ecosystems. This review aims to explore the sources and mechanisms of PAHs and MPs mixed contamination in marine environments. Understanding the released sources of PAHs and MPs is crucial for proposing appropriate regulations on the release of these contaminants. Additionally, the mechanisms of co-occurrence and the role of MPs in distributing PAHs in marine ecosystems were investigated in detail. Moreover, the chemical affinity between PAHs and MPs was proposed, highlighting the potential mechanisms that lead to their persistence in marine ecosystems. Moreover, we delve into the various factors influencing the co-occurrence, chemical affinity, and distribution of mixed contaminants in marine ecosystems. These factors, including environmental characteristics, MPs properties, PAHs molecular weight and hydrophobicity, and microbial interactions, were critically examined. The co-contamination raises concerns about the potential synergistic effects on their degradation and toxicity. Interesting, few studies have reported the enhanced photodegradation and biodegradation of contaminants under mixed contamination compared to their individual remediation. However, currently, the remediation strategies reported for PAHs and MPs mixed contamination are scarce and limited. While there have been some initiatives to remove PAHs and MPs individually, there is a lack of research specifically targeting the removal of mixed contaminants. This deficiency highlights the need for further investigation and the development of effective remediation approaches for the efficient remediation of PAHs and MPs from marine ecosystems.

Redox potential model for guiding moderate oxidation of polycyclic aromatic hydrocarbons in soils

In-situ chemical oxidation is an important approach to remediate soils contaminated with persistent organic pollutants, e.g., polycyclic aromatic hydrocarbons (PAHs). However, massive oxidants are added into soils without an explicit model for predicting the redox potential (Eh) during soil remediation, and overdosed oxidants would pose secondary damage by disturbing soil organic matter and acidity. Here, a soil redox potential (Eh) model was first established to quantify the relationship among oxidation parameters, crucial soil properties, and pollutant elimination. The impacts of oxidant types and doses, soil pH, and soil organic carbon contents on soil Eh were systematically clarified in four commonly used oxidation systems (i.e., KMnO4, H2O2, fenton, and persulfate). The relative error of preliminary Eh model was increased from 48-62% to 4-16% after being modified with the soil texture and dissolved organic carbon, and this high accuracy was verified by 12 actual PAHs contaminated soils. Combining the discovered critical oxidation potential (COP) of PAHs, the moderate oxidation process could be regulated by the guidance of the soil Eh model in different soil conditions. Moreover, the product analysis revealed that the hydroxylation of PAHs occurred most frequently when the soil Eh reached their COP, providing a foundation for further microorganism remediation. These results provide a feasible strategy for selecting oxidants and controlling their doses toward moderate oxidation of contaminated soils, which will reduce the consumption of soil organic matter and protect the main structure and function of soil for future utilization. ENVIRONMENTAL IMPLICATIONS: This study provides a novel insight into the moderate chemical oxidation by the Eh model and largely reduces the secondary risks of excessive oxidation and oxidant residual in ISCO. The moderate oxidation of PAHs could be a first step to decrease their toxicity and increase their bioaccessibility, favoring the microbial degradation of PAHs. Controlling the soil Eh with the established model here could be a promising approach to couple moderate oxidation of organic contaminants with microbial degradation. Such an effective and green soil remediation will largely preserve the soil's functional structure and favor the subsequent utilization of remediated soil.

Co-occurrence, toxicity, and biotransformation pathways of metformin and its intermediate product guanylurea: Current state and future prospects for enhanced biodegradation strategy

Accumulation of metformin and its biotransformation product "guanylurea" are posing an increasing concern due to their low biodegradability under natural attenuated conditions. Therefore, in this study, we reviewed the unavoidable function of metformin in human body and the route of its release in different water ecosystems. In addition, metformin and its biotransformation product guanylurea in aquatic environments caused certain toxic effects on aquatic organisms which include neurotoxicity, endocrine disruption, production of ROS, and acetylcholinesterase disturbance in aquatic organisms. Moreover, microorganisms are the first to expose and deal with the release of these contaminants, therefore, the mechanisms of biodegradation pathways of metformin and guanylurea under aerobic and anaerobic environments were studied. It has been reported that certain microbes, such as Aminobacter sp. and Pseudomonas putida can carry potential enzymatic pathways to degrade the dead-end product "guanylurea", and hence guanylurea is no longer the dead-end product of metformin. However, these microbes can easily be affected by certain geochemical cycles, therefore, we proposed certain strategies that can be helpful in the enhanced biodegradation of metformin and its biotransformation product guanylurea. A better understanding of the biodegradation potential is imperative to improve the use of these approaches for the sustainable and cost-effective remediation of the emerging contaminants of concern, metformin and guanylurea in the near future.