Degradation Products of Polychlorinated Biphenyls and Their In Vitro Transformation by Ligninolytic Fungi

New insights in the biodegradation of high-cyclic polycyclic aromatic hydrocarbons with crude enzymes of Trametes versicolor

High-cyclic polycyclic aromatic hydrocarbons (PAHs), with complex fused aromatic structures, are widespread, refractory and harmful in soil, but the current remediation technologies for high-cyclic PAHs are often inefficient and costly. This study focused on the biodegradation process of high-cyclic benzo[a]pyrene by Trametes versicolor crude enzymes. The crude enzymes exhibited high laccase activity (22112 U/L) and benzo[a]pyrene degradation efficiency (42.21%) within a short reaction time. Through the actual degradation and degradation kinetics, the degradation efficiency of PAHs decreased with the increase of aromatic rings. And the degradation conditions (temperature, pH, Cu2+ concentration, mediator) were systematically optimised. The optimum degradation conditions (1.5 mM Cu2+, 28℃ and pH 6) showed significant degradation efficiency for the low and medium concentrations of benzo[a]pyrene. In addition, complete degradation of benzo[a]pyrene could be achieved using only 0.2 mM of HBT mediator compared with crude enzymes alone. Collectively, these results showed the high-cyclic PAHs degradation potential of Trametes versicolor crude enzymes, and provided references to evaluate applicable prospects of white rot fungus crude enzymes in PAHs-contaminated soils.

Under explored roles of microbial ligninolytic enzymes in aerobic polychlorinated biphenyl transformation

Polychlorinated biphenyls (PCBs) are persistent organic pollutants in the environment that are responsible for many adverse health effects. Bioremediation appears to be a healthy and cost-effective alternative for remediating PCB-contaminated environments. While some microbial species have been observed to be capable of transforming PCBs, only two different microbial pathways (rdh and bph pathways) have been described to be involved in PCB transformations. Ligninolytic enzymes have been observed or are under suspicion in some microbial PCB transformations. However, the role of these promising PCB-transforming enzymes, which are produced by fungi and some aerobic bacteria, is still unclear. The present review describes their role by identifying microbial PCB-transforming species and their reported ligninolytic enzymes whether proven or suspected to be involved in PCB transformations. There are several lines of evidence that ligninolytic enzymes are responsible for PCB transformations such as (1) the ability of purified laccases from Myceliophthora thermophila, Pycnoporus cinnabarinus, Trametes versicolor, Cladosporium sp, and Coprinus cumatus to transform hydroxy-PCBs; (2) the increased production of laccases and peroxidases by many fungi in the presence of PCBs; and (3) the enhanced PCB transformation by Pseudomonas stutzeri and Sinorhizobium meliloti NM after the addition of ligninolytic enzyme enhancers. However, if the involvement of ligninolytic enzymes in PCB transformation is clearly demonstrated in some fungal species, it does not seem to be implicated in all microbial species suggesting other still unknown metabolic pathways involved in PCB transformation and different from the bph and rdh pathways. Therefore, PCB transformation may involve several metabolic pathways, some involving ligninolytic enzymes, bph or rdh genes, and some still unknown, depending on the microbial species. In addition, current knowledge does not fully clarify the role of ligninolytic enzymes in PCB oxidation and dechlorination. Therefore, further studies focusing on purified ligninolytic enzymes are needed to clearly elucidate their role in PCB transformation.

Transformation of hydroxylated polychlorinated biphenyls by bacterial 2-hydroxybiphenyl 3-monooxygenase

Monohydroxylated PCBs (OH-PCBs) are an (eco)toxicologically significant group of compounds, as they arise from the oxidation of polychlorinated biphenyls (PCBs) and, at the same time, may exert even more severe toxic effects than their parent PCB molecules. Despite having been widely detected in environmental samples, plants, and animals, information on the fate of OH-PCBs in the environment is scarce, including on the enzymatic machinery behind their degradation. To date, only a few bacterial taxa capable of OH-PCB transformation have been reported. In this study, we aimed to obtain a deeper insight into the transformation of OH-PCBs in soil bacteria and isolated a Pseudomonas sp. strain P1B16 based on its ability to use o-phenylphenol (2-PP) which, when exposed to the Delor 103-derived OH-PCB mixture, depleted a wide spectrum of mono-, di, and trichlorinated OH-PCBs. In the P1B16 genome, a region designated as hbp was identified, which bears a set of putative genes involved in the transformation of OH-PCBs, namely hbpA encoding for a putative flavin-dependent 2-hydroxybiphenyl monooxygenase, hbpC (2,3-dihydroxybiphenyl-1,2-dioxygenase), hbpD (2-hydroxy-6-oxo-6-phenylhexa-2,4-dienoate hydrolase), and the transcriptional activator-encoding gene hbpR. The hbpA coding sequence was heterologously expressed, purified, and its substrate specificity was investigated towards the Delor 103-derived OH-PCB mixture, individual OH-PCBs, and multiple (chlorinated) phenolics. Apart from 2-PP and 2-chlorophenol, HbpA was also demonstrated to transform a range of OH-PCBs, including a 3-hydroxy-2,2',4',5,5'-pentachlorobiphenyl. Importantly, this is the first direct evidence of HbpA homologs being involved in the degradation of OH-PCBs. Moreover, using a P1B16-based biosensor strain, the specific induction of hbp genes by 2-PP, 3-phenylphenol, 4-phenylphenol, and the OH-PCB mixture was demonstrated. This study provides direct evidence on the specific enzymatic machinery responsible for the transformation of OH-PCBs in bacteria, with many implications in ecotoxicology, environmental restoration, and microbial ecology in habitats burdened with PCB contamination.

Recent advances in fungal xenobiotic metabolism: enzymes and applications

Fungi have been extensively studied for their capacity to biotransform a wide range of natural and xenobiotic compounds. This versatility is a reflection of the broad substrate specificity of fungal enzymes such as laccases, peroxidases and cytochromes P450, which are involved in these reactions. This review gives an account of recent advances in the understanding of fungal metabolism of drugs and pollutants such as dyes, agrochemicals and per- and poly-fluorinated alkyl substances (PFAS), and describes the key enzymes involved in xenobiotic biotransformation. The potential of fungi and their enzymes in the bioremediation of polluted environments and in the biocatalytic production of important compounds is also discussed.

Current and emerging trends in bioaugmentation of organic contaminated soils: A review

Organic contaminated soils constitute an important environmental problem, whereas field applicability of existing physical-chemical methods has encountered numerous obstacles, such as high chemical cost, large energy consumption, secondary pollution, and soil degradation. Bioaugmentation is an environmentally friendly and potentially economic technology that efficiently removes toxic pollutants from organic contaminated soils by microorganisms or their enzymes and bioremediation additives. This review attempted to explore the recent advances in bioaugmentation of organic contaminated soils and provided a comprehensive summary of various bioaugmentation methods, including bacterial, fungus, enzymes and bioremediation additives. The practical application of bioaugmentation is frequently limited by soil environmental conditions, microbial relationships, enzyme durability and remediation cycles. To tackle these problems, the future of bioaugmentation can be processed from sustainability of broad-spectrum bioremediation carriers, microbial/enzyme agents targeting combined contaminants, desorption of environmentally friendly additives and small molecular biological stimulants. Findings of this research are expected to provide new references for bioaugmentation methods that are practically feasible and economically potential.

Recent advances in PCB removal from historically contaminated environmental matrices

Despite being drastically restricted in the 1970s, polychlorinated biphenyls (PCBs) still belong among the most hazardous contaminants. The chemical stability and dielectric properties of PCBs made them suitable for a number of applications, which then lead to their ubiquitous presence in the environment. PCBs are highly bioaccumulative and persistent, and their teratogenic, carcinogenic, and endocrine-disrupting features have been widely reported in the literature. This review discusses recent advances in different techniques and approaches to remediate historically contaminated matrices, which are one of the most problematic in regard to decontamination feasibility and efficiency. The current knowledge published in the literature shows that PCBs are not sufficiently removed from the environment by natural processes, and thus, the suitability of some approaches (e.g., natural attenuation) is limited. Physicochemical processes are still the most effective; however, their extensive use is constrained by their high cost and often their destructiveness toward the matrices. Despite their limited reliability, biological methods and their application in combinations with other techniques could be promising. The literature reviewed in this paper documents that a combination of techniques differing in their principles should be a future research direction. Other aspects discussed in this work include the incompleteness of some studies. More attention should be given to the evaluation of toxicity during these processes, particularly in terms of monitoring different modes of toxic action. In addition, decomposition mechanisms and products need to be sufficiently clarified before combined, tailor-made approaches can be employed.