Hydroxy-PCBs, methoxy-PCBs and hydroxy-methoxy-PCBs: metabolites of polychlorinated biphenyls formed in vitro by tobacco cells

Environmental fate of sulfonated-PCBs: Soil partitioning properties, bioaccumulation, persistence, and mobility

Two new classes of PCB metabolites were recently discovered: sulfonated-polychlorinated biphenyls (sulfonated-PCBs) and hydroxy-sulfonated-polychlorinated biphenyls (OH-sulfonated-PCBs). These metabolites, originating from PCB degradation, seem to possess more polar characteristics than their parent compounds. However, no other information, such as their chemical identity (CAS number) or their ecotoxicity or toxicity, is available so far, although more than about one hundred different chemicals were observed in soil samples. In addition, their physico-chemical properties are still uncertain since only estimations are available. Here we show the first evidence on the fate of these new classes of contaminants in the environment, producing results from several experiments, to evaluate sulfonated-PCBs and OH-sulfonated-PCBs soil partition coefficients, degradation in soil after 18 months of rhizoremediation, uptake into plant roots and earthworms, as well as a preliminary analytical method to extract and concentrate these chemicals from water. The results give an overview of the expected environmental fate of these chemicals and open questions for further studies.

Polyethylene mesh knitted fabrics mulching the soil to mitigate China's haze: A potential source of PBDEs

The fate of polybrominated diphenyl ethers (PBDEs) from polyethylene mesh knitted fabrics (PMKFs) to mulched soil and nearby plants was studied. PBDEs in the soil sample collected from Tianjin University of Commerce in April 2019 increased significantly after 6 months of PMKF mulching owing to PMKFs as the main input source. The compositional profiles/congener patterns of the PBDEs in the soil and PMKFs became similar after 6 months. High correlations were found between ΣPBDEs in the soil and PMKFs in October 2019, with no significant correlation in April. Plants could take up, accumulate and biotransform PBDEs in contaminated soil. The uptake of BDE-209 by plants was the highest compared with other lesser brominated PBDE congeners, due to its higher log Kow value and molecular weight or size. BDE-47 taken up in the plant was biotransformed via hydroxylation. These results prove that the government's PMKF solution to haze is causing environmental problems in bare soil, i.e., PBDE pollution in both soil and nearby plants. The present study provides important pieces of evidence for government and policymakers, and it is recommended that one environmental problem is not solved by creating another.

Bioremediation strategies with biochar for polychlorinated biphenyls (PCBs)-contaminated soils: A review

Polychlorinated biphenyls (PCBs) are hazardous organic contaminants threatening human health and environmental safety due to their toxicity and carcinogenicity. Biochar (BC) is an eco-friendly carbonaceous material that can extensively be utilized for the remediation of PCBs-contaminated soils. In the last decade, many studies reported that BC is beneficial for soil quality enhancement and agricultural productivity based on its physicochemical characteristics. In this review, the potential of BC application in PCBs-contaminated soils is elaborated as biological strategies (e.g., bioremediation and phytoremediation) and specific mechanisms are also comprehensively demonstrated. Further, the synergy effects of BC application on PCBs-contaminated soils are discussed, in view of eco-friendly, beneficial, and productive aspects.

Interconversion between methoxylated, hydroxylated and sulfated metabolites of PCB 3 in whole poplar plants

Methoxylated polychlorinated biphenyls (MeO-PCBs) are overlooked metabolites of PCBs. In general, they are more toxic to plants than their parent congeners. However, information on the fate of MeO-PCBs and the relationship between methoxylated, hydroxylated and sulfated metabolites of PCBs in plants is scarce. In this work, poplar plants (Populus deltoides × nigra, DN34) were hydroponically and separately exposed to 4'-methoxy-4-monochlorobiphenyl (4'-MeO-PCB 3) and 4'-PCB 3 sulfate for 10 days to investigate the uptake, translocation and metabolism of MeO-PCBs and the relationship between methoxy-PCBs, hydroxyl-PCBs and PCB sulfates within plants. Results showed that 4'-MeO-PCB 3 and 4'-PCB 3 sulfate were taken up by the roots of poplar plants and translocated from roots to shoots and leaves. 4'-OH-PCB 3 and 4'-PCB 3 sulfate were identified as the hydroxylated metabolite and sulfate metabolite of 4'-MeO-PCB 3 in poplar, respectively. In the backward reaction, 4'-OH-PCB 3 and 4'-MeO-PCB 3 were found as metabolites of 4'-PCB 3 sulfate. For exposure groups, the yields of 4'-OH-PCB 3 produced from 4'-MeO-PCB 3 and 4'-PCB 3 sulfate were 1.29% and 0.13% respectively. The yield of 4'-PCB 3 sulfate which originated from 4'-MeO-PCB 3 in wood and root samples of exposure groups was only 0.02%. Only 0.04% of the initial mass of 4'-PCB 3 sulfate was transformed to 4'-MeO-PCB 3 in the exposure groups. The sulfation yield of 4'-OH-PCB 3 was higher than hydrolysis yield of 4'-PCB 3 sulfate, indicating that formation of PCB sulfates was predominant over the reverse reaction, the formation of hydroxy-PCBs. These results provide new perspective on the transport, metabolism, and fate of MeO-PCBs, and also help to better understand sources of OH-PCBs and PCB sulfates in the environment. This study provides the first evidence of interconversion of sulfate metabolites from methoxy-PCBs and methoxy-PCBs from PCB sulfates.

Protective effect of ginger (Zingiber officinale) against PCB-induced acute hepatotoxicity in male rats

After absorption by the organism, polychlorinated biphenyls (PCBs) cross cellular membranes and pass into blood vessels and the lymphatic system. It is generally in the liver, adipose tissues, brain and skin that we find the strongest concentrations of PCBs. Herbal medicine remains as a discipline intended to treat and to prevent certain functional disorders and/or pathologies caused by oxidative stress, which can be induced by pesticides, medicines or pollutants. The objective of this study is to verify the toxic and oxidative effects of PCBs and to investigate the protective effect of ginger (Zingiber officinale) in the liver of male rats of the “Wistar” strain. These rats are divided into 6 groups: a control group (T), two groups treated with PCB at two different concentrations (P₁ and P₂), a group treated with ginger extract (G), a group pretreated with ginger extract and then injected with the first concentration of PCBs (P₁G), and a group pretreated with ginger and then injected with the second concentration of PCBs (P₂G). The results showed that the administration of PCBs led to an increase in the relative weight of the liver, and a significant increase in all of the hepatic biomarker levels (glucose, cholesterol, triglycerides, AST, ALT, and LDH) in the serum. Furthermore, an increase in the rate of lipid peroxidation and a decrease in the antioxidant enzyme activities (catalase, superoxide dismutase and glutathione peroxidase) were observed under the influence of PCBs in the liver. The histological test showed that the PCBs induced hepatocyte vacuolization, prominent and peripheralized nuclei, hepatocellular hypertrophy and turgor of the vein in the centriacinar regions. Pretreatment with ginger extract restored all of the biochemical and oxidative parameters to the normal values and reduced the injuries caused by the PCBs. In conclusion, in our experimental conditions, ginger effectively protects the liver against the hepatotoxic effects induced by PCBs.

After absorption by the organism, polychlorinated biphenyls (PCBs) cross cellular membranes and pass into blood vessels and the lymphatic system.

Effect of copper on the translocation and transformation of polychlorinated biphenyls in rice

Contamination of organic pollutants in the environment is usually accompanied by heavy metals. However, a little information on the influences of heavy metals on the uptake, translocation and transformation of organic pollutants in plants is available. In this study, ten-day hydroponic exposure was conducted to explore the influence of copper (Cu) on the bioaccumulation and biotransformation of polychlorinated biphenyls (PCBs) in intact young rice (Oryza sativa L.). Low dose of Cu (≤100 μmol/L) increased the accumulation of CB-61 in rice plants, while excess concentrations of Cu (>100 μmol/L) inhibited uptake and translocation of CB-61. Effect of Cu on the uptake of CB-61 was attributed to the Cu-triggered damage to the roots of rice plants. The presence of a moderate dose of Cu (50 μmol/L) enhanced the formation of hydroxylated polychlorinated biphenyls (OH-PCBs) and methoxylated polychlorinated biphenyls (MeO-PCBs), whereas excess concentrations of Cu (250 μmol/L) inhibited the metabolism of CB-61. The effect of Cu on the interconversion between 4'-OH-CB-61 and 4'-MeO-CB-61 was also concentration dependent: the biotransformation was promoted by a moderate concentration of Cu but inhibited by excess concentrations of Cu. The activities of Cytochrome P450 (CYP450) and S-adenosyl-l-methionine (SAM)-dependent methyltransferase in the roots of rice plants exposed to Cu and CB-61 or its derivatives were consistent with the pattern and trend of the metabolites observed in rice roots. These results could provide valuable insights into the interactions and combined effects of PCBs and heavy metals in plants.

Polychlorinated biphenyls in alfalfa: Accumulation, sorption and speciation in different plant parts

The accumulation, chemical speciation and distribution of polychlorinated biphenyls (PCBs) were investigated in various parts of alfalfa. Moreover, the adsorption characteristics for PCB 28 by alfalfa and the influencing factors of the adsorption characteristics were studied. There were different degrees of PCB accumulation in alfalfa roots, root nodules and shoots. The decreasing order of the accumulation of PCBs in plant tissues was root nodules > roots > shoots, and the decreasing order of the total PCB contents was roots > shoots > root nodules, indicating that the roots were the main sink for PCB accumulation. There were three modes of PCB speciation in alfalfa roots and root nodules, comprising strong sorption (78%) and weak sorption (19%) on tissue surfaces and absorption within tissues (2%). The adsorption isotherms of PCB 28 indicate that the adsorption capacities of root nodules and shoots were both significantly higher than that of the roots. Both lipids and carbohydrates, and especially lipids, affected the PCB adsorption capacities of the tissues. These results may help in the elucidation of the mechanisms of sorption and accumulation of PCBs in the plants and their main influencing factors and thus contribute to the development of phytoremediation technologies for PCB-contaminated soils.

Dechlorination and chlorine rearrangement of 1,2,5,5,6,9,10-heptachlorodecane mediated by the whole pumpkin seedlings

Short chain chlorinated paraffins (SCCPs) are ubiquitously present as persistent organic pollutants in the environment. However, little information on the interaction of SCCPs with plants is currently available. In this work, young pumpkin plants (Cucurbita maxima × C. Moschata) were hydroponically exposed to the congener of chlorinated decane, 1,2,5,5,6,9,10-heptachlorodecane (1,2,5,5,6,9,10-HepCD), to investigate the uptake, translocation and transformation of chlorinated decanes in the intact plants. It was found that parent HepCD was taken up by the pumpkin roots, translocated from root to shoots, and phytovolatilized from pumpkin plants to air via the plant transpiration flux. Our data suggested that dechlorination of 1,2,5,5,6,9,10-HepCD to lower chlorinated decanes and rearrangement of chlorine atoms in the molecule were all mediated by the whole pumpkin seedlings. Chlorinated decanes were found in the shoots and roots of blank controls, indicating that chlorinated decanes in the air could be absorbed by leaves and translocated from shoots to roots. Lower chlorinated congeners (C₁₀H₁₇Cl₅) tended to detain in air compared to higher chlorinated congeners (C₁₀H₁₆Cl₆ and other C₁₀H₁₅Cl₇). Potential transformation pathway and behavior of 1,2,5,5,6,9,10-HepCD in pumpkin were proposed based on these experiments.

Phyto-rhizoremediation of polychlorinated biphenyl contaminated soils: An outlook on plant-microbe beneficial interactions

Polychlorinated biphenyls (PCBs) are toxic chemicals, recalcitrant to degradation, bioaccumulative and persistent in the environment, causing adverse effects on ecosystems and human health. For this reason, the remediation of PCB-contaminated soils is a primary issue to be addressed. Phytoremediation represents a promising tool for in situ soil remediation, since the available physico-chemical technologies have strong environmental and economic impacts. Plants can extract and metabolize several xenobiotics present in the soil, but their ability to uptake and mineralize PCBs is limited due to the recalcitrance and low bioavailability of these molecules that in turn impedes an efficient remediation of PCB-contaminated soils. Besides plant degradation ability, rhizoremediation takes into account the capability of soil microbes to uptake, attack and degrade pollutants, so it can be seen as the most suitable strategy to clean-up PCB-contaminated soils. Microbes are in fact the key players of PCB degradation, performed under both aerobic and anaerobic conditions. In the rhizosphere, microbes and plants positively interact. Microorganisms can promote plant growth under stressed conditions typical of polluted soils. Moreover, in this specific niche, root exudates play a pivotal role by promoting the biphenyl catabolic pathway, responsible for microbial oxidative PCB metabolism, and by improving the overall PCB degradation performance. Besides rhizospheric microbial community, also the endophytic bacteria are involved in pollutant degradation and represent a reservoir of microbial resources to be exploited for bioremediation purposes. Here, focusing on plant-microbe beneficial interactions, we propose a review of the available results on PCB removal from soil obtained combining different plant and microbial species, mainly under simplified conditions like greenhouse experiments. Furthermore, we discuss the potentiality of "omics" approaches to identify PCB-degrading microbes, an aspect of paramount importance to design rhizoremediation strategies working efficiently under different environmental conditions, pointing out the urgency to expand research investigations to field scale.

Effects of Secondary Plant Metabolites on Microbial Populations: Changes in Community Structure and Metabolic Activity in Contaminated Environments

Secondary plant metabolites (SPMEs) play an important role in plant survival in the environment and serve to establish ecological relationships between plants and other organisms. Communication between plants and microorganisms via SPMEs contained in root exudates or derived from litter decomposition is an example of this phenomenon. In this review, the general aspects of rhizodeposition together with the significance of terpenes and phenolic compounds are discussed in detail. We focus specifically on the effect of SPMEs on microbial community structure and metabolic activity in environments contaminated by polychlorinated biphenyls (PCBs) and polyaromatic hydrocarbons (PAHs). Furthermore, a section is devoted to a complex effect of plants and/or their metabolites contained in litter on bioremediation of contaminated sites. New insights are introduced from a study evaluating the effects of SPMEs derived during decomposition of grapefruit peel, lemon peel, and pears on bacterial communities and their ability to degrade PCBs in a long-term contaminated soil. The presented review supports the "secondary compound hypothesis" and demonstrates the potential of SPMEs for increasing the effectiveness of bioremediation processes.

Plants Rather than Mineral Fertilization Shape Microbial Community Structure and Functional Potential in Legacy Contaminated Soil

Plant-microbe interactions are of particular importance in polluted soils. This study sought to determine how selected plants (horseradish, black nightshade and tobacco) and NPK mineral fertilization shape the structure of soil microbial communities in legacy contaminated soil and the resultant impact of treatment on the soil microbial community functional potential. To explore these objectives, we combined shotgun metagenomics and 16S rRNA gene amplicon high throughput sequencing with data analysis approaches developed for RNA-seq. We observed that the presence of any of the selected plants rather than fertilization shaped the microbial community structure, and the microbial populations of the root zone of each plant significantly differed from one another and/or from the bulk soil, whereas the effect of the fertilizer proved to be insignificant. When we compared microbial diversity in root zones versus bulk soil, we observed an increase in the relative abundance of Alphaproteobacteria, Betaproteobacteria, Gammaproteobacteria or Bacteroidetes, taxa which are commonly considered copiotrophic. Our results thus align with the theory that fast-growing, copiotrophic, microorganisms which are adapted to ephemeral carbon inputs are enriched in the vegetated soil. Microbial functional potential indicated that some genetic determinants associated with signal transduction mechanisms, defense mechanisms or amino acid transport and metabolism differed significantly among treatments. Genetic determinants of these categories tend to be overrepresented in copiotrophic organisms. The results of our study further elucidate plant-microbe relationships in a contaminated environment with possible implications for the phyto/rhizoremediation of contaminated areas.

Interconversion between Methoxylated and Hydroxylated Polychlorinated Biphenyls in Rice Plants: An Important but Overlooked Metabolic Pathway

To date, there is limited knowledge on the methoxylation of polychlorinated biphenyls (PCBs) and the relationship between hydroxylated polychlorinated biphenyls (OH-PCBs) and methoxylated polychlorinated biphenyls (MeO-PCBs) in organisms. In this study, rice (Oryza sativa L.) was chosen as the model organism to determine the metabolism of PCBs in plants. Limited para-substituted 4'-OH-CB-61 (major metabolite) and 4'-MeO-CB-61 (minor metabolite) were found after a 5-day exposure to CB-61, while ortho- and meta-substituted products were not detected. Interconversion between OH-PCBs and MeO-PCBs in organisms was observed for the first time. The demethylation ratio of 4'-MeO-CB-61 was 18 times higher than the methylation ratio of 4'-OH-CB-61, indicating that formation of OH-PCBs was easier than formation of MeO-PCBs. The transformation products were generated in the roots after 24 h of exposure. The results of in vivo and in vitro exposure studies show that the rice itself played a key role in the whole transformation processes, while endophytes were jointly responsible for hydroxylation of PCBs and demethylation of MeO-PCBs. Metabolic pathways of PCBs, OH-PCBs, and MeO-PCBs in intact rice plants are proposed. The findings are important in understanding the fate of PCBs and the source of OH-PCBs in the environment.

Metabolism of Doubly para-Substituted Hydroxychlorobiphenyls by Bacterial Biphenyl Dioxygenases

In this work, we examined the profile of metabolites produced from the doubly para-substituted biphenyl analogs 4,4'-dihydroxybiphenyl, 4-hydroxy-4'-chlorobiphenyl, 3-hydroxy-4,4'-dichlorobiphenyl, and 3,3'-dihydroxy-4,4'-chlorobiphenyl by biphenyl-induced Pandoraea pnomenusa B356 and by its biphenyl dioxygenase (BPDO). 4-Hydroxy-4'-chlorobiphenyl was hydroxylated principally through a 2,3-dioxygenation of the hydroxylated ring to generate 2,3-dihydro-2,3,4-trihydroxy-4'-chlorobiphenyl and 3,4-dihydroxy-4'-chlorobiphenyl after the removal of water. The former was further oxidized by the biphenyl dioxygenase to produce ultimately 3,4,5-trihydroxy-4'-chlorobiphenyl, a dead-end metabolite. 3-Hydroxy-4,4'-dichlorobiphenyl was oxygenated on both rings. Hydroxylation of the nonhydroxylated ring generated 2,3,3'-trihydroxy-4'-chlorobiphenyl with concomitant dechlorination, and 2,3,3'-trihydroxy-4'-chlorobiphenyl was ultimately metabolized to 2-hydroxy-4-chlorobenzoate, but hydroxylation of the hydroxylated ring generated dead-end metabolites. 3,3'-Dihydroxy-4,4'-dichlorobiphenyl was principally metabolized through a 2,3-dioxygenation to generate 2,3-dihydro-2,3,3'-trihydroxy-4,4'-dichlorobiphenyl, which was ultimately converted to 3-hydroxy-4-chlorobenzoate. Similar metabolites were produced when the biphenyl dioxygenase of Burkholderia xenovorans LB400 was used to catalyze the reactions, except that for the three substrates used, the BPDO of LB400 was less efficient than that of B356, and unlike that of B356, it was unable to further oxidize the initial reaction products. Together the data show that BPDO oxidation of doubly para-substituted hydroxychlorobiphenyls may generate nonnegligible amounts of dead-end metabolites. Therefore, biphenyl dioxygenase could produce metabolites other than those expected, corresponding to dihydrodihydroxy metabolites from initial doubly para-substituted substrates. This finding shows that a clear picture of the fate of polychlorinated biphenyls in contaminated sites will require more insights into the bacterial metabolism of hydroxychlorobiphenyls and the chemistry of the dihydrodihydroxylated metabolites derived from them.

Metabolism of (14)C-labeled polychlorinated biphenyl congeners by wheat cell suspension cultures

The metabolism of [UL-(14)C]-2,2',5,5'-tetrachlorobiphenyl ((14)C-PCB-52), [UL-(14)C]-2,2',4,4',5,5'-hexachlorobiphenyl ((14)C-PCB-153, and a congeneric mixture of [UL-(14)C]-labeled polychlorinated biphenyls ((14)C-PCB-Mix) was studied in cell suspension cultures of wheat (Triticum aestivum L. cv. 'Heines Koga II'). About 50% of applied (14)C-PCB-52 (20 μg/assay) was transformed during 96 h of incubation. While 7.6% on non-extractable residues emerged, turnover of (14)C-PCB-52 was mainly due to soluble polar metabolites. These were subjected to chemical glycoside cleavage. In the resulting hydrolysate, four aglycons were identified by GC-EIMS, namely four tetrachloro-hydroxy-biphenyl isomers (C6H6Cl4O, M(+·) at m/z = 306, 308, 310 and 312), and one trichloro-hydroxy-biphenyl (C6H7Cl3O, M(+·) at m/z = 272, 274 and 276). Number and character of hydroxylated products pointed to cytochromes P450 as enzymatic catalysts of hydroxylation. (14)C-PCB-153 was metabolized by wheat to minor degree if at all. Due to GC-EIMS analysis, of (14)C-PCB-Mix consisted of biphenyl, one mono-, four di-, seven tri-, eleven tetra-, and four pentachlorobiphenyls besides traces of further mono- and hexachlorobiphenyls. Among these were PCB-28, PCB-52, PCB 101, and PCB-118 (identified by seven key congeners standard). The mixture resembled industrial products Clophen A30 or Aroclor 1016. Metabolic turnover of applied (14)C-PCB-Mix (15 μg/assay) was 30% after 96 h; 8.4% of non-extractable residues emerged. Using DDE (p,p'-dichlorodiphenyl-dichloroethylene) as internal standard it was demonstrated that biphenyl, one monochloro-, two dichloro-, and one trichlorobiphenyl were completely metabolized to polar products. Partial metabolization occurred with one di-, five tri-, and four tetrachlorobiphenyls. Two tri-, four tetra-, and all pentachlorbiphenyls proved to be stable. Due to strong interference by matrix, evaluation of three congeners was not possible. In addition to wheat, results of similar experiments with cell cultures of other species are briefly mentioned.

Enantioselective transport and biotransformation of chiral hydroxylated metabolites of polychlorinated biphenyls in whole poplar plants

Hydroxylated metabolites of polychlorinated biphenyls (OH-PCBs) have been found to be ubiquitous in the environment due to the oxidative metabolism of their parent PCBs. With more polarity, OH-PCBs may be more toxic and mobile than their parent compounds. However, the behavior and fate of OH-PCBs have been neglected in the environment because they are not the original contaminants. Some of these hydroxylated metabolites are chiral, and chiral compounds can be used to probe biological metabolic processes. Therefore, chiral OH-PCBs were selected to study their uptake, translocation, transformation, and enantioselectivity in plants in this work. Poplars (Populus deltoides × nigra, DN34), a model plant with complete genomic sequence, were hydroponically exposed to 5-hydroxy-2,2',3,4',6-pentachlorobiphenyl (5-OH-PCB91) and 5-hydroxy-2,2',3,5',6-pentachlorobiphenyl (5-OH-PCB95) for 10 days. Chiral 5-OH-PCB91 and 5-OH-PCB95 were clearly shown to be sorbed, taken up, and translocated in whole poplars, and they were detected in various tissues of whole poplars. However, the enantioselectivity of poplar for 5-OH-PCB91 and 5-OH-PCB95 proved to be quite different. The second-eluting enantiomer of OH-PCB95, separated on a chiral column (Phenomenex Lux Cellulose-1), was enantioselectively removed in whole poplar. Enantiomeric fractions in the middle xylem, top bark, top xylem, and stem, reached 0.803 ± 0.022, 0.643 ± 0.110, 0.835 ± 0.087, and 0.830 ± 0.029, respectively. Therefore, 5-OH-PCB95 was significantly enantioselectively biotransformed inside poplar tissues, in contrast to nearly racemic mixtures of 5-OH-PCB95 remaining in hydroponic solutions. Unlike 5-OH-PCB95, 5-OH-PCB91 remained nearly racemic in most tissues of whole poplars during 10 day exposure, suggesting the enantiomers of 5-OH-PCB91 were equally transported and metabolized in whole poplars. This is the first evidence of enantioselectivity of chiral OH-PCBs and suggests that poplars can enantioselectively biotransform at least one chiral OH-PCB: namely, 5-OH-PCB95.

Transformation of hydroxylated derivatives of 2,5-dichlorobiphenyl and 2,4,6-trichlorobiphenyl by Burkholderia xenovorans LB400

The polychlorinated biphenyl (PCB)-degrading bacterium, Burkholderia xenovorans LB400, was capable of transforming three hydroxylated derivatives of 2,5-dichlorobiphenyl (2,5-DCB) (2'-hydroxy- (2'-OH-), 3'-OH-, and 4'-OH-2,5-DCB) when biphenyl was used as the carbon source (i.e., biphenyl pathway-inducing condition), although only 2'-OH-2,5-DCB was transformed when the bacterium was growing on succinate (i.e., condition non-inductive of the biphenyl pathway). On the contrary, hydroyxlated derivatives of 2,4,6-trichlorobiphenyl (2,4,6-TCB) (2'-OH-, 3'-OH-, and 4'-OH-2,4,6-TCB) were not significantly transformed by B. xenovorans LB400, regardless of the carbon source used. Gene expression analyses showed a clear correlation between the transformation of OH-2,5-DCBs and expression of genes of the biphenyl pathway. The PCB metabolite, 2,5-dichlorobenzoic acid (2,5-DCBA), was produced following the transformation of OH-2,5-DCBs. 2,5-DCBA was not further transformed by B. xenovorans LB400. The present study is significant because it provides evidence that PCB-degrading bacteria are capable of transforming hydroxylated derivatives of PCBs, which are increasingly considered as a new class of environmental contaminants.

Plant-microorganism interactions in bioremediation of polychlorinated biphenyl-contaminated soil

During the second half of the last century a large amount of substances toxic for higher organisms was released to the environment. Physicochemical methods of pollutant removal are difficult and prohibitively expensive. Using biological systems such as microorganisms, plants, or consortia microorganisms-plants is easier, cheaper, and more environmentally friendly. The aim of this study was to isolate, characterize and identify microorganisms from contaminated soil and to find out the effect of plants on microbial diversity in the environment. Microorganisms were isolated by two approaches with the aim to find all cultivable species and those able to utilise biphenyl as a sole source of carbon and energy. The first approach was direct extraction and the second was isolation of bacteria after enrichment cultivation with biphenyl. Isolates were biochemically characterized by NEFERMtest 24 and then the composition of ribosomal proteins in bacterial cells was determined by MALDI-TOF mass spectrometry. Ribosomal proteins can be used as phylogenetic markers and thus MALDI-TOF MS can be exploited also for taxonomic identification because the constitution of ribosomal proteins in bacterial cells is specific for each bacterial species. Identification of microorganisms using this method is performed with the help of database Bruker Daltonics MALDI BioTyper. Isolated bacteria were analyzed from the point of the bphA gene presence. Bacteria with detected bphA gene were then taxonomically identified by 16S rRNA sequence. The ability of two different plant species, tobacco (Nicotiana tabacum) and nightshade (Solanum nigrum), to accumulate PCBs was studied as well. It was determined that various plant species differ in the PCBs accumulation from the contaminated soil. Also the content of PCBs in various plant tissues was compared. PCBs were detected in roots and aboveground biomass including leaves and berries.

Prospects for using combined engineered bacterial enzymes and plant systems to rhizoremediate polychlorinated biphenyls

The fate of polychlorinated biphenyls (PCBs) in soil is driven by a combination of interacting biological processes. Several investigations have brought evidence that the rhizosphere provides a remarkable ecological niche to enhance the PCB degradation process by rhizobacteria. The bacterial oxidative enzymes involved in PCB degradation have been investigated extensively and novel engineered enzymes exhibiting enhanced catalytic activities toward more persistent PCBs have been described. Furthermore, recent studies suggest that approaches involving processes based on plant-microbe associations are very promising to remediate PCB-contaminated sites. In this review emphasis will be placed on the current state of knowledge regarding the strategies that are proposed to engineer the enzymes of the PCB-degrading bacterial oxidative pathway and to design PCB-degrading plant-microbe systems to remediate PCB-contaminated soil.

Metabolites of 2,2'-dichlorobiphenyl and 2,6-dichlorobiphenyl in hairy root culture of black nightshade Solanum nigrum SNC-9O

The hairy root culture of black nightshade (Solanum nigrum) SNC-9O was exposed to 2,2'-dichlorobiphenyl (PCB 4) and 2,6-dichlorobiphenyl (PCB 10) to follow the metabolites produced. The analytical standards of 4-hydroxy-2,2'-dichlorobiphenyl, 5'-hydroxy-2,2'-dichlorobiphenyl, 4-hydroxy-2,6-dichlorobiphenyl, 2-hydroxy-2',6'-dichlorobiphenyl, 3-hydroxy-2',6'-dichlorobiphenyl and 4-hydroxy-2',6'-dichlorobiphenyl have been synthesized. Hydroxy-metabolites of both PCB 4 and PCB 10 were present in the biomass. These appeared mainly as conjugates rather than as free hydroxy-PCBs, both maintained in plant cells. The concentrations of non-conjugated hydroxy-PCBs ranged between 0.9 and 35.2 μg kg(-1) of biomass fresh weight and the concentration of the conjugated ones ranged between 2.0 and 113.0 μg kg(-1) depending on the position of hydroxyl. The para- position of biphenyl (4 or 4') seems to be preferred for hydroxylation. Methoxy-PCBs and hydroxy-methoxy-PCBs have also been identified in plant cells. Hydroxyl in the meta-position (3, 3', 5 or 5') appears to be preferred for methylation in hydroxy-PCBs. Hydroxy-methoxy-PCBs have occurred in the conjugated form as well.

Stable isotope probing in the metagenomics era: a bridge towards improved bioremediation

Microbial biodegradation and biotransformation reactions are essential to most bioremediation processes, yet the specific organisms, genes, and mechanisms involved are often not well understood. Stable isotope probing (SIP) enables researchers to directly link microbial metabolic capability to phylogenetic and metagenomic information within a community context by tracking isotopically labeled substances into phylogenetically and functionally informative biomarkers. SIP is thus applicable as a tool for the identification of active members of the microbial community and associated genes integral to the community functional potential, such as biodegradative processes. The rapid evolution of SIP over the last decade and integration with metagenomics provide researchers with a much deeper insight into potential biodegradative genes, processes, and applications, thereby enabling an improved mechanistic understanding that can facilitate advances in the field of bioremediation.

Cloning the bacterial bphC gene into Nicotiana tabacum to improve the efficiency of phytoremediation of polychlorinated biphenyls

The aim of this work was to construct transgenic plants with increased capabilities to degrade organic pollutants, such as polychlorinated biphenyls. The environmentally important gene of bacterial dioxygenase, the bphC gene, was chosen to clone into a plant of Nicotiana tabacum. The chosen bphC gene encodes 2,3-dihydroxybiphenyl-1,2-dioxygenase, which cleaves the aromatic ring of dihydroxybiphenyl, and we cloned it in fusion with the gene for β-glucuronidase (GUS), luciferase (LUC) or with a histidine tail. Several genetic constructs were designed and prepared and the possible expression of desired proteins in tobacco plants was studied by transient expression. We used genetic constructs successfully expressing dioxygenase's genes we used for preparation of transgenic tobacco plants by agrobacterial infection. The presence of transgenic DNA , mRNA and protein was determined in parental and the first filial generation of transgenic plants with the bphC gene. Properties of prepared transgenic plants will be further studied.

Phyto/rhizoremediation studies using long-term PCB-contaminated soil

PURPOSE

Polychlorinated biphenyls (PCBs) represent a large group of recalcitrant environmental pollutants, differing in the number of chlorine atoms bound to biphenyl ring. Due to their excellent technological properties, PCBs were used as heat-transfer media, for filling transformers and condensers, as paint additives, etc. With increasing knowledge of their toxicity, transfer to food chains and accumulation in living organisms, their production ended in most countries in the 1970s and in 1984 in the former Czechoslovakia. But even a quarter of century after the PCB production ceased, from contaminated areas, the volatile PCBs evaporate and contaminate much larger areas even at very distant parts of the world. For this reason, PCBs still represent a global problem. The main method of PCB removal from contaminated environment is at present the expensive incineration at high temperatures. With the aim of finding effective alternative approaches, we are studying biological methods for PCB removal from the environment. In this paper, we summarise 10 years of studies using long-term PCB-contaminated soil from a dumpsite in South Bohemia, targeted for the use of plants (phytoremediation) and their cooperation with microorganisms in the root zone (rhizoremediation).

MATERIALS AND METHODS

Long-term contaminated soil from Lhenice dumpsite, more than hundred kilograms of homogenised material, was used in microcosms (pots and buckets), and field plots were established at the site. Tested plants include among others tobacco, black nightshade, horseradish, alfalfa and willow. Aseptic plant cell and tissue cultures were from the collection of the IOCB. Microorganisms were our own isolates. The paper summarises experiments done between 1998 and 2008 with real contaminated soil, both vegetated and non-vegetated. PCB analysis was performed by GC-ECD, metabolic products identified mostly using 2D-GC/MS-MS and synthetic standards, whereas molecular methods included quantitative PCR and sequencing.

RESULTS

The soil was used both for preparation of field plots at the site and for greenhouse and laboratory tests in microcosms. The results include analyses of changes in PCB content in untreated and vegetated soil, PCB uptake and distribution in different parts of various plant species, analysis of products formed, identification and characterisation of cultivable and non-cultivable bacteria both in rhizosphere and in bulk soil. Different treatments and amendments were also tested. Experiments in real contaminated soil were accompanied by in vitro experiments using aseptic cultures of plant biomass, genetically modified (GM) plants and bacteria, to allow identification of players responsible for PCB metabolisation in soil. The time-span of the experiments allows extrapolating some of the results and drawing conclusions concerning the effectivity of exploitation of various plant species and treatments to remove PCBs from soils.

DISCUSSION

The approach using plants proved to represent a viable alternative to costly incineration of PCB-contaminated soils. The recent studies using molecular methods show that plants are responsible for the composition of consortia of microorganisms present in their root zone, including those with ability to degrade the chlorinated aromatic compounds.

CONCLUSIONS

In addition to uptake, accumulation and partial metabolisation of PCBs by plants, compounds produced by plants allow survival of microorganisms even in poor soils, serve as carbon and energy source, and can even induce the degradation pathways of different xenobiotics. Thus, the choice of proper plant species is crucial for effective cleaning of different polluted sites. Our study shows how the efficiency of PCB removal is dependent on the plant used.

RECOMMENDATIONS AND PERSPECTIVES

The use of plants in biological remediation of different organic xenobiotics proved to be a useful approach. Further improvement can be expected by application of specifically tailored GM plants and use of selective conditions ensuring high remediation potential based on optimal composition of the soil microbial consortia designed for the needs of given site.

In vivo biotransformation of 3,3',4,4'-tetrachlorobiphenyl by whole plants-poplars and switchgrass

Polychlorinated biphenyls (PCBs) are widely distributed persistent organic pollutants. In vitro research has shown that plant cell cultures might transform lower chlorinated congeners to hydroxylated PCBs, but there are few studies on in vivo metabolism of PCBs by intact whole plants. In this research, poplar plants (Populus deltoides x nigra, DN34) and switchgrass (Panicum vigratum, Alamo) were hydroponically exposed to 3,3',4,4'-tetrachlorobiphenyl (CB77). Metabolism in plants occurred rapidly, and metabolites were detected after only a 24 h exposure. Rearrangement of chlorine atoms and dechlorination of CB77 by plants was unexpectedly observed. In addition, poplars were able to hydroxylate CB77 and the metabolite 6-hydroxy-3,3,4,4'-tetrachlorobiphenyl 6-OH-CB77) was identified and quantified. Hybrid poplar was able to hydroxylate CB77, but switchgrass was not, suggesting that enzymatic transformations are plant specific. Sulfur-containing metabolites (from the action of sulfotransferases) were investigated in this study, but they were not detected in either poplar or switchgrass.

Biphenyl-metabolizing bacteria in the rhizosphere of horseradish and bulk soil contaminated by polychlorinated biphenyls as revealed by stable isotope probing

DNA-based stable isotope probing in combination with terminal restriction fragment length polymorphism was used in order to identify members of the microbial community that metabolize biphenyl in the rhizosphere of horseradish (Armoracia rusticana) cultivated in soil contaminated with polychlorinated biphenyls (PCBs) compared to members of the microbial community in initial, uncultivated bulk soil. On the basis of early and recurrent detection of their 16S rRNA genes in clone libraries constructed from [(13)C]DNA, Hydrogenophaga spp. appeared to dominate biphenyl catabolism in the horseradish rhizosphere soil, whereas Paenibacillus spp. were the predominant biphenyl-utilizing bacteria in the initial bulk soil. Other bacteria found to derive carbon from biphenyl in this nutrient-amended microcosm-based study belonged mostly to the class Betaproteobacteria and were identified as Achromobacter spp., Variovorax spp., Methylovorus spp., or Methylophilus spp. Some bacteria that were unclassified at the genus level were also detected, and these bacteria may be members of undescribed genera. The deduced amino acid sequences of the biphenyl dioxygenase alpha subunits (BphA) from bacteria that incorporated [(13)C]into DNA in 3-day incubations of the soils with [(13)C]biphenyl are almost identical to that of Pseudomonas alcaligenes B-357. This suggests that the spectrum of the PCB congeners that can be degraded by these enzymes may be similar to that of strain B-357. These results demonstrate that altering the soil environment can result in the participation of different bacteria in the metabolism of biphenyl.

Transgenic plants to improve rhizoremediation of polychlorinated biphenyls (PCBs)

Recent investigations have shown that the three components of the biphenyl dioxygenase and the 2,3-dihydroxybiphenyl dioxygenase can be produced actively in transgenic plants. Both enzymes catalyze critical steps of the bacterial polychlorinated biphenyl (PCB) degrading pathway. On the basis of these observations, optimized plant-microbe bioremediation processes in which transgenic plants would initiate PCB metabolism and release the metabolites for further degradation by rhizobacteria has been proposed. Since this is still a relatively new approach for PCB remediation, its successful application will require efforts first, to engineer improved PCB-degrading enzymes; second, to co-ordinately express these enzymes' components in plants; and third, to better understand the mechanisms by which plants and rhizobacteria interact to degrade organic pollutants.