Polycyclic aromatic hydrocarbon biodegradation rates: a structure-based study

Asphalt dust influence on the distribution of polycyclic aromatic hydrocarbon in crankcase oil stains of a concrete, covered parking structure

Used crankcase oil is an important source of environmental polycyclic aromatic hydrocarbons (PAHs). Here, we use gas chromatography-mass spectrometry (GC-MS) to measure and compare the concentration of PAHs, including alkylated PAHs, in used oil against new and old oil stains and parking dust collected from a concrete, covered, open parking structure to understand the distribution of PAH in crankcase oil stains. PAH concentration in used oils ranges from 606 ng/mg to 1,592 ng/mg. The PAH distribution in used oil does not match that observed in parking oils stains, parking background, or parking dust. A comparison with PAH distributions in traffic related dusts extracted from the literature and dust collected from a neighboring open asphalt-paved parking suggests that covered parking dust includes substantial contributions from asphalt-paved parking dust, road dust, and/or coal tar dust. The parking dust is the most concentrated source of PAHs in the covered parking structure (PAHs up to 4,371 ng/mg), a small contribution of which can alter the distribution of PAHs in oil stains. Even with this contribution, we were able to observe a significant decrease of the ratio of low molecular weight PAHs to high molecular weight PAHs, and a significant increase in values of the phenanthrene/anthracene and fluoranthene/(fluoranthene + pyrene) ratios when oil stains age, suggesting biodegradation is an active attenuation process in covered, open parking structures.

PAH bioremediation with Rhodococcus rhodochrous ATCC 21198: Impact of cell immobilization and surfactant use on PAH treatment and post-remediation toxicity

Polycyclic aromatic hydrocarbons (PAHs) are prevalent environmental contaminants that are harmful to ecological and human health. Bioremediation is a promising technique for remediating PAHs in the environment, however bioremediation often results in the accumulation of toxic PAH metabolites. The objectives of this research were to demonstrate the cometabolic treatment of a mixture of PAHs by a pure bacterial culture, Rhodococcus rhodochrous ATCC 21198, and investigate PAH metabolites and toxicity. Additionally, the surfactant Tween ® 80 and cell immobilization techniques were used to enhance bioremediation. Total PAH removal ranged from 70-95% for fluorene, 44-89% for phenanthrene, 86-97% for anthracene, and 6.5-78% for pyrene. Maximum removal was achieved with immobilized cells in the presence of Tween ® 80. Investigation of PAH metabolites produced by 21198 revealed a complex mixture of hydroxylated compounds, quinones, and ring-fission products. Toxicity appeared to increase after bioremediation, manifesting as mortality and developmental effects in embryonic zebrafish. 21198's ability to rapidly transform PAHs of a variety of molecular structures and sizes suggests that 21198 can be a valuable microorganism for catalyzing PAH remediation. However, implementing further treatment processes to address toxic PAH metabolites should be pursued to help lower post-remediation toxicity in future studies.

Quantitative structure-biotransformation relationships of organic micropollutants in aerobic and anaerobic wastewater treatments

Biotransformation is one of the dominant processes to remove organic micropollutants (OMPs) in wastewater treatment. However, studies on the role of molecular structure in determining the biotransformation rates of OMPs are limited. We evaluated the biotransformation of 14 OMPs belonging to different chemical classes under aerobic and anaerobic conditions, and then explored the quantitative structure-biotransformation relationships (QSBRs) of the OMPs based on biotransformation rates using valid molecular structure descriptors (electrical and physicochemical parameters). Pseudo-first-order kinetic modeling was used to fit the biotransformation rate, and only 2 of the 14 OMPs showed that the biotransformation rate constant (kbio) values were higher under anaerobic conditions than aerobic conditions, indicating that aerobic conditions were more favorable for biotransformation of most OMPs. QSBRs infer that the electrophilicity index (ω) is a reliable predictor for OMPs biotransformation under aerobic conditions. ω corresponds to the interaction between OMPs and microbial enzyme active sites, this process is the rate-limiting step of biotransformation. However, under anaerobic conditions the QSBR based on ω was not significant, indicating that specific functional groups may be more critical than electrophilicity. In conclusion, QSBRs can serve as alternative tools for the prediction of the biotransformation of OMPs and provide further insights into the factors that influence biotransformation.

Biosurfactants and chemotaxis interplay in microbial consortium-based hydrocarbons degradation

Hydrocarbons are routinely detected at low concentrations, despite the degrading metabolic potential of ubiquitous microorganisms. The potential drivers of hydrocarbons persistence are lower bioavailability and mass transfer limitation. Recently, bioremediation strategies have developed rapidly, but still, the solution is not resilient. Biosurfactants, known to increase bioavailability and augment biodegradation, are tightly linked to bacterial surface motility and chemotaxis, while chemotaxis help bacteria to locate aromatic compounds and increase the mass transfer. Harassing the biosurfactant production and chemotaxis properties of degrading microorganisms could be a possible approach for the complete degradation of hydrocarbons. This review provides an overview of interplay between biosurfactants and chemotaxis in bioremediation. Besides, we discuss the chemical surfactants and biosurfactant-mediated biodegradation by microbial consortium.

Structural Selectivity of PAH Removal Processes in Soil, and the Effect of Metal Co-Contaminants

Polycyclic aromatic hydrocarbons (PAHs) form a convenient structural series of molecules with which to examine the selectivity exerted on their removal by soil microbiota. It is known that there is an inverse relationship between PAH molecular size and degradation rates in soil. In this paper, we look at how the magnitude of the slope for this relationship, m, can be used as an indicator of the effect of metal co-contaminants on degradation rates across a range of PAH molecular weights. The analysis utilises data collected from our previous microcosm study (Deary, M.E.; Ekumankama, C.C.; Cummings, S.P. Development of a novel kinetic model for the analysis of PAH biodegradation in the presence of lead and cadmium co-contaminants. Journal of Hazard Materials 2016, 307, 240–252) in which we followed the degradation of the 16 US EPA PAHs over 40 weeks in soil microcosms taken from a high organic matter content woodland soil. The soil was amended with a PAH mixture (total concentration of 2166 mg kg−1) and with a range of metal co-contaminant concentrations (lead, up to 782 mg kg−1; cadmium up to 620 mg kg−1; and mercury up to 1150 mg kg−1). It was found that the magnitude of m increases in relation to the applied concentration of metal co-contaminant, indicating a more adverse effect on microbial communities that participate in the removal of higher molecular weight PAHs. We conclude that m is a useful parameter by which we might measure the differential effects of environmental contaminants on the PAH removal. Such information will be useful in planning and implementing remediation strategies.

Tracing historical changes, degradation, and original sources of airborne polycyclic aromatic hydrocarbons (PAHs) in Jilin Province, China, by Abies holophylla and Pinus tabuliformis needle leaves

Due to their wide distribution and availability, plant leaves can be considered interesting candidates as biomonitoring substrates for the evaluation of atmospheric pollution. In addition, some species can also retain historical information, for example, related to environmental pollution, due to their leaf class age. In this study, the content of polycyclic aromatic hydrocarbons (PAHs) in Abies holophylla and Pinus tabuliformis needle samples in the function of their class age has been investigated to obtain information regarding the degradation constant for each PAH under investigation (α values ranging from 0.173 to 1.870) and to evaluate the possibility to correlate the presence of PAHs in needles with some important pollution environmental factors. Considering air pollutant variables registered in Jilin Province, interesting correlations (at 95% confidence level) have been found between coal consumption per year and anthracene contents in needles, while fluorene, phenanthrene, and anthracene results correlated with coal consumption. Furthermore, it has been demonstrated that the total PAH concentration in needles, for both species, increased with their age (from 804 to 3604 ng g^-1 dry weight), showing a general tendency to accumulate these substances through years. PAH degradation rates increased instead with molecular complexity. This study could be considered a first trial to obtain historical environmental information by pine needles biomonitoring.

Persistence, bioaccumulation and toxicity-assessment of petroleum UVCBs: A case study on alkylated three-ring PAHs

Substances with (very) persistent, (very) bioaccumulative, and/or toxic properties (PBT/vPvB) are of environmental concern and are identified via hazard-based PBT-assessment approaches. The PBT-assessment of well-defined substances is optimized over the past decades, but is under development for substances of unknown or variable composition, complex reaction products or biological materials (UVCBs). Particularly, the large number of constituents and variable composition complicate the PBT-assessment of UVCBs. For petroleum UVCBs, the use of the hydrocarbon block method (HBM) is proposed. Within this method, groups of constituents with similar physicochemical properties and structure are treated as a single entity and are expected to have comparable environmental fate and hazard properties. So far, however, there is a lack of experience with the application of the HBM for PBT-assessment purposes. The aim of this study is to investigate the suitability of the HBM for the PBT-assessment of petroleum UVCBs by evaluating the group of alkylated three-ring polycyclic aromatic hydrocarbons (PAHs). The presented approach is based on experimental data and model predictions and followed the guidelines of the European Chemicals Agency. Because of a lack of relevant experimental data, relative trend analyses were applied. The results indicate that alkylated three-ring PAHs are more persistent, bioaccumulative, and toxic than the parent three-ring PAHs. As the parent three-ring PAHs are currently identified within Europe as PBT/vPvB substances, the alkylated three-ring PAHs could also be considered as PBT/vPvB. Accordingly, this case study provides the prospects for the application of the HBM for the PBT-assessment of UVCBs using trend analysis.

A comprehensive feasibility study of effectiveness and environmental impact of PAH bioremediation using an indigenous microbial degrader consortium and a novel strain Stenotrophomonas maltophilia CPHE1 isolated from an industrial polluted soil

Polycyclic Aromatic Hydrocarbons (PAHs) are major toxic and recalcitrant pollutants in the environment. This study assessed the capacity of an isolated soil microbial consortium (OMC) to biodegrade PAHs. OMC was able to reach 100% biodegradation of naphthalene, acenaphthylene, acenaphthene, fluorene and phenanthrene in solution, and up to 76% and 50% of anthracene and fluoranthene, respectively, from a mix of 16 PAHs. To measure phenanthrene (PHE) mineralization, OMC and eight strains isolated from OMC were used and identified by PCR amplification of the gene 16S ribosomal RNA. A novel Stenotrophomonas maltophilia CPHE1, not previously described as a PAH degrader, was able to mineralize almost 40% PHE and biodegrade 90.5% in solution, in comparison to OMC that reached 100% PHE degradation, but only 18.8% mineralization. Based on metabolites identified during PHE degradation and on the detection of two genes (PAH RHDα and nahAc) in OMC consortium, two possible via were described for its degradation, through salicylic and phthalic acid. PAH RHDα, which codified the first step on PHE biodegradation pathway, was also found in the DNA of S. maltophilia CPHE1. An ecotoxicology study showed that PHE bioremediation after inoculating S. maltophilia CPHE1 for 30 days decreased by half the solution toxicity.

Using Vinegar Residue-Based Carrier Materials to Improve the Biodegradation of Phenanthrene in Aqueous Solution

A large amount of vinegar residue (VR) is generated every year in China, causing serious environmental pollutions. Meanwhile, as a kind of persistent organic pollutants, polycyclic aromatic hydrocarbons (PAHs) ubiquitously exist in environments. With a goal of reusing VR and reducing PAHs pollutions, we herein isolated one B. subtilis strain, ZL09-26, which can degrade phenanthrene and produce biosurfactants. Subsequently, raw VR was dried under different temperatures (50 °C, 80 °C, 100 °C and 120 °C) or pyrolyzed under 350 °C and 700 °C, respectively. After being characterized by various approaches, the treated VR were mixed with ZL09-26 as carriers to degrade phenanthrene. We found that VR dried at 50 °C (VR50) was the best in promoting the growth of ZL09-26 and the degradation of phenanthrene. This result may be attributed to the residual nutrients, suitable porosity and small surface charge of VR50. Our results demonstrate the potential of VR in the biodegradation of phenanthrene, which may be meaningful for developing new VR-based approaches to remove PAHs in aqueous environments.

Enhancement of Toxic Efficacy of Alkylated Polycyclic Aromatic Hydrocarbons Transformed by Sphingobium quisquiliarum

Alkylated polycyclic aromatic hydrocarbons (PAHs) are abundant in crude oils and refined petroleum products and are considered as major contributors to the toxicity of spilled oils. In this study, the microbial degradation of model (alkylated) PAHs (i.e., phenanthrene, 3-methylphenanthrene, 3,6-dimethylphenanthrene (36DMPhe), pyrene, and 1-methylpyrene (1MP)) by the bacterium Sphingobium quisquiliarum EPA505, a known degrader of PAHs, was studied. To evaluate the toxic potential of the metabolic products, reaction mixtures containing metabolites of 36DMPhe and 1MP were fractionated by high-performance liquid chromatography, and their effects on the luminescence inhibition of Aliivibrio fischeri were evaluated. Although the luminescence inhibition of 36DMPhe and 1MP at their solubility levels was not observed, inhibition was observed in their metabolite fractions at the solubility limit of their parent molecule. This indicates that initial biotransformation increases the toxicity of alkylated PAHs because of the increased solubility and/or inherent toxicity of metabolites. Qualitative analysis of the metabolite fractions suggested that mono-oxidation of the methyl group is the main metabolic pathway of 36DMPhe and 1MP.

Disentanglement of the chemical, physical, and biological processes aids the development of quantitative structure-biodegradation relationships for aerobic wastewater treatment

Attenuation of organic compounds in sewage treatment plants (STPs) is affected by a complex interplay between chemical (e.g. ionization, hydrolysis), physical (e.g. sorption, volatilization), and biological (e.g. biodegradation, microbial acclimation) processes. These effects should be accounted for individually, in order to develop predictive cheminformatics tools for STPs. Using measured data from 70 STPs in the Netherlands for 69 chemicals (pharmaceuticals, herbicides, etc.), we highlighted the influences of 1) chemical ionization, 2) sorption to sludge, and 3) acclimation of the microbial consortia on the primary removal of chemicals. We used semi-empirical corrections for each of these influences to deduce biodegradation rate constants upon which quantitative structure-biodegradation relationships (QSBRs) were developed. As shown by a global QSBR, biodegradation in STPs generally relates to structural complexity, size, energetics, and charge distribution. Statistics of the global QSBR were reasonable, being R²_training=0.69 (training set of 51 compounds) and R²_validation=0.50 (validation set of 18 compounds). Class-specific QSBRs utilized electronic properties potentially relating to rate-limiting enzymatic steps. For class-specific QSBRs, values of R² of in between 0.7 and 0.8 were obtained. With caution, environmental risk assessment methodologies may apply these models to estimate biodegradation rates for 'data-poor' compounds. The approach also highlights 'meta data' on STP operational parameters needed to develop QSBRs of better predictability in the future.

Aluminium removal and recovery from wastewater and soil using isolated indigenous bacteria

This paper elucidates the capability of isolated indigenous bacteria to remove aluminium from wastewater and soil. Two indigenous species of Brochothrix thermosphacta and Vibrio alginolyticus were isolated from an aluminium-contaminated site. These two species were used to treat aluminium-containing wastewater and contaminated soil using the bioaugmentation method. B. thermosphacta showed the highest aluminium removal of 57.87 ± 0.45% while V. alginolyticus can remove aluminium up to 59.72 ± 0.33% from wastewater. For aluminium-contaminated soil, B. thermosphacta and V. alginolyticus, showed a highest removal of only 4.58 ± 0.44% and 5.48 ± 0.58%, respectively. The bioaugmentation method is more suitable to be used to treat aluminium in wastewater compared to contaminated soil. The produced biomass separation after wastewater treatment was so much easier and applicable, compared to the produced biomass handling from contaminated soil treatment. A 48.55 ± 2.45% and 40.12 ± 4.55% of aluminium can be recovered from B. thermosphacta and V. alginolyticus biomass, respectively, with 100 mg/L initial aluminium concentration in wastewater.

Changes in the Bacterioplankton Community Structure from Southern Gulf of Mexico During a Simulated Crude Oil Spill at Mesocosm Scale

The southern Gulf of Mexico (sGoM) is highly susceptible to receiving environmental impacts due to the recent increase in oil-related activities. In this study, we assessed the changes in the bacterioplankton community structure caused by a simulated oil spill at mesocosms scale. The 16S rRNA gene sequencing analysis indicated that the initial bacterial community was mainly represented by Gamma-proteobacteria, Alpha-proteobacteria, Flavobacteriia, and Cyanobacteria. The hydrocarbon degradation activity, measured as the number of culturable hydrocarbonoclastic bacteria (CHB) and by the copy number of the alkB gene, was relatively low at the beginning of the experiment. However, after four days, the hydrocarbonoclastic activity reached its maximum values and was accompanied by increases in the relative abundance of the well-known hydrocarbonoclastic Alteromonas. At the end of the experiment, the diversity was restored to similar values as those observed in the initial time, although the community structure and composition were clearly different, where Marivita, Pseudohongiella, and Oleibacter were detected to have differential abundances on days eight–14. These changes were related with total nitrogen (p value = 0.030 and r² = 0.22) and polycyclic aromatic hydrocarbons (p value = 0.048 and r² = 0.25), according to PERMANOVA. The results of this study contribute to the understanding of the potential response of the bacterioplankton from sGoM to crude oil spills.

The experimental determination of reliable biodegradation rates for mono-aromatics towards evaluating QSBR models

Quantitative Structure Biodegradation Relationships (QSBRs) are a tool to predict the biodegradability of chemicals. The objective of this work was to generate reliable biodegradation data for mono-aromatic chemicals in order to evaluate and verify previously developed QSBRs models. A robust biodegradation test method was developed to estimate specific substrate utilization rates, which were used as a proxy for biodegradation rates of chemicals in pure culture. Five representative mono-aromatic chemicals were selected that spanned a wide range of biodegradability. Aerobic biodegradation experiments were performed for each chemical in batch reactors seeded with known degraders. Chemical removal, degrader growth and CO₂ production were monitored over time. Experimental data were interpreted using a full carbon mass balance model, and Monod kinetic parameters (Y, K_s, q_max and μ_max) for each chemical were determined. In addition, stoichiometric equations for aerobic mineralization of the test chemicals were developed. The theoretically estimated biomass and CO₂ yields were similar to those experimentally observed; 35% (s.d ± 8%) of the recovered substrate carbon was converted to biomass, and 65% (s.d ± 8%) was mineralised to CO₂. Significant correlations were observed between the experimentally determined specific substrate utilization rates, as represented by q_max and q_max/K_s, at high and low substrate concentrations, respectively, and the first order biodegradation rate constants predicted by a previous QSBR study. Similarly, the correlation between q_max and selected molecular descriptors characterizing the chemicals structure in a previous QSBR study was also significant. These results suggest that QSBR models can be reliable and robust in prioritising chemical half-lives for regulatory screening purposes.

Electrophilicity index as a critical indicator for the biodegradation of the pharmaceuticals in aerobic activated sludge processes

Improving biodegradation of pharmaceuticals during wastewater treatment is critical to control the release of emerging micropollutants to natural waters. In this study, biodegradation of six model pharmaceuticals was investigated at different initial concentrations in two discrete activated sludge systems, and moreover, the correlation was explored between the biodegradation rate and key molecular properties of the contaminants. First, the biodegradation rates of the pharmaceuticals were measured fitting a pseudo first-order kinetic model to the experimental kinetic data. The degradation rate constants (k_bio) were found to negatively correlate to the initial concentration of the chemicals, indicating an inhibitory effect on the microorganisms by the pharmaceuticals. Further examinations of the rate data against the key molecular properties of the pharmaceuticals revealed, for the first time, that the electrophilicity index (ω), a measure of electrophilic power, served as a better indicator of the biodegradability and predictive parameter for the k_bio than the conventional log K_OW (a measure of hydrophobicity) in the two discrete aerobic activated sludge systems. However, the correlation strength (goodness‒of‒fit) between ω and k_bio deteriorated when the reactor turned from aerobic to anoxic and anaerobic conditions, suggesting that electron transfer from pharmaceutical molecules to enzymes was inhibited when dissolved oxygen was deficit or absent. Our results show that ω can potentially serve as a straightforward and robust indicator for predicting the biodegradability of pharmaceutical in conventional activated sludge processes.

A quantitative structure-biodegradation relationship (QSBR) approach to predict biodegradation rates of aromatic chemicals

The objective of this work was to develop a QSBR model for the prioritization of organic pollutants based on biodegradation rates from a database containing globally harmonized biodegradation tests using relevant molecular descriptors. To do this, we first categorized the chemicals into three groups (Group 1: simple aromatic chemicals with a single ring, Group 2: aromatic chemicals with multiple rings and Group3: Group 1 plus Group 2) based on molecular descriptors, estimated the first order biodegradation rate of the chemicals using rating values derived from the BIOWIN3 model, and finally developed, validated and defined the applicability domain of models for each group using a multiple linear regression approach. All the developed QSBR models complied with OECD principles for QSAR validation. The biodegradation rate in the models for the two groups (Group 2 and 3 chemicals) are associated with abstract molecular descriptors that provide little relevant practical information towards understanding the relationship between chemical structure and biodegradation rates. However, molecular descriptors associated with the QSBR model for Group 1 chemicals (R² = 0.89, Q²_loo = 0.87) provided information on properties that can readily be scrutinised and interpreted in relation to biodegradation processes. In combination, these results lead to the conclusion that QSBRs can be an alternative tool to estimate the persistence of chemicals, some of which can provide further insights into those factors affecting biodegradation.

Co-metabolic degradation of naphthalene and pyrene by acclimated strain and competitive inhibition kinetics

A dominant strain named Ochrobactrum sp. was isolated from soils contaminated with coal tar. The batch experiments were carried out to study the co-metabolic degradation of pyrene by Ochrobactrum MB-2 with naphthalene as the main substrate and the effects of several significant parameters such as naphthalene concentration, pH and temperature on removal efficiency were explored. The results showed that Ochrobactrum MB-2 effectively degraded naphthalene and that the addition of naphthalene favored the degradation of pyrene. The maximum elimination efficiency of naphthalene (10 mg L^-1) and pyrene (1 mg L^-1) was achieved at pH 7 and 25 °C, and the corresponding values were 99 and 41%, respectively. A competitive inhibition model based on the Michaelis-Menten equation was used to characterize the inhibitory effect of pyrene on naphthalene degradation. The values of the half-saturation coefficient for naphthalene (K_S) and dissociation constant of enzyme-inhibitor complex (K_C) were determined to be 4.93 and 1.38 mg L^-1, respectively.

Prediction of biopersistence of hydrocarbons using a single parameter

Aerobic biodegradation is an important attenuation process for petroleum hydrocarbons (PHCs) in the natural environment. It has also been frequently used in engineered systems to remediate PHC-contaminated sites. A model such as a quantitative structure property relationship (QSPR) that can predict the biodegradation rate of PHCs would be helpful prior to implementing any extensive environmental measurements and bioremediation strategies. Existing QSPRs either have a large number of predictor variables that may cause overfitting or are based on a small dataset of PHCs. The goal of this study is to develop a simple, portable QSPR that has only a few predicator variables but can accurately predict the biodegradation half-lives of a large group of PHCs. To this end, more than 500 molecular variables were screened, and candidate variables were refined by a feature selection method and fitted to biodegradation data of a group of structurally heterogeneous PHCs (n = 173). The model was established by means of hierarchical clustering and classification and regression tree algorithms, which was optimized by an internal validation procedure and validated by an external dataset. The optimal QSPR model, containing only one predictor variable (the number of bonds that do not contain hydrogen), was able to accurately predict biodegradation half-lives for a wide variety of PHCs. The internal validation test indicated an overall prediction accuracy of 93%, and predictions applied to an independent external set of 64 PHCs yielded 95% accuracy. The new model is transparent and easily portable from one user to another.

Biodegradation of hydrocarbon mixtures in surface waters at environmentally relevant levels - Effect of inoculum origin on kinetics and sequence of degradation

Biodegradation is a dominant removal process for many organic pollutants, and biodegradation tests serve as tools for assessing their environmental fate within regulatory risk assessment. In simulation tests, the inoculum is not standardized, varying in microbial quantity and quality, thereby potentially impacting the observed biodegradation kinetics. In this study we investigated the effect of inoculum origin on the biodegradation kinetics of hydrocarbons for five inocula from surface waters varying in urbanization and thus expected pre-exposure to petroleum hydrocarbons. A new biodegradation method for testing mixtures of hydrophobic chemicals at trace concentrations was demonstrated: Aqueous solutions containing 9 hydrocarbons were generated by passive dosing and diluted with surface water resulting in test systems containing native microorganisms exposed to test substances at ng-μg/L levels. Automated Headspace Solid Phase Microextraction coupled to GC-MS was applied directly to these test systems to determine substrate depletion relative to abiotic controls. Lag phases were generally less than 8 days. First order rate constants were within one order of magnitude for each hydrocarbon in four of the five waters but lower in water from a rural lake. The sequence of degradation between the 9 hydrocarbons showed similar patterns in the five waters indicating the potential for using selected hydrocarbons for benchmarking between biodegradation tests. Degradation half-times were shorter than or within one order of magnitude of BioHCwin predictions for 8 of 9 hydrocarbons. These results showed that location choice is important for biodegradation kinetics and can provide a relevant input to aquatic exposure and fate models.

Surfactant-enhanced remediation of polycyclic aromatic hydrocarbons: A review

Polycyclic aromatic hydrocarbons (PAHs) are toxic, mutagenic and carcinogenic organic compounds that are widely present in the environment. The bioremediation of PAHs is an economical and environmentally friendly remediation technique, but it is limited because PAHs have low water solubility and fewer bioavailable properties. The solubility and bioavailability of PAHs can be increased by using surfactants to reduce surface tension and interfacial tension; this method is called surfactant-enhanced remediation (SER). The SER of PAHs is influenced by many factors such as the type and concentration of surfactants, PAH hydrophobicity, temperature, pH, salinity, dissolved organic matter and microbial community. Furthermore, as mixed micelles have a synergistic effect on PAH solubilisation, selecting the optimum ratio of mixed surfactants leads to effective PAH remediation. Although the use of surfactants inhibits microbial activities in some cases, this could be avoided by choosing an optimum combination of surfactants and a proper microbial community for the targeted PAH(s), resulting in up to 99.99% PAH removal. This article reviews the literature on SER of PAHs, including surfactant types, the synergistic effect of mixed micelles on PAH removal, the impact of surfactants on the PAH biodegradation process, factors affecting the SER process, and the mechanisms of surfactant-enhanced solubilisation of PAHs.

Adsorption and Biodegradation of Aromatic Chemicals by Bacteria Encapsulated in a Hydrophobic Silica Gel

An adsorbent silica biogel material was developed via silica gel encapsulation of Pseudomonas sp. NCIB 9816-4, a bacterium that degrades a broad spectrum of aromatic pollutants. The adsorbent matrix was synthesized using silica precursors methyltrimethoxysilane and tetramethoxysilane to maximize the adsorption capacity of the matrix while maintaining a highly networked and porous microstructure. The encapsulated bacteria enhanced the removal rate and capacity of the matrix for an aromatic chemical mixture. Repeated use of the material over four cycles was conducted to demonstrate that the removal capacity could be maintained with combined adsorption and biodegradation. The silica biogel can thus be used extensively without the need for disposal, as a result of continuous biodegradation by the encapsulated bacteria. However, an inverse trend was observed with the ratio of biodegradation to adsorption as a function of log K_ow, suggesting increasing mass-transport limitation for the most hydrophobic chemicals used (log K_ow > 4).

Bioremediation of PAH-contamined soils: Consequences on formation and degradation of polar-polycyclic aromatic compounds and microbial community abundance

A bioslurry batch experiment was carried out over five months on three polycyclic aromatic compound (PAC) contaminated soils to study the PAC (PAH and polar-PAC) behavior during soil incubation and to evaluate the impact of PAC contamination on the abundance of microbial communities and functional PAH-degrading populations. Organic matter characteristics and reactivity, assessed through solvent extractable organic matter and PAC contents, and soil organic matter mineralization were monitored during 5 months. Total bacteria and fungi, and PAH-ring hydroxylating dioxygenase genes were quantified. Results showed that PAHs and polar-PACs were degraded with different degradation dynamics. Differences in degradation rates were observed among the three soils depending on PAH distribution and availability. Overall, low molecular weight compounds were preferentially degraded. Degradation selectivity between isomers and structurally similar compounds was observed which could be used to check the efficiency of bioremediation processes. Bacterial communities were dominant over fungi and were most likely responsible for PAC degradation. Abundance of PAH-degrading bacteria increased during incubations, but their proportion in the bacterial communities tended to decrease. The accumulation of some oxygenated-PACs during the bioslurry experiment underlines the necessity to monitor these compounds during application of remediation treatment on PAH contaminated soils.

Persistence and biodegradation of oil at the ocean floor following Deepwater Horizon

The 2010 Deepwater Horizon disaster introduced an unprecedented discharge of oil into the deep Gulf of Mexico. Considerable uncertainty has persisted regarding the oil's fate and effects in the deep ocean. In this work we assess the compound-specific rates of biodegradation for 125 aliphatic, aromatic, and biomarker petroleum hydrocarbons that settled to the deep ocean floor following release from the damaged Macondo Well. Based on a dataset comprising measurements of up to 168 distinct hydrocarbon analytes in 2,980 sediment samples collected within 4 y of the spill, we develop a Macondo oil "fingerprint" and conservatively identify a subset of 312 surficial samples consistent with contamination by Macondo oil. Three trends emerge from analysis of the biodegradation rates of 125 individual hydrocarbons in these samples. First, molecular structure served to modulate biodegradation in a predictable fashion, with the simplest structures subject to fastest loss, indicating that biodegradation in the deep ocean progresses similarly to other environments. Second, for many alkanes and polycyclic aromatic hydrocarbons biodegradation occurred in two distinct phases, consistent with rapid loss while oil particles remained suspended followed by slow loss after deposition to the seafloor. Third, the extent of biodegradation for any given sample was influenced by the hydrocarbon content, leading to substantially greater hydrocarbon persistence among the more highly contaminated samples. In addition, under some conditions we find strong evidence for extensive degradation of numerous petroleum biomarkers, notably including the native internal standard 17α(H),21β(H)-hopane, commonly used to calculate the extent of oil weathering.

Bacteria capable of degrading anthracene, phenanthrene, and fluoranthene as revealed by DNA based stable-isotope probing in a forest soil

Information on microorganisms possessing the ability to metabolize different polycyclic aromatic hydrocarbons (PAHs) in complex environments helps in understanding PAHs behavior in natural environment and developing bioremediation strategies. In the present study, stable-isotope probing (SIP) was applied to investigate degraders of PAHs in a forest soil with the addition of individually (13)C-labeled phenanthrene, anthracene, and fluoranthene. Three distinct phylotypes were identified as the active phenanthrene-, anthracene- and fluoranthene-degrading bacteria. The putative phenanthrene degraders were classified as belonging to the genus Sphingomona. For anthracene, bacteria of the genus Rhodanobacter were the putative degraders, and in the microcosm amended with fluoranthene, the putative degraders were identified as belonging to the phylum Acidobacteria. Our results from DNA-SIP are the first to directly link Rhodanobacter- and Acidobacteria-related bacteria with anthracene and fluoranthene degradation, respectively. The results also illustrate the specificity and diversity of three- and four-ring PAHs degraders in forest soil, contributes to our understanding on natural PAHs biodegradation processes, and also proves the feasibility and practicality of DNA-based SIP for linking functions with identity especially uncultured microorganisms in complex microbial biota.

Development of a novel kinetic model for the analysis of PAH biodegradation in the presence of lead and cadmium co-contaminants

We report on the results of a 40 week study in which the biodegradation of 16 US EPA polycyclic aromatic hydrocarbons (PAHs) was followed in microcosms containing soil of high organic carbon content (11%) in the presence and absence of lead and cadmium co-contaminants. The total spiked PAH concentration was 2166mg/kg. Mercury amendment was also made to give an abiotic control. A novel kinetic model has been developed to explain the observed biphasic nature of PAH degradation. The model assumes that PAHs are distributed across soil phases of varying degrees of bioaccessibility. The results of the analysis suggest that overall percentage PAH loss is dependent on the respective rates at which the PAHs (a) are biodegraded by soil microorganisms in pore water and bioaccessible soil phases and (b) migrate from bioaccessible to non-bioaccessible soil phases. In addition, migration of PAHs to non-bioaccessible and non-Soxhlet-extractable soil phases associated with the humin pores gives rise to an apparent removal process. The presence of metal co-contaminants shows a concentration dependent inhibition of the biological degradation processes that results in a reduction in overall degradation. Lead appears to have a marginally greater inhibitory effect than cadmium.

Accelerated degradation of PAHs using edaphic biostimulants obtained from sewage sludge and chicken feathers

We studied in the laboratory the bioremediation effects over a 100-day period of three edaphic biostimulants (BS) obtained from sewage sludge (SS) and from two different types of chicken feathers (CF1 and CF2), in a soil polluted with three polycyclic aromatic hydrocarbons (PAH) (phenanthrene, Phe; pyrene, Py; and benzo(a)pyrene, BaP), at a concentration of 100 mg kg(-1) soil. We determined their effects on enzymatic activities and on soil microbial community. Those BS with larger amounts of proteins and a higher proportion of peptides (<300 daltons), exerted a greater stimulation on the soil biochemical properties and microbial community, possibly because low molecular weight proteins can be easily assimilated by soil microorganisms. The soil dehydrogenase, urease, β-glucosidase and phosphatase activities and microbial community decreased in PAH-polluted soil. This decrease was more pronounced in soils contaminated with BaP than with Py and Phe. The application of the BS to PAH-polluted soils decreased the inhibition of the soil biological properties, principally at 7 days into the experiment. This decrease was more pronounced in soils contaminated with BaP than with Py and Phe and was higher in polluted soils amended with CF2, followed by SS and CF1, respectively.

Engineering of a silica encapsulation platform for hydrocarbon degradation using Pseudomonas sp. NCIB 9816-4

Industrial application of encapsulated bacteria for biodegradation of hydrocarbons in water requires mechanically stable materials. A silica gel encapsulation method was optimized for Pseudomonas sp. NCIB 9816-4, a bacterium that degrades more than 100 aromatic hydrocarbons. The design process focused on three aspects: (i) mechanical property enhancement; (ii) gel cytocompatibility; and (iii) reduction of the diffusion barrier in the gel. Mechanical testing indicated that the compressive strength at failure (σf ) and elastic modulus (E) changed linearly with the amount of silicon alkoxide used in the gel composition. Measurement of naphthalene biodegradation by encapsulated cells indicated that the gel maintained cytocompatibility at lower levels of alkoxide. However, significant loss in activity was observed due to methanol formation during hydrolysis at high alkoxide concentrations, as measured by FTIR spectroscopy. The silica gel with the highest amount of alkoxide (without toxicity from methanol) had a biodegradation rate of 285 ± 42 nmol/L-s, σf  = 652 ± 88 kPa, and E = 15.8 ± 2.0 MPa. Biodegradation was sustained for 1 month before it dropped below 20% of the initial rate. In order to improve the diffusion through the gel, polyvinyl alcohol (PVA) was used as a porogen and resulted in a 48 ± 19% enhancement in biodegradation, but it impacted the mechanical properties negatively. This is the first report studying how the silica composition affects biodegradation of naphthalene by Pseudomonas sp. NCIB 9816-4 and establishes a foundation for future studies of aromatic hydrocarbon biodegradation for industrial application.

Prediction of the Fate of Organic Compounds in the Environment From Their Molecular Properties: A Review

A comprehensive review of quantitative structure-activity relationships (QSAR) allowing the prediction of the fate of organic compounds in the environment from their molecular properties was done. The considered processes were water dissolution, dissociation, volatilization, retention on soils and sediments (mainly adsorption and desorption), degradation (biotic and abiotic), and absorption by plants. A total of 790 equations involving 686 structural molecular descriptors are reported to estimate 90 environmental parameters related to these processes. A significant number of equations was found for dissociation process (pKa), water dissolution or hydrophobic behavior (especially through the KOW parameter), adsorption to soils and biodegradation. A lack of QSAR was observed to estimate desorption or potential of transfer to water. Among the 686 molecular descriptors, five were found to be dominant in the 790 collected equations and the most generic ones: four quantum-chemical descriptors, the energy of the highest occupied molecular orbital (EHOMO) and the energy of the lowest unoccupied molecular orbital (ELUMO), polarizability (α) and dipole moment (μ), and one constitutional descriptor, the molecular weight. Keeping in mind that the combination of descriptors belonging to different categories (constitutional, topological, quantum-chemical) led to improve QSAR performances, these descriptors should be considered for the development of new QSAR, for further predictions of environmental parameters. This review also allows finding of the relevant QSAR equations to predict the fate of a wide diversity of compounds in the environment.

Quantitative structure-biodegradability relationships for biokinetic parameter of polycyclic aromatic hydrocarbons

Prediction of the biodegradability of organic pollutants is an ecologically desirable and economically feasible tool for estimating the environmental fate of chemicals. In this paper, stepwise multiple linear regression analysis method was applied to establish quantitative structure biodegradability relationship (QSBR) between the chemical structure and a novel biodegradation activity index (qmax) of 20 polycyclic aromatic hydrocarbons (PAHs). The frequency B3LYP/6-311+G(2df,p) calculations showed no imaginary values, implying that all the structures are minima on the potential energy surface. After eliminating the parameters which had low related coefficient with qmax, the major descriptors influencing the biodegradation activity were screened to be Freq, D, MR, EHOMO and ToIE. The evaluation of the developed QSBR mode, using a leave-one-out cross-validation procedure, showed that the relationships are significant and the model had good robustness and predictive ability. The results would be helpful for understanding the mechanisms governing biodegradation at the molecular level.

Theoretical study of the molecular properties of dimethylanthracenes as properties for the prediction of their biodegradation and mutagenicity

There is little information available on methyl derivatives of anthracene and their interaction with the enzymes of bacterial consortia that could be found in petroleum sludge. In this study a theoretical investigation of all dimethylanthracenes (DMA) isomers and their relation to biodegradation are presented. Equilibrium geometries, ionization potentials (IP), electronic affinities (EA), dipole moments and electronic dipole polarizabilities of DMA isomers calculated by Density Functional Theory (DFT) methods are reported. The calculated IP and EA values vary little along the series of isomers. The polarizability values (〈α〉, Δα, and αyy) increase on passing from meso,meso- and α,meso- to β,β-DMA isomers. The computed polarizability values of DMAs can be used as predictors in determining differences in biodegradation rates of DMAs. The summation over Raman activity ∑ARaman over 3N-6 vibrational modes is sensitive to the position of the methyl substituent. The ∑ARaman values of 1-methylanthracene (MA), 2-MA, 2,9-DMA and 9,10-DMA are consistent with observed mutagenic activities in Salmonella Typhimurium strains TA98 and TA100.

Static and dynamic electronic (hyper)polarizabilities of dimethylnaphthalene isomers: characterization of spatial contributions by density analysis

Static and frequency-dependent electronic (hyper)polarizabilities of the dimethylnaphthalene (DMN) isomers were computed in vacuum using the Coulomb-attenuating Density Functional Theory method. The nonlinear optical Second Harmonic Generation (SHG) and Electro-Optical Pockels Effect (EOPE) were investigated at the characteristic Nd:YAG laser wavelength of 1064 nm. The response electric properties especially the longitudinal polarizability, polarizability anisotropy, and first-order hyperpolarizability are significantly affected by the position of the methyl groups. The SHG and EOPE techniques can be potentially useful to discriminate the α,α-DMN isomers (2,6-DMN < 2,7-DMN < 2,3-DMN) as well as the β,β-DMN isomers (1,5-DMN < 1,4-DMN < 1,8-DMN). The (hyper)polarizability differences among the investigated DMNs were elucidated through density analysis calculations. The predicted polarizabilities exhibit good linear relationships with the experimental first-order biomass-normalized rate coefficient, a physicochemical property connected to the rates of biodegradation processes of polycyclic aromatic hydrocarbons.

Characterization of pyrene degradation by Pseudomonas sp. strain Jpyr-1 isolated from active sewage sludge

Using pyrene as a sole carbon, a new polycyclic aromatic hydrocarbons (PAHs)-degrading bacterial strain was isolated from the active sewage sludge. This strain was identified as Pseudomonas sp. Jpyr-1 by 16S rRNA gene sequence analysis. The maximum degradation rate of pyrene was 3.07 mg L(-1)h(-1) in 48 h incubation with initial pyrene concentration of 200 mg L(-1). Moreover, in binary system consisting of pyrene and another PAH, the enzyme system of Jpyr-1 showed a preference toward pyrene. Furthermore, competitive inhibition of pyrene degradation by other PAH compounds occurred in the binary system. Jpyr-1 could also rapidly degrade other PAHs, such as benzanthracene, chrysene and benzo[a]pyrene. Moreover, several metabolites were detected during pyrene degradation which indicated that Jpyr-1 degraded pyrene through the o-phthalate pathway. Taken together, these results indicated that Pseudomonas sp. Jpyr-1 was a new PAHs-degrading strain that might be useful in the bioremediation of sites contaminated with PAHs.

Kinetics of heavy metal inhibition of 1,2-dichloroethane biodegradation in co-contaminated water

Sites co-contaminated with heavy metals and 1,2-DCA may pose a greater challenge for bioremediation, as the heavy metals could inhibit the activities of microbes involved in biodegradation. Therefore, this study was undertaken to quantitatively assess the effects of heavy metals (arsenic, cadmium, mercury, and lead) on 1,2-DCA biodegradation in co-contaminated water. The minimum inhibitory concentrations (MICs) and concentrations of the heavy metals that caused half-life doubling (HLDs) of 1,2-DCA as well as the degradation rate coefficient (k(1)) and half-life (t(½)) of 1,2-DCA were measured and used to predict the toxicity of the heavy metals in the water microcosms. An increase in heavy metal concentration resulted in a progressive increase in the t(½) and relative t(½) and a decrease in k(1). The MICs and HLDs of the heavy metals were found to vary, depending on the heavy metals type. In addition, the presence of heavy metals was shown to inhibit 1,2-DCA biodegradation in a dose-dependent manner, with the following order of decreasing inhibitory effect: Hg(2+)  > As(3+)  > Cd(2+)  > Pb(2+). Findings from this study have significant implications for the development of bioremediation strategies for effective degradation of 1,2-DCA and other related compounds in wastewater co-contaminated with heavy metals.

Characteristic molecular signature for the early detection and prediction of polycyclic aromatic hydrocarbons in rat liver

Predictions of toxicity are central for the assessment of chemical toxicity, and the effects of environmental toxic compounds are still a major issue for predicting potential human health risks. Among the various environmental toxicants, polycyclic aromatic hydrocarbons (PAHs) are an important class of environmental pollutant, and many PAHs are known or suspected carcinogens. In the present study, to investigate whether characteristic expression profiles of PAHs exist in rat liver and whether a characteristic molecular signature can discriminate and predict among different PAHs at an early exposure time, we analyzed the genome-wide expression profiles of rat livers exposed to PAHs [benzo[a]anthracene (BA), benzo[a]pyrene (BP), phenanthrene (PA) and naphthalene (NT)]. At early time-point PAH exposure, large-scale gene expression analysis resulted in characteristic molecular signatures for each PAH, and supervised analysis identified 1183 outlier genes as a distinct molecular signature discerning PAHs from the normal control group. We identified 158 outlier genes as early predictive and surrogate markers for predicting each tested PAH by combination of two different multi-classification algorithms with 100% accuracy through a leave-one out cross-validation method. In conclusion, the characteristic gene expression signatures from a rat model system could be used as predictable and discernible gene-based biomarkers for the detection and prediction of PAHs, and these molecular markers may provide insights into the underlying mechanisms for genotoxicity of exposure to PAHs from environmental aspect.

Low molecular weight components in an aquatic humic substance as characterized by membrane dialysis and orbitrap mass spectrometry

Suwannee River fulvic acid (SRFA) was dialyzed through a 100-500 molecular weight cutoff dialysis membrane, and the dialysate and retentate were analyzed by UV-visible absorption and high-resolution Orbitrap mass spectrometry (MS). A significant fraction (36% based on dissolved organic carbon) of SRFA passed through the dialysis membrane. The fraction of SRFA in the dialysate had a different UV-visible absorption spectrum and was enriched in low molecular weight molecules with a more aliphatic composition relative to the initial SRFA solution. Comparison of the SRFA spectra collected by Orbitrap MS and Fourier transform ion cyclotron resonance MS (FT-ICR MS) demonstrated that the mass accuracy of the Orbitrap MS is sufficient for determination of unique molecular formulas of compounds with masses <600 Da in a complex mixture, such as SRFA. The most intense masses detected by Orbitrap MS were found in the 100-200 Da mass range. Many of these low molecular masses corresponded to molecular formulas of previously identified compounds in organic matter, lignin, and plants, and the use of the standard addition method provided an upper concentration estimate of selected target compounds in SRFA. Collectively, these results provide evidence that SRFA contains low molecular weight components that are present individually or in loosely bound assemblies.

Relationship between biodegradation and sorption of phthalate esters and their metabolites in natural sediments

Regulatory evaluations of commercial chemicals in Canada, the United States, the European Union, and other countries aim to identify biodegradation rates of chemicals in natural soils and sediments. However, commonly used biodegradation testing methods are limited in their capacity to determine biodegradation rates under natural environmental conditions. As a result, widely varying biodegradation rates have been reported for many very hydrophobic substances. This variability causes difficulties in regulatory evaluations, potentially leading to chemical misclassification. In the present study, the authors developed a model of the relationship between biodegradation, sorption, and hydrophobicity, and tested the model in experiments that measured the biodegradation rates of a range of di-phthalate esters (DPEs) and mono-phthalate esters (MPEs) in natural sediments. The results indicate that DPEs and MPEs have the inherent capacity to be quickly degraded by microbes in sediments at a common rate, but that DPE biodegradation rates in natural sediments decrease with increasing phthalate ester sorption to sediments. The results show that inherently biodegradable substances that are subject to a high degree of sorption can be expected to exhibit long half-lives in natural sediments. The model provides a potential methodology for assessing biodegradation rates in natural sediments from inherent biodegradation rates measured in screening tests by accounting for chemical sorption. The present study indicates that a reduced rate of biodegradation is due to a reduced fraction of freely dissolved chemical concentration in the interstitial water, and that the environmental significance of sorption-reduced biodegradation rates needs to be viewed in the context of risk in chemical evaluations.

Comparison of a bioremediation process of PAHs in a PAH-contaminated soil at field and laboratory scales

A laboratory experiment was carried on the same initial soil and at the same time than a windrow treatment in order to compare results at field and laboratory scales for a soil mainly contaminated with PAHs. After 6 months, laboratory experiments gave similar but less scattered results than those obtained in the field indicating that the field biotreatment was well optimised. The total amount of PAHs degraded after 6 months was ca. 90% and degradation rates followed a negative exponential trend. Relative degradation rates of 3- and 4-ring PAHs were about 32 and 7.2 times greater than those of 5- and 6-ring PAHs, respectively. With respect to the bacterial community, bacteria belonging to Gamma-proteobacteria persisted whereas Beta-proteobacteria appeared after three months of biotreatment when PAH concentration was low enough to render the soil non-ecotoxic.

The electronic properties of trimethylnaphthalenes as properties for the prediction of biodegradation rates: ab initio and DFT study

There is little information on trimethylnaphthalenes (TMNs) which are constituents of diesel fuel and bitumen emissions. In this study, a theoretical investigation of the electronic properties of all trimethylnaphthalene (TMN) isomers and their relation to biodegradation are presented. Equilibrium geometries, ionization potentials (IPs), electron affinities (EAs), dipole moments and electronic dipole polarizabilities of TMN isomers calculated by ab initio and Density Functional Theory (DFT) methods are reported. Polarizability and dipole moment computations have been performed in gas and in water solution using the polarizable continuum model (PCM). The results obtained show that the IP value varies little along the series of isomers while averaged static dipole polarizabilities (<α>) increase on passing from α,α,α-TMN to β,β,β-TMN isomers. This indicates that the binding affinity between TMNs and active site of bacterial enzymes is mainly determined by dispersive and inductive effects. Therefore, the computed polarizability values of TMNs can be used as predictors of the rates of biodegradation of TMNs.

Dynamic passive dosing for studying the biotransformation of hydrophobic organic chemicals: microbial degradation as an example

Biotransformation plays a key role in hydrophobic organic compound (HOC) fate, and understanding kinetics as a function of (bio)availability is critical for elucidating persistence, accumulation, and toxicity. Biotransformation mainly occurs in an aqueous environment, posing technical challenges for producing kinetic data because of low HOC solubilities and sorptive losses. To overcome these, a new experimental approach based on passive dosing is presented. This avoids using cosolvent for introducing the HOC substrate, buffers substrate depletion so biotransformation is measured within a narrow and defined dissolved concentration range, and enables high compound turnover even at low concentrations to simplify end point measurement. As a case study, the biodegradation kinetics of two model HOCs by the bacterium Sphingomonas paucimobilis EPA505 were measured at defined dissolved concentrations ranging over 4 orders of magnitude, from 0.017 to 658 μg L(-1) for phenanthrene and from 0.006 to 90.0 μg L(-1) for fluoranthene. Both compounds had similar mineralization fluxes, and these increased by 2 orders of magnitude with increasing dissolved concentrations. First-order mineralization rate constants were also similar for both PAHs, but decreased by around 2 orders of magnitude with increasing dissolved concentrations. Dynamic passive dosing is a useful tool for measuring biotransformation kinetics at realistically low and defined dissolved HOC concentrations.

The EmhABC efflux pump decreases the efficiency of phenanthrene biodegradation by Pseudomonas fluorescens strain LP6a

Pseudomonas fluorescens strain LP6a, designated here as strain WEN (wild-type PAH catabolism, efflux positive), utilizes the polycyclic aromatic hydrocarbon phenanthrene as a carbon source but also extrudes it into the extracellular medium using the efflux pump EmhABC. Because phenanthrene is considered a nontoxic carbon source for P. fluorescens WEP, its energy-dependent efflux seems counter-productive. We hypothesized that the efflux of phenanthrene would decrease the efficiency of its biodegradation. Indeed, an emhB disruptant strain, wild-type PAH catabolism, efflux negative (WEN), biodegraded 44% more phenanthrene than its parent strain WEP during a 6-day incubation. To determine whether efflux affected the degree of oxidation of phenanthrene, we quantified the conversion of ¹⁴C-phenanthrene to radiolabeled polar metabolites and ¹⁴CO₂. The emhB⁻ WEN strain produced approximately twice as much ¹⁴CO₂ and radiolabeled water-soluble metabolites as the WEP strain. In contrast, the mineralization of ¹⁴C-glucose, which is not a known EmhB efflux substrate, was equivalent in both strains. An early open-ring metabolite of phenanthrene, trans-4-(1-hydroxynaphth-2-yl)-2-oxo-3-butenoic acid, also was found to be a substrate of the EmhABC pump and accumulated in the supernatant of WEP but not WEN cultures. The analogous open-ring metabolite of dibenzothiophene, a heterocyclic analog of phenanthrene, was extruded by EmhABC plus a putative alternative efflux pump, whereas the end product 3-hydroxy-2-formylbenzothiophene was not actively extruded from either WEP or WEN cells. These results indicate that the active efflux of phenanthrene and its early metabolite(s) decreases the efficiency of phenanthrene degradation by the WEP strain. This activity has implications for the bioremediation and biocatalytic transformation of polycyclic aromatic hydrocarbons and heterocycles.