Degradation of polycyclic aromatic hydrocarbons by an immobilized mixed bacterial culture

The mixed bacterial culture MK1 was capable of degrading a wide spectrum of aromatic compounds both as free and as immobilized cells. By offering anthracene oil or a defined mixture of phenol, naphthalene, phenanthrene, anthracene and pyrene (in concentrations of 0.1–0.2 mm, respectively) as sources of carbon and energy, a specific degradation pattern correlating with the condensation degree was observed. Regarding the defined mixture of aromatic hydrocarbons, complete metabolism was reached for phenol (0.1 mm) after 1 day, for naphthalene (0.1 mm) after 2 days and for phenanthrene (0.1 mm) after 15 days of cultivation. The conversion of anthracene (0.1 mm) and pyrene (0.1 mm) resulted in minimal residual concentrations, analogous to fluoranthene and pyrene of the anthracene oil (0.1%). Maximal total degradation for the tricyclic compounds dibenzofurane, fluorene, dibenzothiophene, phenanthrene and anthracene of the anthracene oil (0.1%) occurred after 5 days. In general, a significant metabolisation of the tetracyclic aromatic hydrocarbons fluoranthene and pyrene was observed after the degradation of phenol, naphthalene and most of the tricyclic compounds. Doubling the start concentrations of the polycyclic aromatic hydrocarbons effected higher degradation rates. Cell growth occurred simultaneously with the conversion of phenol, naphthalene and the tricyclic compounds. The immobilized cells showed stable growth and, compared to freely suspended cells, the same degradation sequence as well as an equivalent degradation potential — even in a model soil system. Correspondence to: I. Wiesel     

Dynamics of microbial community during bioremediation of phenanthrene and chromium(VI)-contaminated soil microcosms

The combined effect of phenanthrene and Cr(VI) on soil microbial activity, community composition and on the efficiency of bioremediation processes has been studied. Biometer flask systems and soil microcosm systems contaminated with 2,000 mg of phenanthrene per kg of dry soil and different Cr(VI) concentrations were investigated. Temperature, soil moisture and oxygen availability were controlled to support bioremediation. Cr(VI) inhibited the phenanthrene mineralization (CO₂ production) and cultivable PAH degrading bacteria at levels of 500–2,600 mg kg⁻¹. In the bioremediation experiments in soil microcosms the degradation of phenanthrene, the dehydrogenase activity and the increase in PAH degrading bacteria counts were retarded by the presence of Cr(VI) at all studied concentrations (25, 50 and 100 mg kg⁻¹). These negative effects did not show a correlation with Cr(VI) concentration. Whereas the presence of Cr(VI) had a negative effect on the phenanthrene elimination rate, co-contamination with phenanthrene reduced the residual Cr(VI) concentration in the water exchangeable Cr(VI) fraction (WEF) in comparison with the soil microcosm contaminated only with Cr(VI). Clear differences were found between the denaturing gradient gel electrophoresis (DGGE) patterns of each soil microcosm, showing that the presence of different Cr(VI) concentrations did modulate the community response to phenanthrene and caused perdurable changes in the structure of the microbial soil community.     

Degradation of High Molecular Weight PAHs in Contaminated Soil by a Bacterial Consortium: Effects on Microtox and Mutagenicity Bioassays

Bioaugmentation of polycyclic aromatic hydrocarbon (PAH)-contaminated soil was investigated using a mixed bacterial culture (community five) isolated from an abandoned industrial site. Community five was inoculated into contaminated soil containing a total PAH (two- to five-ring compounds) concentration of approximately 820 mg/kg soil. PAH degradation by the indigenous microbial population was restricted to the lower molecular weight compounds (naphthalene, acenaphthene, fluorene and phenanthrene) even with yeast extract addition: these compounds decreased by 14 to 37%, in soil hydrated to 50% water capacity, following 91 days of incubation at 24°C. Inoculation of community five into this PAH-contaminated soil resulted in significant decreases in the concentration of all PAHs over the incubation period: greater than 86% of naphthalene, acenaphthene, fluorene, and phenanthrene were degraded after 91 days, while anthracene, fluoranthene, and pyrene were degraded to lesser extents (51.7 to 57.6%). A lag period of 48 to 63 days was observed before the onset of benz[a]anthracene, benzo[a]pyrene, and dibenz[a,h]anthracene removal. However, significant decreases in the concentration of these compounds (32.6, 25.2, and 18.5%, respectively) were observed after 91 days. No significant decrease in the mutagenic potential of organic soil extracts (as measured by the Ames Test) was observed after incubation of the soil with the indigenous microflora; however, the Microtox toxicity of aqueous soil extracts was reduced sevenfold. In contrast, extracts from contaminated soil inoculated with community five underwent a 43% decrease in mutagenic potential and the toxicity was reduced 170-fold after 91 days incubation. These observations suggest that community five could be utilised for the detoxification of PAH-contaminated soil.     

Bacterial Diversity of a Consortium Degrading High-Molecular-Weight Polycyclic Aromatic Hydrocarbons in a Two-Liquid Phase Biosystem

High-molecular-weight (HMW) polycyclic aromatic hydrocarbons (PAHs) are pollutants that persist in the environment due to their low solubility in water and their sequestration by soil and sediments. Although several PAH-degrading bacterial species have been isolated, it is not expected that a single isolate would exhibit the ability to degrade completely all PAHs. A consortium composed of different microorganisms can better achieve this. Two-liquid phase (TLP) culture systems have been developed to increase the bioavailability of poorly soluble substrates for uptake and biodegradation by microorganisms. By combining a silicone oil–water TLP system with a microbial consortium capable of degrading HMW PAHs, we previously developed a highly efficient PAH-degrading system. In this report, we characterized the bacterial diversity of the consortium with a combination of culture-dependent and culture-independent methods. Polymerase chain reaction (PCR) of part of the 16S ribosomal RNA gene (rDNA) sequences combined with denaturing gradient gel electrophoresis was used to monitor the bacterial population changes during PAH degradation of the consortium when pyrene, chrysene, and benzo[a]pyrene were provided together or separately in the TLP cultures. No substantial changes in bacterial profiles occurred during biodegradation of pyrene and chrysene in these cultures. However, the addition of the low-molecular-weight PAHs phenanthrene or naphthalene in the system favored one bacterial species related to Sphingobium yanoikuyae. Eleven bacterial strains were isolated from the consortium but, interestingly, only one—IAFILS9 affiliated to Novosphingobium pentaromativorans—was capable of growing on pyrene and chrysene as sole source of carbon. A 16S rDNA library was derived from the consortium to identify noncultured bacteria. Among 86 clones screened, 20 were affiliated to different bacterial species–genera. Only three strains were represented in the screened clones. Eighty-five percent of clones and strains were affiliated to Alphaproteobacteria and Betaproteobacteria; among them, several were affiliated to bacterial species known for their PAH degradation activities such as those belonging to the Sphingomonadaceae. Finally, three genes involved in the degradation of aromatic molecules were detected in the consortium and two in IAFILS9. This study provides information on the bacterial composition of a HWM PAH-degrading consortium and its dynamics in a TLP biosystem during PAH degradation.     

Characterization of a Polycyclic Aromatic Hydrocarbon-Degrading Microbial Consortium from a Petrochemical Sludge Landfarming Site

Anthracene, phenanthrene, and pyrene are polycyclic aromatic hydrocarbon (PAHs) that display both mutagenic and carcinogenic properties. They are recalcitrant to microbial degradation in soil and water due to their complex molecular structure and low solubility in water. This study presents the characterization of an efficient PAH (anthracene, phenanthrene, and pyrene)-degrading microbial consortium, isolated from a petrochemical sludge landfarming site. Soil samples collected at the landfarming area were used as inoculum in Warburg flasks containing soil spiked with 250 mg kg^-1 of anthracene. The soil sample with the highest production of CO₂-C in 176 days was used in liquid mineral medium for further enrichment of anthracene degraders. The microbial consortium degraded 48%, 67%, and 22% of the anthracene, phenanthrene, and pyrene in the mineral medium, respectively, after 30 days of incubation. Six bacteria, identified by 16S rRNA sequencing as Mycobacterium fortuitum, Bacillus cereus, Microbacterium sp., Gordonia polyisoprenivorans, two Microbacteriaceae bacteria, and a fungus identified as Fusarium oxysporum were isolated from the enrichment culture. The consortium and its monoculture isolates utilized a variety of hydrocarbons including PAHs (pyrene, anthracene, phenanthrene, and naftalene), monoaromatics hydrocarbons (benzene, ethylbenzene, toluene, and xylene), aliphatic hydrocarbons (1-decene, 1-octene, and hexane), hydrocarbon mixtures (gasoline and diesel oil), intermediary metabolites of PAHs degradation (catechol, gentisic acid, salicylic acid, and dihydroxybenzoic acid) and ethanol for growth. Biosurfactant production by the isolates was assessed by an emulsification index and reduction of the surface tension in the mineral medium. Significant emulsification was observed with the isolates, indicating production of high-molecular-weigh surfactants. The high PAH degradation rates, the wide spectrum of hydrocarbons utilization, and emulsification capacities of the microbial consortium and its member microbes indicate that they can be used for biotreatment and bioaugumentation of soils contaminated with PAHs.     

Pilot Scale Bioremediation of Creosote-Contaminated Soil—Efficacy of Enhanced Natural Attenuation and Bioaugmentation Strategies

In this study, the efficacy of bioremediation strategies (enhanced natural attenuation with nitrate and phosphate addition [ENA] and bioaugmentation) for the remediation of creosote-contaminated soil (7767 ± 1286 mg kg^?1 of the 16 EPA priority PAHs) was investigated at pilot scale. Bioaugmentation of creosote-contaminated soil with freshly grown or freeze dried Mycobacterium sp. strain 1B (a PAH degrading microorganism) was applied following bench scale studies that indicated that the indigenous soil microflora had a limited PAH metabolic activity. After 182 days, the total PAH concentration in creosote-contaminated soil was reduced from 7767 ± 1286 mg kg^?1 to 5579 ± 321 mg kg^?1, 2250 ± 71 mg kg^?1, 2050 ± 354 mg kg^?1 and 1950 ± 70 mg kg^?1 in natural attenuation (no additions) and ENA biopiles and biopiles augmented with freshly grown or freeze dried Mycobacterium sp. strain 1B respectively. In ENA and bioaugmentation biopiles, between 82% and 99% of three-ring compounds (acenaphthene, anthracene, fluorene, phenanthrene) were removed while four-ring PAH removal ranged from 33 to 81%. However, the extent of PAH degradation did not vary significantly between the ENA treatment and biopiles augmented with Mycobacterium sp. strain 1B. Four-ring PAH removal followed the order fluoranthene > pyrene > benz[a]anthracene > chrysene. The high residual concentration of some four-ring PAHs may be attributable to bioavailability issues rather than a lack of microbial catabolic activity. Comparable results between ENA and bioaugmentation at pilot scale were surprising given the limited degradative capacity of the microbial consortia enriched from the creosote-contaminated soil.     

Containment of a genetically engineered microorganism during a field bioremediation application

A field release of a genetically engineered microorganism was performed at the Field Lysimeter Site on the Oak Ridge Reservation. Six large lysimeters were filled with soil that had been contaminated with a mixture of naphthalene, phenanthrene, and anthracene. A genetically engineered bacterial strain, Pseudomonas fluorescens HK44, was sprayed onto the surface of the soil during soil loading. This strain contains a fusion between the lux genes of Vibrio fischeri and the promoter for the lower pathway of naphthalene degradation, enabling the strain to become bioluminescent when it is degrading naphthalene. Release of the bacteria outside the lysimeters was monitored, using selective agar plates and one-stage Anderson air samplers. Although approximately 10¹⁴ bacteria were sprayed during the loading process, escape was only detected sporadically; the highest incidence of bacterial escape was found when the relative humidity and wind speed were low. Received: 6 March 1998 /thinsp;Received revision: 16 September 1998 / Accepted: 16 October 1998     

Degradation of polycyclic aromatic hydrocarbons in soil by a two-step sequential treatment

The objectives of this work were to isolate the microorganisms responsible for a previously observed degradation of polycyclic aromatic hydrocarbons (PAH) in soil and to test a method for cleaning a PAH-contaminated soil. An efficient PAH degrader was isolated from an agricultural soil and designated as Mycobacterium LP1. In liquid culture, it degraded phenanthrene (58%), pyrene (24%), anthracene (21%) and benzo(a)pyrene (10%) present in mixture (initial concentration 50 μg ml⁻¹ each) and phenanthrene (92%) and pyrene (94%) as sole carbon sources after 14 days of incubation at 30°C. In soil, Mycobacterium LP1 mineralised ¹⁴C-phenanthrene (45%) and ¹⁴C-pyrene (65%) after 10 days. The good ability of this Mycobacterium was combined with the benzo(a)pyrene oxidation effect obtained by 1% w/w rapeseed oil in a sequential treatment of a PAH-spiked soil (total PAH concentration 200 mg kg⁻¹). The first step was incubation with the bacterium for 12 days and the second step was the addition of the rapeseed oil after this time and a further incubation of 22 days. Phenanthrene (99%), pyrene (95%) and anthracene (99%) were mainly degraded in the first 12 days and a total of 85% of benzo(a)pyrene was transformed during the whole process. The feasibility of the method is discussed.     

Effects of soil amendment with different carbon sources and other factors on the bioremediation of an aged PAH-contaminated soil

Carbon supplementation, soil moisture and soil aeration are believed to enhance in situ bioremediation of PAH-contaminated soils by stimulating the growth of indigenous microorganisms. However, the effects of added carbon and nitrogen together with soil moisture and soil aeration on the dissipation of PAHs and on associated microbial counts have yet to be fully assessed. In this study the effects on bioremediation of carbon source, carbon-to-nitrogen ratio, soil moisture and aeration on an aged PAH-contaminated agricultural soil were studied in microcosms over a 90-day period. Additions of starch, glucose and sodium succinate increased soil bacterial and fungal counts and accelerated the dissipation of phenanthrene and benzo(a)pyrene in soil. Decreases in phenanthrene and benzo(a)pyrene concentrations were effective in soil supplemented with glucose and sodium succinate (both 0.2 g C kg⁻¹ dry soil) and starch (1.0 g C kg⁻¹ dry soil). The bioremediation effect at a C/N ratio of 10:1 was significantly higher (P < 0.05) than at a C/N of either 25:1 or 40:1. Soil microbial counts and PAH dissipation were lower in the submerged soil but soil aeration increased bacterial and fungal counts, enhanced indigenous microbial metabolic activities, and accelerated the natural degradation of phenanthrene and benzo(a)pyrene. The results suggest that optimizing carbon source, C/N ratio, soil moisture and aeration conditions may be a feasible remediation strategy in certain PAH contaminated soils with large active microbial populations.     

REDUCTION IN SOIL POLYCYCLIC AROMATIC HYDROCARBONS BY ARBUSCULAR MYCORRHIZAL LEEK PLANTS

The effect of arbuscular mycorrhizal fungi (AMF) on the reduction of soil polycyclic aromatic hydrocarbon (PAH), nutrient uptake, and growth of leek (Allium porrum L. cv. Musselburgh) plants was studied under greenhouse conditions. This experiment was a 3 × 2 × 2 × 4 factorial design including three mycorrhizal treatments (non-AMF, Glomus intraradices, and G. versiforme strains), two microorganism statuses (with and without soil bacteria), two PAH chemicals (anthracene and phenanthrene), and four PAH concentrations (three concentrations added and one control). Leek growth was reduced significantly in soils spiked with anthracene or phenanthrene. Inoculation with either Glomus intraradices or G. versiforme not only increased N and P uptake and plant growth, but also enhanced PAH disappearance in soil. After 12 weeks of potcultures, the anthracene and phenanthrene concentrations in soils were decreased as compared to their initial level, 9%–31% versus 43%–88%, respectively. Reductions in concentration were larger for phenanthrene than anthracene. The addition of a soil microorganism (SM) extract in potcultures accelerated the disappearance of PAHs. The decrease of PAHs in soil was mainly attributed to the enhanced nutrient uptake by AMF, leading to improved plant growth, which, in turn, may stimulate soil microbial activity. This study shows the interrelationships between AMF, plants, other SMs, and PAH disappearance in soil. The phytoremediation of soil contaminated with PAHs can be accelerated through inoculation with AMF and other SMs.     

Microbial consortium bioaugmentation of a polycyclic aromatic hydrocarbons contaminated soil

In this study we evaluated the capacity of a defined microbial consortium (five bacteria: Mycobacterium fortuitum, Bacillus cereus, Microbacterium sp., Gordonia polyisoprenivorans, Microbacteriaceae bacterium, Naphthalene-utilizing bacterium; and a fungus identified as Fusarium oxysporum) isolated from a PAHs contaminated landfarm site to degrade and mineralize different concentrations (0, 250, 500 and 1000 mg kg(-1)) of anthracene, phenanthrene and pyrene in soil. PAHs degradation and mineralization was evaluated by gas chromatography and respirometry, respectively. The microbial consortium degraded on average, 99%, 99% and 96% of the different concentrations of anthracene, phenanthrene and pyrene in the soil, in 70 days, respectively. This consortium mineralized 78%, on average, of the different concentrations of the 3 PAHs in soil after 70 days. Contrarily, the autochthonous soil microbial population showed no substantial mineralization of the PAHs. Bacterial and fungal isolates from the consortium, when inoculated separately to the soil, were less effective in anthracene mineralization compared to the consortium. This signifies synergistic promotion of PAHs mineralization by mixtures of the monoculture isolates (the microbial consortium).     

Phenanthrene toxicity and dissipation in rhizosphere of grassland plants (Lolium perenne L. and Trifolium pratense L.) in three spiked soils

Rhizodegradation is a technique involving plants that offers interesting potential to enhance biodegradation of persistent organic pollutants such as polycyclic aromatic hydrocarbons (PAHs). Nevertheless, the behaviour of PAHs in plant rhizosphere, including micro-organisms and the physico-chemical soil properties, still needs to be clarified. The present work proposes to study the toxicity and the dissipation of phenanthrene in three artificially contaminated soils (1 g kg^-1 DW). Experiments were carried out after 2 months of soil aging. They consisted in using different systems with two plant species (Ryegrass—Lolium perenne L. var. Prana and red clover—Trifolium pratense L. var. fourragère Caillard), three kinds of soils (a silty-clay-loam soil “La Bouzule”, a coarse sandy-loam soil “Chenevières” and a fine sandy-loam soil “Maconcourt”). Phenanthrene was quantified by HPLC in the beginning (T ₀) and the end of the experiments (30 days). Plant biomass, microbial communities including mycorrhizal fungi, Rhizobium and PAH degraders were also recorded. Generally phenanthrene contamination did not affect plant biomass. Only the red clover biomass was enhanced in Chenevières and La Bouzule polluted soils. A stimulation of Rhizobium red clover colonisation was quantified in spiked soils whereas a drastic negative phenanthrene effect on the mycorrhization of ryegrass and red clover was recorded. The number of PAH degraders was stimulated by the presence of phenanthrene in all tested soils. Both in ryegrass and red clover planted soils, the highest phenanthrene dissipation due to the rhizosphere was measured in La Bouzule soils. On the contrary, in non-planted soils, La Bouzule soils had also the lowest pollutant dissipation. Thus, in rhizospheric and non-rhizospheric soils the phenanthrene dissipation was found to depend on soil clay content.     

Biodegradation of polycyclic aromatic hydrocarbons by an acidophilic Stenotrophomonas maltophilia strain AJH1 isolated from a mineral mining site in Saudi Arabia

The present study aims at analyzing the degradation of polycyclic aromatic hydrocarbons (PAHs) at acidic conditions (pH = 2) by acidophilic Stenotrophomonas maltophilia strain AJH1 (KU664513). The strain AJH1 was obtained from an enrichment culture obtained from soil samples of mining area in the presence of PAH as sole sources of carbon and energy. Strain AJH1was able to degrade low (anthracene, phenanthrene, naphthalene, fluorene) and high (pyrene, benzo(e)pyrene and benzo(k)fluoranthene) molecular weight PAHs in acidophilic mineral salt medium at pH 2, with removal rates of up to 95% (LMW PAH) and 80% (HMW PAH), respectively. In addition, strain AJH1 treated petroleum wastewater with 89 ± 1.1% COD removal under acidic condition (pH 2) in a continuously stirred reactor. Acidophilic S. maltophilia strain AJH1, hence holds the promise as an effective degrader for biological treatment of PAHs contaminated wastewater at acidic pH.

     

Enhanced bioremediation of soil contaminated with viscous oil through microbial consortium construction and ultraviolet mutation

This study focused on enhancing the bioremediation of soil contaminated with viscous oil by microorganisms and evaluating two strategies. Construction of microbial consortium and ultraviolet mutation were both effective applications in the remediation of soil contaminated with viscous oil. Results demonstrated that an interaction among the microorganisms existed and affected the biodegradation rate. Strains inoculated equally into the test showed the best remediation, and an optimal microbial consortium was achieved with a 7 days’ degradation rate of 49.22%. On the other hand, the use of ultraviolet mutation increased one strain’s degrading ability from 41.83 to 52.42% in 7 days. Gas chromatography and mass spectrum analysis showed that microbial consortium could treat more organic fractions of viscous oil, while ultraviolet mutation could be more effect on increasing one strain’s degrading ability.     

Effects of co-occurring aromatic hydrocarbons on degradation of individual polycyclic aromatic hydrocarbons in marine sediment slurries

Rates of polycyclic aromatic hydrocarbon (PAH) degradation and mineralization were influenced by preexposure to alternate PAHs and a monoaromatic hydrocarbon at relatively high (100 ppm) concentrations in organic-rich aerobic marine sediments. Prior exposure to three PAHs and benzene resulted in enhanced [14C]naphthalene mineralization, while [14C]anthracene mineralization was stimulated only by benzene and anthracene preexposure. Preexposure of sediment slurries to phenanthrene stimulated the initial degradation of anthracene. Prior exposure to naphthalene stimulated the initial degradation of phenanthrene but had no effect on either the initial degradation or mineralization of anthracene. For those compounds which stimulated [14C]anthracene or [14C]naphthalene mineralization, longer preexposures (2 weeks) to alternative aromatic hydrocarbons resulted in an even greater stimulation response. Enrichment with individual PAHs followed by subsequent incubation with one or two PAHs showed no alteration in degradation patterns due to the simultaneous presence of PAHs. The evidence suggests that exposure of marine sediments to a particular PAH or benzene results in the enhanced ability of these sediments to subsequently degrade that PAH as well as certain other PAHs. The enhanced degradation of a particular PAH after sediments have been exposed to it may result from the selection and proliferation of specific microbial populations capable of degrading it. The enhanced degradation of other PAHs after exposure to a single PAH suggests that the populations selected have either broad specificity for PAHs, common pathways of PAH degradation, or both.     

Microbiological characterization of a coke oven contaminated site and evaluation of its potential for bioremediation

The soil microbial population of a coke oven site was investigated in order to evaluate its potential for bioremediation. The study was carried out in soil samples with distinct polynuclear aromatic hydrocarbon (PAH) contamination levels, comparing the population profiles constituted by total heterotrophic and PAH-utilizing strains. Isolation of degrading strains was performed with phenanthrene or pyrene as sole carbon sources. The ability to degrade other PAHs, such as anthracene, fluorene and fluoranthene was also investigated. The results showed a reduction of 30% in species diversity and microbial density drops one order of magnitude in contaminated samples. Furthermore, the number of PAH-utilizing colonies was higher in the contaminated area and about 20% of the isolates were able to degrade phenanthrene and pyrene, while this value decreased to 0.15% in uncontaminated samples. Three PAH-degrader strains were identified as: CDC gr. IV C-2, Aeromonas sp. and Pseudomonas vesicularis. The ability of these strains to degrade other PAHs was also investigated.     

Effects of co-occurring aromatic hydrocarbons on degradation of individual polycyclic aromatic hydrocarbons in marine sediment slurries.

Rates of polycyclic aromatic hydrocarbon (PAH) degradation and mineralization were influenced by preexposure to alternate PAHs and a monoaromatic hydrocarbon at relatively high (100 ppm) concentrations in organic-rich aerobic marine sediments. Prior exposure to three PAHs and benzene resulted in enhanced [14C]naphthalene mineralization, while [14C]anthracene mineralization was stimulated only by benzene and anthracene preexposure. Preexposure of sediment slurries to phenanthrene stimulated the initial degradation of anthracene. Prior exposure to naphthalene stimulated the initial degradation of phenanthrene but had no effect on either the initial degradation or mineralization of anthracene. For those compounds which stimulated [14C]anthracene or [14C]naphthalene mineralization, longer preexposures (2 weeks) to alternative aromatic hydrocarbons resulted in an even greater stimulation response. Enrichment with individual PAHs followed by subsequent incubation with one or two PAHs showed no alteration in degradation patterns due to the simultaneous presence of PAHs. The evidence suggests that exposure of marine sediments to a particular PAH or benzene results in the enhanced ability of these sediments to subsequently degrade that PAH as well as certain other PAHs. The enhanced degradation of a particular PAH after sediments have been exposed to it may result from the selection and proliferation of specific microbial populations capable of degrading it. The enhanced degradation of other PAHs after exposure to a single PAH suggests that the populations selected have either broad specificity for PAHs, common pathways of PAH degradation, or both.     

Kinetics of Polycyclic Aromatic Hydrocarbon (PAH) Degradation in Long-term Polluted Soils during Bioremediation

Bioremediation experiments with ten different soil samples from former industrial sites which were long-term polluted with polycyclic aromatic hydrocarbons (PAHs) were carried out using outdoor pot trials. The degradation of 15 PAHs according to the US EPA was investigated for 168 weeks through repeated soil sampling and determination of the total PAH concentration. On average, degradation was largest for acenaphthene (88%; 63 to 99%) and smallest for anthracene (22%; no significant degradation to 89%). For most of the PAH single substances, degradation kinetics were characterised by a first initial phase of fast degradation. In a subsequent second phase, degradation diminished and residual PAH concentrations were approached within 168 weeks, resulting in a similar PAH pattern in the ten soil samples. Degradation kinetics was calculated through the selection of the appropriate differential rate equation from a set of seven equations. Kinetics of PAH degradation was best fitted by single and two coupled first order exponential equations with median R2 of 0.71 (0.01 to 1.00). Degradation rate constants of the rapid phase (k ₁) ranged from 0.05×10⁻² week⁻¹ for benzo[k]fluoranthene to 18.3 week⁻¹ for naphthalene and for the subsequent slow degradation phase (k ₂) they ranged from 0.01×10⁻² week⁻¹ for benzo[a]anthracene to 2.3×10⁻² week⁻¹ for fluoranthene. Degradation was governed by desorption and diffusion processes of different rates, while microbial activity did not influence the kinetics. Median disappearance times (DT₅₀) ranged from 6.1 weeks for naphthalene to 522 weeks for benzo[k]fluoranthene. With the exception of the 6-ring PAHs dibenzo[ah]anthracene and indeno[1,2,3-cd]pyrene, this sequence followed the PAHs’ degree of condensation. The total initial PAH concentration and the residual concentration were correlated with R2 of 0.69, with larger initial PAH concentrations leading to larger residual concentrations and degradation rates.     

Biodegradation of Phenanthrene by Pseudomonas Bacteria Bearing Rhizospheric Plasmids in Model Plant–Microbial Associations

The consumption of phenanthrene in soil by model plant–microbial associations including natural and transconjugant plasmid-bearing rhizospheric strains of Pseudomonas fluorescens and P. aureofaciens degrading polycyclic aromatic hydrocarbons was studied. It was shown that phytoremediation of soil polluted with phenanthrene in the rhizosphere of barley (Hordeum sativum L.) was inefficient in the absence of the degrading strains. Inoculation of barley seeds with both natural and transconjugant plasmid-bearing Pseudomonas strains able to degrade polycyclic aromatic hydrocarbons (PAH) protected plants from the phytotoxic action of phenanthrene and favored its degradation in soil. Rape (Brassica napus L.) was shown to be an appropriate sentinel plant, sensitive to phenanthrene, which can be used for testing the efficiency of phenanthrene degradation in soil. Biological testing with the use of sensitive rape plants can be applied for estimation of the efficiency of phyto/bioremediation of PAH-polluted soils.     

Activation of an indigenous microbial consortium for bioaugmentation of pentachlorophenol/creosate contaminated soils

Soil activation, a concept based on the cultivation of biomass from a fraction of a comtaminated soil for subsequent use as an inoculum for bioaugmentation of the same soil, was studied as a method for the aerobic biodegradation of pentachlorophenol (PCP) and polycyclic hydrocarbons (PAH) in contaminated soils. A microbial consortium able to degrade PCP and PAH in contaminated soil from wood-preserving facilities was isolated and characterized for PCP degradation and resistance. To obtain an active consortium from the contaminated soil in a fed-batch bioreactor, the presence of soil as a support or source of nutrients was found to be essential. During the 35 days of bioreactor operation, residual PCP in solution remained near zero up to a loading rate of 700mg/l per day. The PCP meneralization rate increased from 70 mg/l per day when no PCP was added to the bioreactor to 700 mg/l per day at the maximum loading rate. The consortium tolerated a PCP concentration of 400 mg/l in batch experiments. Production of a PCP-degrading consortium in a fed-batch slurry bioreactor enhanced the activity of PCP biodegradation by a factor of ten. PAH biodegradation increased, during the same time period, by a factor of 30 and 81 for phenanthrene and pyrene, respectively. Preliminary laboratory-scale results indicated that a significant reduction in the time required for degradation of PCP and PAH in contaminated soil could be achieved using activated soil as an inoculum.Issued as NRC 33861 correspondence to: R. Samson