B. Mahro

University of Hamburg, Hamburg, Hamburg, Germany

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Publications (15)21.66 Total impact

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    ABSTRACT: Stable carbon isotopic measurements were employed to characterize the transformation of a 13C-labeled polycyclic aromatic hydrocarbon (PAH), anthracene, in a closed soil bioreactor system. The 13C-label was used to calculate a carbon mass balance including mineralization and the formation of nonextractable soil-bound residues. Similar results were obtained from 13C-labeled carbon and 14C-labeled carbon mass balance calculations for separate batch experiments with labeled anthracene. In concentration ranges typical for real PAH-contaminated sites, the sensitivity of the 13C tracer method meets the requirements of classical radiotracer experiments. Therefore, our balancing method based on stable isotope-labeled chemicals may supplement or substitute radiotracer experiments under many circumstances. One major advantage of using stable isotope-labeled tracers is the possible application in transformation studies where the use of radioactive substances is of environmental concern. The transformation of 13C-labeled PAH into nonextractable residues clearly depends on the metabolic activity of the soil microflora and occurs during an early phase of biodegradation. Successive contamination of the soil by anthracene leads to a progressive adaptation of the microflora to a complete mineralization of anthracene in the soil. The extent of residue formation is controlled by the capability of the microflora to degrade the contaminant. Results of long-term experiments indicate that nonextractable residues are relatively stable over time.
    Environmental Science & Technology - ENVIRON SCI TECHNOL. 09/1999; 33(21).
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    ABSTRACT: It has been recognized during recent years that polycyclic aromatic hydrocarbons (PAH) may form nonextractable residues in soil and that this process may be stimulated by microbial activities. To use that process intentionally for soil bioremediation, one must ensure that the formed nonextractable PAH residues will not be released from the soil on the long run. The long-term stability of four different nonextractable [14C]PAH residues ([14C]naphthalene, [14C]anthracene, [14C]pyrene, and [14C]benzo[a]pyrene) was therefore monitored under different ecological stress conditions. It was found that a considerable fraction of the total [14C]PAH residues could be released as 14CO2 from the soil being partly due to a biogenic reduction of the nonextractable 14C residue fraction. The turnover of this fraction was comparable to the natural turnover rate for humic substances. Neither the addition of humus-degrading microorganisms nor a mechanical stress treatment of the soil structure by freezing and thawing led to a mobilization of the nonextractable [14C]PAH residues. However, a significant mobilization of the nonextractable 14C activity occurred when EDTA was added to the soil. The metal−organic soil complexes were destabilized by this complexing agent and released 14C activity that was attached to colloidal or dissolved organic matter.
    Environmental Science & Technology - ENVIRON SCI TECHNOL. 07/1998; 32(17).
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    ABSTRACT: The formation of non-extractable residues during biodegradation and humification processes in soils and sediments represents a major sink for organic contaminants. The mode of incorporation of polycyclic aromatic hydrocarbons (PAH) and their metabolites into macromolecular organic matter during microbial degradation was studied applying 13C-labelled compounds. Mineralisation rates were determined by measuring the 13CO2 production. An incorporation of 13C-PAH-fragments into humic material could be traced by isotopic analysis of the bulk organic matter. Furthermore, selective chemical degradation reactions were performed to analyse the precise chemical structure of covalently bound 13C-labelled PAH fragments in soil humic substances. Structural assignments by GC-MS combined with isotope measurements on the bulk organic carbon and at the molecular level (Isotope Ratio Monitoring-GC-MS) provided useful information on the fate of xenobiotics within the soil.
    Chemosphere 05/1998; 36(10):2211-24. · 3.14 Impact Factor
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    ABSTRACT: Degradation of polycyclic aromatic hydrocarbons (PAHs) and survival of bacteria in soil was investigated by applying different inoculation protocols. The soil was inoculated with Sphingomonas paucimobilis BA 2 and strain BP 9, which are able to degrade anthracene and pyrene, respectively. CFU of soil bacteria and of the introduced bacteria were monitored in native and sterilized soil at different pHs. Introduction with mineral medium inhibited PAH degradation by the autochthonous microflora and by the strains tested. After introduction with water (without increase of the pore water salinity), no inhibition of the autochthonous microflora was observed and both strains exhibited PAH degradation.
    Applied and Environmental Microbiology 02/1998; 64(1):359-62. · 3.95 Impact Factor
  • M Kästner, B Mahro
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    ABSTRACT: This paper describes the degradation of naphthalene, phenanthrene, anthracene, fluoranthene, and pyrene in soil and soil/compost mixtures. Compost addition facilitated the degradation of 500 mg naphthalene/kg soil and 100 mg/kg each of other polycyclic aromatic hydrocarbons (PAH) within 25 days in soil systems with water contents below the water-holding capacity. By means of a humic acid extraction, it was demonstrated that the decrease of PAH concentrations after compost addition was not caused by a sorption to organic matter preventing PAH analysis. The enhanced PAH degradation was examined in a series of batch experiments with contaminated soil to evaluate whether the effect of compost addition is caused by the microorganisms of the compost itself, by the properties of the organic matrix of the compost material, or by water-soluble fertilising substances. The experiments revealed that the release of fertilising substances from the compost and the shift of soil pH brought about by the compost did not cause the stimulatory effect. The microorganisms inherent to the compost were also not necessary for the enhanced degradation. Sterilised compost was recolonised by soil microorganisms after a lagphase yielding a degradation activity similar to that of the non-sterilised compost. The presence of the solid organic matrix of the compost seemed to be essential for the enhanced degradation. The soil/compost microflora, which was separated from the organic matrix in liquid cultures, exhibited a much lower degrading activity than in the presence of the solid organic material.
    Applied Microbiology and Biotechnology 02/1996; 44(5):668-75. · 3.81 Impact Factor
  • Abstracts of Papers of the American Chemical Society. 01/1996; 212:151-ENVR.
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    ABSTRACT: Experiments were carried out to evaluate the impact of the addition of ripe compost on the degradation of two 14C-labeled hydrocarbon model compounds (anthracene and hexadecane) in soil. The addition of mature compost (20% dry wt./dry wt) stimulated significantly the disappearance of the extractable fraction of both compounds. With compost, 23% of the labeled anthracene was transformed into 14CO2 and 42% was fixed to the soil matrix irreversibly. In the unsupplemented control reactor, more than 88% of the original anthracene could be recovered by either of two organic extraction procedures. THe formation of non-extractable bound residues was significantly less with 14C-hexadecane since only 21% of the labeled carbon had become non-extractable after 103 days. The results presented show that compost could stimulate the depletion of hydrocarbons by either mineralization or the formation of unextractable bound residues (humification). The latter process might be a significant route of depletion in soil especially, for those hydrocarbons that are mineralized only slowly. The meaning of this finding for the assessment of soil bioremediation is discussed.
    Applied Microbiology and Biotechnology 12/1995; 43(6):1128-35. · 3.81 Impact Factor
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    ABSTRACT: Humic acids isolated from soils contaminated by polycyclic aromatic hydrocarbons (PAH) and from non-contaminated soils were investigated by analytical pyrolysis to examine the abundance and structure of soil bound PAH-residues. PAHs which are not extractable by organic solvents could be liberated from humic substances by analytical pyrolysis. Pyrolysates of humic acids derived from biodegradation experiments with PAH contaminated soils revealed a 2 to 8 fold enrichment of PAHs compared to those from non-polluted soils. This enrichment was also observed in humic substances isolated from contaminated field samples during a remediation experiment. Environmental implications for the formation of soil bound residues during bioremediation are discussed.
    Chemosphere 01/1995; · 3.14 Impact Factor
  • B. Mahro, K. Rode, V. Kasche
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    ABSTRACT: The influence of the precultivation with different carbon sources on the ability of three different bacterial strains (Sphingomonas sp. strain BA2, Gordona sp. strain BP9, Mycobacterium sp. strain VF1) to grow on phenanthrene. anthracene, pyrene or fluoranthene as the sole source of carbon and energy were studied. The strains were found to maintain their ability to grow on two of the four PAH after 30 serial transfers in liquid nutrient broth medium without selective pressure. The ability to grow on these PAH as the sole carbon and energy source was also maintained after curing experiments with acridine orange. The high stability of the PAH-degradation phenotype suggests that the tested strains carry at least parts of the PAH-degradation pathway genes on the chromosome. The PAH-degradation versatility of the strains was also influenced by the carbon source being used for precultivation. Possible reasons for the particularly good impact of the precultivation on hexadecane on the PAH degradation are discussed in this paper.
    Acta Biotechnologica 12/1994; 15(4):337 - 345.
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    ABSTRACT: The use of a plate screening technique allowed the direct isolation and quantification of polycylic aromatic hydrocarbon (PAH)-degrading bacteria from different soil sites. Bacteria that were able to grow on anthracene, phenanthrene, fluoranthene or pyrene as a sole carbon source were found with numbers between 103 and 105 colony-forming units (cfu)/g of soil dry weight, but only in samples that originated from PAH-contaminated sites. No isolates were found that could grow on perylene, triphenylene, benzo(a)pyrene or chrysene as sole carbon source. Bacteria that had been selected on the same PAH substrate showed a related degradation pattern for both other PAH and oil compounds and carbohydrate substrates even if they had been collected at distant soil sites. Based on these findings the isolates could be clustered into four different catabolic and taxonomic similarity groups. Taxonomic determination of representative isolates suggested that nocardioform actinomycetes of the genera Mycobacterium, Rhodococcus and Gordona represented a major part of the soil microflora able to mineralize PAH. Three new isolates able to grow on anthracene, pyrene or fluoranthene as the sole carbon source, respectively, have been isolated and identified (Sphingomonas paucimobilis BA2, Gordona sp. BP9, Mycobacterium sp. VF1). The ubiquitous presence of a potent and versatile mineralizing microflora in PAH-contaminated soils indicated that the microflora is not the limiting factor for the degradation of PAH with up to four rings.
    Applied Microbiology and Biotechnology 03/1994; 41(2):267-273. · 3.81 Impact Factor
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    ABSTRACT: A new and efficient protocol for the extraction of polycyclic aromatic hydrocarbons (PAH) from soil samples is described. The method is based on two successive steps, including an organic solvent extraction and a methanolic hydrolysis (saponification) of the soil sample. By use of the new technique the microbial degradation of PAH in soil batch cultures was monitored. The application of the method helps to recover significant higher amounts of PAH from soil than it was possible with an organic solvent extraction only. An explanation for the observed effect could be that PAH molecules which got trapped in the humic polymer are released due to the cleavage of ester bonds which form parts of humic macromolecules.
    Chemosphere. 01/1994;
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    ABSTRACT: The type of association between pollutants and humic substances of soils, sediments and river waters has been investigated. Metabolites, which can arise from the microbiological degradation of polyaromatic hydrocarbons (PAH) and polychlorinated biphenyls (PCB), were cleaved from the macromolecular matrix by selective chemical degradation techniques (OH−, BCl3, Rh/H2). Hydrolysis reactions performed with Na18OH proved that some metabolites of pollutants form stable ester bonds by condensation processes with functional groups of humic substances, a phenomenon which has major implications for transport, toxicity and bioavailability of xenobiotica.
    Organic Geochemistry. 01/1994;
  • Microbial Releases. 01/1992; 1:79-85.
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    ABSTRACT: The successful cloning of the bioluminescence genes from marine bacteria offers the chance to use the bioluminescence phenotype as a marker for environmental monitoring purposes. This work describes a video camera based technique which allows both the sensitive detection and determination of the spatial distribution of luminescent Streptomyces coelicolor pellets and filaments within liquid and soil samples.
    Acta Biotechnologica 02/1991; 11:541 - 546.
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