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XENOMIC Résumé de rapport

Project ID: 500956
Financé au titre de: FP6-MOBILITY
Pays: Denmark

Final Activity Report Summary - XENOMIC (Characterisation of Microbial Communities degrading Xenobiotics)

Man-made chemicals of organic origin (Xenobiotics) are a persistent problem in Europe, partly from legacy industry, and from continuing human activity. These include emulsifying agents such as nonylphenols and nonylphenol ethoxylates (NPE), polycyclic aromatic hydrocarbons (PAHs) derived from incomplete combustion processes, and phthalates, which are used as additives in plastics. These chemicals can have negative impacts both on the quality of human life and on the natural environment. The aim of this study was to characterise the microorganisms capable to degrade xenobiotics.

Because of the high difficulty to analyse xenobiotic compounds in aqueous samples, new analytical procedures were implemented and optimised to accurately measure trace level concentrations of PAEs/PAHs/NPs in sludge. Environmental samples from various origins of complex anaerobic habitats were screened for their ability to degrade xenobiotics (lab-scale reactors, sludge, soils, household wastes). Only anaerobic sludge showed significant ability to degrade xenobiotics. High microbial diversity was observed in the degrading ecosystems, and no xenobiotic degraders were clearly identified. It was therefore attempted to enrich the microbial populations using advanced microbial cultivation techniques. Experiments of enrichment in lab-scale reactors were specifically designed and, for the first time, several anaerobic microbial communities degrading xenobiotics were clearly identified.

Specific microbial populations were able to degrade significantly xenobiotic compounds under anaerobic conditions in a range of concentrations from 10 to 500 mg/L. In addition, the enrichment techniques were compared and continuous conditions showed the best performances for the selection of highly specific microorganisms. All enriched cultures were fully characterised, and molecular tools were developed for further monitoring of the dominant degraders. The isolation of pure degrading microorganisms was then attempted. While four pure bacteria were isolated on PAEs, co-cultures of two to three partners were obtained for NP and PAH compounds, suggesting the presence of symbiotic bacteria. The isolates were characterised phylogenetically, physiologically and phenologically. DNA probes were specifically developed to target these microorganisms.

Furthermore, metabolic pathways were partly identified by the detection of specific intermediates, such as phthalic acids, carboxylated PAH and nonylphenol for PAEs, PAHs and NPs, respectively. Kinetic models as well as thermodynamics assumption were also developed for better assumption of the behaviour of these compounds in anaerobic environments. At last, bioaugmentation tests were carried out in batch reactors. Better bioaugmentation performances were obtained with single bacteria (PAE isolates) than with co-cultures. Indeed, bioaugmentation of non-degrading sludge by the PAE isolated bacteria showed a significant enhancement of the PAE degradability, while a slight and no significant enhancement of NP/PAH removal was observed after addition of the isolated co-cultures.

Similar results were obtained in continuous reactors simulating full-scale anaerobic sludge digesters. It was shown that an active process of cell lysis occurred in the host reactor, such as cell predation. Various experimental strategies to protect the isolates throughout the bioaugmentation procedure were attempted, but no significant enhancement of the removal was obtained. The enrichment techniques, the use of the isolated cultures in bioaugmented reactors, as well as the molecular tools described in this work have been patented for further use in full-scale plants.


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