| Most biospheric nitroaromatic compounds (NACs) are industrial chemicals, such as explosives, dyes, polyurethane foams, herbicides, insecticides, and solvents, that often are recalcitrant to biological treatments and persist in the environment, constituting a hazard due to their toxicity and mutagenicity. In the present study, we evaluated perspectives for bioremediation and phytoremediation strategies at a nitroaromatic-contaminated site. Our research focussed mainly on 2,4-DNT and TNT. Two consortia, isolated by selective enrichment from a soil sample from a TNT-contaminated site, metabolised 2,4-DNT as their sole nitrogen source without accumulating one or more detectable intermediates. Though originating from the same sample, the optimised consortia had no common members, so that selective enrichment could not be repeated. Consortium 1 had four bacterial species and consortium 3 contained six, but both had two members that could collectively degrade 2,4-DNT. Variovorax paradoxus VM685 (consortium 1) and Pseudomonas sp. VM908 (consortium 3) initiated the catabolism of 2,4-DNT by an oxidation step, thereby releasing nitrite and forming 4-methyl-5-nitrocatechol (MNC). Both strains contained a homologue to the dntAa gene encoding 2,4-DNT dioxygenase. They subsequently metabolised MNC to 2-hydroxy-5-methylquinone (HMQ) and nitrite, indicative of DntB or MNC monooxygenase activity. A second consortium member, Pseudomonas marginalis VM683 (consortium 1) or P. aeruginosa VM903, Sphingomonas sp. VM904, Stenotrophomonas maltophilia VM905 or P. viridiflava VM907 (consortium 3), was indispensable for efficient growth on 2,4-DNT and for the metabolisation of the intermediates MNC and HMQ. We have limited knowledge about the interactions in this degradative step. However, a gene homologous to the dntD gene of Burkholderia sp. strain DNT that catalyses ring fission was demonstrated by DNA-DNA hybridisation in the second member strains. Attempts were unsuccessful at selective enrichment on other nitroaromatic compounds as the sole nitrogen source (i.e., TNT, 2,6-DNT, 2-NT, and 4-NT) with soil and water samples from this industrial site. To select a consortium for further applications, we then compared the ability of each consortium to break down TNT. Consortium 3 performed the best, towards a fast and complete TNT transformation, decrease in genotoxicity and high co-metabolic activity, even when an excessive amount of TNT was added. Therefore, this consortium was chosen for remediations of TNT-contaminated soil and water (Chapter 7, Chapter 8). (Abstract shortened by UMI.)... |
