Microbial interactions for 2,4,6-Trinitrotoluene biotransformation

Abstract

Much of the research in the past on the biodegradation of TNT has been directed towards pure culture studies with very little emphasis on mixed populations. Researchers have observed that, on many occasions, microorganisms capable of existing in environments containing xenobiotic compounds, function as a community. Microbial "teamwork" is required for the complete biodegradation of certain xenobiotics, since the metabolic diversity of a microbial community is greater than that of any single component species of that community (Bartha, 1990). Pure culture approach to biodegradation of xenobiotic and natural recalcitrant products have numerous advantages and disadvantages. Pure culture studies allow a more detailed evaluation of pathways, enzymes, degradation intermediates, and products, coupled with simplicity and reproducibility for bioremediation of xenobiotics. However, mixed populations have a higher resistance to toxic compounds, higher quantitative and qualitative biodegradative capabilities, and enhance the significance of genetic exchange between different species leading to the evolution of novel degradative activities (Bull, 1980). Nine aerobic bacterial species, isolated from a munitions waste contaminated site in Illinois, were tentatively identified as one Enterobacter sp., one Pseudomonas sp., and seven Alcaligenes sp. Five of these isolates, Ent. sp. 15, Pseu. sp. 17, and three Alc. sp. I 1 3, 11 5 and 122 were selected for this study to investigate the system efficiency of pure and mixed cultures in the aerobic biotransfonnation of TNT. The isolates were mixed in various combinations of doublets, triplets, quadruplets, and quintuplet. The percentages of TNT transformation were higher for the mixed cultures than the average sum of the transformations of the component species. The average percentages of TNT transformations were in the following order: 5-species > 4-species > 3-species > 2species > isolates. The growth, measured as optical density, was more pronounced in the mixed culture reactors as compared to the isolates. The major intermediates identified were 2amDNT and 4amDNT, with the p-nitro group being more preferentially reduced over the o-nitro group. Sorption/desorption processes were observed in certain reactors containing Pseu. sp. 17 in mixed cultures, but not as an isolate. It appears that faster transformation of TNT is observed in mixed cultures as compared to pure cultures.