Evidence for differential effects of 2,4,6-trinitrotoluene and other munitions compounds on specific subpopulations of soil microbial communities

The effects of 2,4,6-trinitrotoluene (TNT) and other munitions compounds on indigenous microbial communities in several soils were examined. Culturable heterotrophs, concentrations of phospholipid fatty acid (PLFA), and basal respiration rates exhibited slight negative correlations with high TNT and 1,3,5-trinitrobenzene (TNB) levels. Heat-shock-resistant culturable heterotrophs, percentage of gram-positive soil isolates, mole percent of branched PLFA, and 10Me18:0 (tuberculostearic acid) were observed to be significantly lower in highly contaminated soils. Total soil nitrogen levels were positively correlated with high TNT and TNB concentrations, whereas total soil carbon exhibited no significant correlation with either compound. Multivariate analysis of PLFA data resulted in distinct separation of soils with respect to their degree of contamination, with specific signature PLFAs for gram-positive bacterial fungi, and protozoa being negatively associated with high contaminant levels. Apparent concentrations of TNT resulting in 50% reductions in indicators of gram-positive populations were much higher than values from pure culture experiments, possibly as a result of low bioavailability due to sorption onto clay and soil organic matter. Few effects of other munitions compounds were observed. Closer examination of a highly contaminated soil revealed that the number of culturable heterotrophs growing on 0.3% molasses plates decreased by 50% when 67 micrograms TNT/ml was added to the medium; a 99% decrease was observed for soil contaminated with less than 20 micrograms TNT/g. Highly contaminated soil harbored a greater number of organisms that were able to grow on plates amended with greater than 10 micrograms TNT/ml. Gram-positive isolates from both soils demonstrated marked growth inhibition when greater than 8-16 micrograms TNT/ml was present in the culture medium. These results indicate that chronic exposure to munitions compounds can dramatically alter soil microbial communities.