Evidence for Differential Effects of 2,4,6-Trinitrotoluene and Other Munitions Compounds on Specific Subpopulations of Soil Microbial Communities

Abstract

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 bacteria, 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 g TNT/ml was added to the medium; a 99% decrease was observed for soil contaminated with less than 20 g TNT/g. Highly contaminated soil harbored a greater number of organisms that were able to grow on plates amended with greater than 10 g TNT/ml. Gram-positive isolates from both soils demonstrated marked growth inhibition when greater than 8-16 g TNT/ml was present in the culture medium. These results indicate that chronic exposure to munitions compounds can dramatically alter soil microbial communities.

Effects and Bioavailability of 2,4,6-Trinitrotoluene in Spiked and Field-Contaminated Soils to Indigenous Microorganisms

“…Supporting our results, several investigators have found that TNT has a much higher toxicity than dinitrotoluenes or the mono-amino metabolites [31,[40][41][42]. Although TNB has a higher toxicity to soil microbes than does TNT [10], TNB was detected in only 27 of 54 samples (11 samples in the dead zones and 16 in the surrounding areas), and its level was always lower than 87 mg/ kg. Unlike the observations of Fuller and Manning [10], no significant correlation was found between TNB levels and microbial activities in our field soils.…”

“…In addition, TNT is cytotoxic and genotoxic to bacterial and mammalian cells [6][7][8]. These laboratory results are supported by two field studies that reported TNT toxicity to cucumber, radish, earthworm, Photobacterium phosphoreum [9], and soil microbial communities [10]. However, most of the studies assessing TNT toxicity are short-term acute assays that use aquatic or terrestrial species not often found at contaminated sites.…”

“…The results presented in Figure 5 are similar to those obtained by Simini et al [9] and Peterson et al [5], both of whom used a linear regression model to correlate logtransformed or nontransformed TNT level with bioassay endpoints. However, a nonlinear power regression model [10] appears to give a better fit to our data as evidenced by higher correlation coefficients (r) using this model than using a linear or logarithmic model (data not shown).…”

“…Although TNB has a higher toxicity to soil microbes than does TNT [10], TNB was detected in only 27 of 54 samples (11 samples in the dead zones and 16 in the surrounding areas), and its level was always lower than 87 mg/ kg. Unlike the observations of Fuller and Manning [10], no significant correlation was found between TNB levels and microbial activities in our field soils. Thus, microbial toxicity in field soils could be attributed to TNT that was the major contaminant detected.…”

“…However, most of the studies assessing TNT toxicity are short-term acute assays that use aquatic or terrestrial species not often found at contaminated sites. Few studies (e.g., [10,11]) have examined the response of indigenous microbes to TNT exposure in soil, and the long-term effect caused by TNT in soil remains unclear.…”