Biodegradation of Nitro-Substituted Explosives 2,4,6-Trinitrotoluene, Hexahydro-1,3,5-Trinitro-1,3,5-Triazine, and Octahydro-1,3,5,7-Tetranitro-1,3,5-Tetrazocine by a Phytosymbiotic Methylobacterium sp. Associated with Poplar Tissues (Populus deltoides × nigra DN34)

Abstract

A pink-pigmented symbiotic bacterium was isolated from hybrid poplar tissues (Populus deltoides × nigra DN34). The bacterium was identified by 16S and 16S-23S intergenic spacer ribosomal DNA analysis as a Methylobacterium sp. (strain BJ001). The isolated bacterium was able to use methanol as the sole source of carbon and energy, which is a specific attribute of the genus Methylobacterium. The bacterium in pure culture was shown to degrade the toxic explosives 2,4,6-trinitrotoluene (TNT), hexahydro-1,3,5-trinitro-1,3,5-triazene (RDX), and octahydro-1,3,5,7-tetranitro-1,3,5,tetrazocine (HMX). [U-ring- 14C]TNT (25 mg liter-1) was fully transformed in less than 10 days. Metabolites included the reduction derivatives amino-dinitrotoluenes and diamino-nitrotoluenes. No significant release of 14CO2 was recorded from [14C]TNT. In addition, the isolated methylotroph was shown to transform [U-14C]RDX (20 mg liter-1) and [U-14C]HMX (2.5 mg liter-1) in less than 40 days. After 55 days of incubation, 58.0% of initial [ 14C]RDX and 61.4% of initial [14C]HMX were mineralized into 14CO2. The radioactivity remaining in solution accounted for 12.8 and 12.7% of initial [14C]RDX and [ 14C]HMX, respectively. Metabolites detected from RDX transformation included a mononitroso RDX derivative and a polar compound tentatively identified as methylenedinitramine. Since members of the genus Methylobacterium are distributed in a wide diversity of natural environments and are very often associated with plants, Methylobacterium sp. strain BJ001 may be involved in natural attenuation or in situ biodegradation (including phytoremediation) of explosive-contaminated sites.