Endophyte-Assisted Phytoremediation of Explosives in Poplar Trees by Methylobacterium populi BJ001^T

Abstract

Phytoremediation is the use of plants for the treatment of environmental pollution. For a long time, bacteria in the rhizosphere have been recognized playing a significant role in the transformation of organic contaminants by higher plants. Although plants are known to detoxify and, to some extent, metabolize organic pollutants, they are autotrophic organisms that are not capable to fully mineralize organic molecules. Plant-associated bacteria can therefore complement the biodegradation capabilities of plants. Increasing interest has been given recently to endophytic bacteria for their potential role in phytoremediation of organic pollutants. In this study, a pink-pigmented symbiotic bacterium was isolated from hybrid poplar tissues (Populus deltoides × nigra DN34) that were used for the biodegradation of the toxic explosives, TNT, RDX, and HMX. On the basis of its physiological, genotypic, and ecological characteristics, the isolate has been recognized as a novel bacterial species, Methylobacterium populi strain BJ001^T. The bacterium in pure culture was shown to degrade the explosives, TNT, RDX, and HMX. TNT was fully transformed in less than 10 days with the production of reduction metabolites including amino-dinitrotoluenes and diamino-nitrotoluenes. No significant mineralization of ¹⁴C-TNT into ¹⁴CO₂ was recorded. The bacterium was also shown to transform RDX and HMX in less than 40 days. After 55 days of incubation, about 60% of initial ¹⁴C-RDX and ¹⁴C-HMX were mineralized into ¹⁴CO₂. The metabolites detected from RDX transformation included a mononitroso derivative and a polar compound tentatively identified as methylenedinitramine. These observations suggest that Methylobacterium populi BJ001^T may play a significant role in the metabolism of explosives in poplar plants.

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