Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) Bioremediation in Groundwater: Are Known RDX-Degrading Bacteria the Dominant Players?

Biodegradation of explosives in groundwater represents a promising remedial approach for these compounds. Although a range of bacteria capable of degrading the explosive hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) in pure culture have been described, the role of these known strains (and the genera they represent) during RDX degradation in groundwater has not been established. RDX-contaminated groundwater was collected from the Pueblo Chemical Depot (CO, USA) and the Picatinny Arsenal (NJ, USA) where bioremediation technologies are being tested. Soil columns and enrichment cultures were derived from Picatinny Arsenal groundwater. Bacteria-specific primers were used to amplify the 16S rRNA genes that were used for phylogenetic analysis. The species detected ranged across multiple genera, many of which have not been previously associated with RDX biodegradation. None of the retrieved sequences were exact matches to previously described RDX-degrading strains, although multiple sequences that grouped with known explosive-degrading strains of Clostridium and Pseudomonas were recovered. Genes previously reported to be associated with RDX degradation, including xplA, hydA, onr, xenA, and xenB, were not detected in any of the groundwater samples. These preliminary results indicate that the previously described RDX-degrading bacteria likely do not capture the microbial diversity associated with RDX bioremediation in groundwater, especially under the general biostimulation approaches used during most remediation efforts.