Microbially Mediated Biodegradation of Hexahydro-1,3,5-Trinitro-1,3,5- Triazine by Extracellular Electron Shuttling Compounds

Abstract

The potential for humic substances to stimulate the reduction of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) was investigated. This study describes a novel approach for the remediation of RDX-contaminated environments using microbially mediated electron shuttling. Incubations without cells demonstrated that reduced AQDS transfers electrons directly to RDX, which was reduced without significant accumulation of the nitroso intermediates. Three times as much reduced AQDS (molar basis) was needed to completely reduce RDX. The rate and extent of RDX reduction differed greatly among electron shuttle/acceptor amendments for resting cell suspensions of Geobacter metallireducens and G. sulfurreducens with acetate as the sole electron donor. AQDS and purified humic substances stimulated the fastest rate of RDX reduction. The nitroso metabolites did not significantly accumulate in the presence of AQDS or humic substances. RDX reduction in the presence of poorly crystalline Fe(III) was relatively slow and metabolites transiently accumulated. However, adding humic substances or AQDS to Fe(III)-containing incubations increased the reduction rates. Cells of G. metallireducens alone reduced RDX; however, the rate of RDX reduction was slow relative to AQDSamended incubations. These data suggest that extracellular electron shuttle-mediated RDX transformation is not organism specific but rather is catalyzed by multiple Fe(III)-and humic-reducing species. Electron shuttle-mediated RDX reduction may eventually become a rapid and effective cleanup strategy in both Fe(III)-rich and Fe(III)-poor environments.

Regulation of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) metabolism in Shewanella halifaxensis HAW-EB4 by terminal electron acceptor and involvement of c-type cytochrome

“…Shewanella is thus an ideal model organism to investigate the molecular mechanisms that underlie the TEA-regulated multiple electron-transfer pathways of anaerobic RDX biotransformation via either initial denitration or initial reduction of -N-NO 2 to -N-NO. In addition to other approaches, such as the recently described microbial electron shuttling (Bhushan et al, 2006;Kwon & Finneran, 2006), the involvement of ctype cytochrome in anaerobic RDX reduction provides a basis for the development of a new strategy to improve RDX bioremediation in the anoxic environment.…”

Involvement of cytochrome c CymA in the anaerobic metabolism of RDX by Shewanella oneidensis MR-1

“…Although MR-1 was not isolated from an RDXcontaminated environment, it was recently proposed that MR-1 can also use RDX as an electron acceptor (Kwon and Finneran 2008) but the pathway for RDX degradation was not determined. Other Fe(III)-reducing bacteria such as Geobacter metallireducens and Geobacter sulfurreducens were shown to directly reduce RDX to its nitroso derivatives (Kwon and Finneran 2006). Additionally, cyclic nitramine transformation mediated by extracellular electron shuttles, such as humic substances or humic substances analog anthraquinone-2,6-disulfonate, is ubiquitous among the keystone Fe (III)-reducing microbial genera Finneran 2008, 2010).…”

“…Additionally, cyclic nitramine transformation mediated by extracellular electron shuttles, such as humic substances or humic substances analog anthraquinone-2,6-disulfonate, is ubiquitous among the keystone Fe (III)-reducing microbial genera Finneran 2008, 2010). Electron shuttling compounds were shown to mediate RDX degradation via initial nitro group reduction (Bhushan et al 2006; Kwon and Finneran 2006). Fig.…”

Extracellular Electron Transfer in in situ Petroleum Hydrocarbon Bioremediation

“…In many studies, anthraquinone disulfonate (AQDS) was used as a model humic substance (Aulenta et al, 2010;Bhushan et al, 2006;Collins and Picardal, 1999;Kwon and Finneran, 2006;Kwon and Finneran, 2008;Scherr et al, 2011) to study extracellular electron transfer in the reduction of halogenated and nitrous organic compounds.…”