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Applied Microbiology and Biotechnology

Springer-Verlag 2005

10.1007/s00253-004-1864-3

Mini-Review

Biodegradation of xenobiotics by anaerobicbacteria

Chunlong Zhang1 and George N. Bennett2

(1) Department of Environmental Sciences, University of Houston-Clear Lake, Houston,

TX 77058, USA

(2) Department of Biochemistry and Cell Biology, Rice University, 6100 Main St., Houston,TX 77005, USA

George N. Bennett

Email: [email protected]

Phone: +1-713-3484920

Fax: +1-713-3485154

Received: 28 September 2004 Revised: 29 November 2004 Accepted:

30 November 2004 Published online: 26 January 2005

Abstract Xenobiotic biodegradation under anaerobic conditions such as in groundwater,

sediment, landfill, sludge digesters and bioreactors has gained increasing attention over the lasttwo decades. This review gives a broad overview of our current understanding of and recentadvances in anaerobic biodegradation of five selected groups of xenobiotic compounds (petroleum

hydrocarbons and fuel additives, nitroaromatic compounds and explosives, chlorinated aliphatic

and aromatic compounds, pesticides, and surfactants). Significant advances have been made

toward the isolation of bacterial cultures, elucidation of biochemical mechanisms, and laboratoryand field scale applications for xenobiotic removal. For certain highly chlorinated hydrocarbons

(e.g., tetrachlorethylene), anaerobic processes cannot be easily substituted with current aerobic

processes. For petroleum hydrocarbons, although aerobic processes are generally used, anaerobicbiodegradation is significant under certain circumstances (e.g., O2-depleted aquifers, oil spilled in

marshes). For persistent compounds including polychlorinated biphenyls, dioxins, and DDT,

anaerobic processes are slow for remedial application, but can be a significant long-term avenuefor natural attenuation. In some cases, a sequential anaerobic-aerobic strategy is needed for total

destruction of xenobiotic compounds. Several points for future research are also presented in this

review.

Introduction
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Anaerobic biodegradation of xenobiotic compounds has been a subject of extensive research

during the last two decades. Consequently, our current understanding of the dissipation

mechanisms of xenobiotics in natural anaerobic environments has considerably improved. Manyanaerobe-based bioreactors and remediation systems have been developed to effectively clean-up

contaminated media. The purpose of this review is to summarize recent advances in our

understanding and briefly describe biotechnological applications for the biodegradation of fivemajor groups of xenobiotic compounds: petroleum hydrocarbons and related fuel additives,

nitroaromatic compounds and explosives, chlorinated aliphatic and aromatic compounds,

pesticides, and surfactants.

The review is not intended to be exhaustive, but focuses on representative anaerobes, theirbiochemical mechanisms, and potential biotechnological and environmental implications. Several

excellent reviews have been published on anaerobic biodegradation of xenobiotics, both in general

(Janke and Fritsche1985; Mogensen et al. 2003a; Schink2002) or focused on specific compoundsincluding petroleum hydrocarbons (Chakraborty and Coates2004; Heider and Fuchs 1997; Prince

1993; Spormann and Widdel2000; Widdel and Rabus 2001), explosives (Ahmad and Hughes

2000; Esteve-Nuez et al.2001; Gorontzy et al.1994; Marvin-Sikkema and de Bont1994; Peresand Agathos 2000), chlorinated compounds (Abramowicz 1990; Bedard 2003; Chen 2004; El

Fantroussi et al. 1998; Fetzner1998; Haggblom et al. 2000; Ohtsubo et al. 2004), and pesticides

(Sethunathan 1973; Williams 1977).

There are several reasons why anaerobic biodegradation of xenobiotics is important to researchersand practitioners. Aerobic processes require expensive O2 delivery systems, maintenance is often

high due to biofouling in subsurface remedial applications (Baker and Herson 1994), and there are

high energy costs and sludge production when bioreactors are employed (Jewell 1987; McCarty

and Smith 1986). In addition, anaerobic conditions naturally prevail in most cases forcontaminated groundwater, and some xenobiotic compounds [e.g., tetrachloroethylene,

polychlorinated biphenyls (PCBs), and nitro-substituted aromatics] can be efficiently transformedor mineralized only by anaerobic bacteria. In some cases, aerobic degradation does not occurwithout a prior anaerobic process (Master et al. 2002).

Major groups of anaerobic organisms involved inxenobiotic biodegradation

Like their aerobic counterparts, anaerobic bacteria able to degrade xenobiotic compounds are

diverse and present in various anaerobic habitats, including sediments, water-laden soils,gastrointestinal contents, reticulo-ruminal contents, feedlot wastes, sludge digesters, groundwater,

and landfill sites (Williams 1977). Anaerobes use natural organics such as proteins, carbohydrates,and many others as carbon and energy sources. Many of the so-called xenobiotic compounds ofenvironmental concern have naturally occurring relatives (Wackett et al.2002). For other

xenobiotics, repeated exposure has resulted in the adaptation and evolution of anaerobic bacteria

capable of metabolizing these man-made compounds.

Table 1 lists the major groups of anaerobic microorganisms involved in biodegradation of selectedxenobiotic compounds. The pure bacterial cultures given in this table are by no means exhaustive
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but are representative of each compound category. In reporting these bacteria with compound-

specific metabolic capability, two classical strategies are commonly employed. Some researchers

have chosen to employ pure cultures of previously isolated anaerobic strains to test with specificcompounds, whereas others have focused on the isolation and identification of new strains from

anaerobic bacterial consortia or enrichment cultures (El Fantroussi et al. 1998). Without a

systematic screening approach, the number of bacterial cultures successfully isolated is limitedsince only a small portion of what is present in the actual microbial habitat has been tested. In other

cases, several syntrophic bacterial strains of a bacterial consortium co-exist to metabolize a

specific compound (El Fantroussi et al. 1998; Janke and Fritsche1985; Williams 1977). Despitethese limitations, the diversity of anaerobic microorganisms able to biodegrade xenobiotic

compounds is apparent.

Table 1 Major groups of anaerobic microorganisms involved in xenobiotic biodegradation. PAH

Polycyclic aromatic hydrocarbon, MTBEmethyl tert-butyl ether, TNTtrinitrotoluene, DNT

dinitrotoluene, RDXhexahydro-1,3,5-trinitro-1,3,5-triazine, HMXoctahydro-1,3,5,7-tetranitro-

1,3,5,7-tetrazocine, PCEtetrachloroethylene, TCEtrichloroethene, DCEcis-dichloroethene, VC

vinyl chloride, PCB polychlorinated biphenyls, PCPpentachlorophenol, LASlinear alkylbenzene

sulfonate,LAEOs linear alcohol ethyoxylates

CompoundsBacteria namea, source of isolationb, chemical

actionReference

Alkane D. oleovorans (P): mineralizes C12C20n-alkane Aeckersberg et al. 1991

Benzene

G. spp. (P): oxidizes benzene in Fe(II)-reducingconditions Coates et al.2001; Rooney-

Varga et al. 1999Dechloromonas spp. (S): mineralizes benzene into

CO2 in 5 days

Toluene

G. metallireducens (S): first pure culture (Fe3+

reducing) for toluene oxidation Chakraborty and Coates2004;

Lovley et al. 1989Azoarcus and Thauera spp. (S/D): facultativetoluene-oxidizing nitrate-reducers

EthylbenzeneThauera-related (S/P): denitrifying bacteria

completely mineralize ethylbenzene

Ball et al. 1996; Rabus and

Widdel1995

XyleneD. acetonicum- andDesulfosarcina variabilis-related: mineralizes o- and m-xylene

Harms et al. 1999; Hess et al.1997; Rabus and Widdel1995

PAHs

Acidovorax,Bordetella,Pseudomonas,

Sphingomonas, and Variovorax (S): degradation

complete for naphthalene and partial for 35 ringPAHs;P. stutzeri and Vibriop pelagius related (S):

mineralizes 720% naphthalene

Eriksson et al. 2003; Rockne etal. 2000

MTBEPure aerobes isolated; slow under anaerobic

conditions, no pure anaerobes isolated

Finneran and Lovley 2001;

Stocking et al.2000

TNT, DNT

Veillonella alkalescens (D): the earliest evidence ofanaerobic TNT degradation C. spp. and

Desulfovibrio spp. (N): most extensively studied

genera transforming TNT

Esteve-Nuez et. 2001; Hughes

et al.1999; McCormick et al.

1976
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ingerlink.com/media/53KGUWVGVJRN7QXQMXFR/Contributions/Y/V/C/E/YVCE0R48P416X2KY_html/fulltext.html#CR78%23CR78http://www.springerlink.com/media/53KGUWVGVJRN7QXQMXFR/Contributions/Y/V/C/E/YVCE0R48P416X2KY_html/fulltext.html#CR84%23CR84http://www.springerlink.com/media/53KGUWVGVJRN7QXQMXFR/Contributions/Y/V/C/E/YVCE0R48P416X2KY_html/fulltext.html#CR143%23CR143http://www.springerlink.com/media/53KGUWVGVJRN7QXQMXFR/Contributions/Y/V/C/E/YVCE0R48P416X2KY_html/fulltext.html#CR50%23CR50http://www.springerlink.com/media/53KGUWVGVJRN7QXQMXFR/Contributions/Y/V/C/E/YVCE0R48P416X2KY_html/fulltext.html#CR151%23CR151http://www.springerlink.com/media/53KGUWVGVJRN7QXQMXFR/Contributions/Y/V/C/E/YVCE0R48P416X2KY_html/fulltext.html#CR61%23CR61http://www.springerlink.com/media/53KGUWVGVJRN7QXQMXFR/Contributions/Y/V/C/E/YVCE0R48P416X2KY_html/fulltext.html#CR174%23CR174http://www.springerlink.com/media/53KGUWVGVJRN7QXQMXFR/Contributions/Y/V/C/E/YVCE0R48P416X2KY_html/fulltext.html#CR54%23CR54http://www.springerlink.com/media/53KGUWVGVJRN7QXQMXFR/Contributions/Y/V/C/E/YVCE0R48P416X2KY_html/fulltext.html#CR90%23CR90http://www.springerlink.com/media/53KGUWVGVJRN7QXQMXFR/Contributions/Y/V/C/E/YVCE0R48P416X2KY_html/fulltext.html#CR121%23CR121


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RDX, HMX

Desulfovibrio spp. (S): uses RDX and HMX as sole

N source

Boopathy et al. 1998; Kitts et al.

1994; Young et al. 1997; Zhang

and Hughes2003; Zhao et al.2002

Providencia sp., andM. morganii (S): transforms

into nitroso derivatives

Serratia marcescens (M): RDX ring cleavage

similar to McCormick s pathway

C. acetobutylicum (N): transforms RDX intoNHOH and NH2 derivatives

K. pneumoniae (D): degrades RDX into HCHO,

CO2 and N2O

PCE, TCE

A. woodii, C. formicoaceticum,Methanolobus

tindarius,Methanosarcina sp.,Methanosarcina

mazei, Methanosarcina thermphila, Sporomusaovata (N): previously known strains transforming

PCE & TCE

El Fantroussi et al. 1998;Fathepure and Boyd 1988;

Jablonski and Ferry1992;Terzenbach and Blaut 1994

Desulfitobacterium sp. (S): transforms PCE to TCEand trace amount of DCE

De Bruin et al. 1992; Gerritse et

al. 1996; Gerritse et al.1999;

Magnuson et al. 2000; Maymo-Gatell et al. 1997; Sung et al.

2003

Dehalobacter restrictus (S): transforms PCE to

ethene

Desulfitobacterium frappieri (S/D): tranforms PCE& TCE into cis-DCE

Dehalococcoides ethenogenes (N): completes PCE

& TCE degradation into etheneDesulfuromonas michiganensis (S): able to grow

on free-phase PCE

VCDehalococcoides sp. (A): able to grow with VC andtransform VC into ethene

He et al. 2003

PCBsDesulfitobacterium dehalogenans (S):

dehalogenates flanking Cl of OH-PCBsWiegel et al.1999

PCP

Desulfitobacterium frappieri (S/D): 9099% PCP

removal forming 3-CPBeaudet et al. 1998; Bouchard

et al.1996; Shelton and Tiedje1984; Tartakovsky et al. 1999

Desulfitobacterium halogenans (S),

Desulfitobacterium chlororespirans (C),Desulfomnile tiedje (N): dechlorinates at o- and m-

position

DioxinsDehalococcoides sp. (S): uses dioxins as the sole

electron acceptorBunge et al. 2003

Chlorinated

pesticide

C. sp. (N): degrades DDT as the sole C source.

Degrades other chlorinated pesticides

Ruppe et al. 2003,2004;

Sethunathan 1973; Williams
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1977

Aerobacter aerogenes,K. pneumoniae,N. vulgaris

(S): DDT-degrading

Dehalospirilum multivorans: preferentially

dechlorinates technical toxaphene

P-based

pesticide

Flavobacterium sp. (S): attacks P-insecticides

including diazino and parathionSethunathan 1973

Carbamatepesticide

K. pneumoniae (D): uses three chlorinateds-triazines as the sole N source

Ernst and Rehm1995;

Dinoseb

pesticide

C. bifermentans (D): utilizes Dinoseb as a sole C

via cometabolismHammill and Crawford 1996

Anionic

surfactant

Strain RZLAS (D): the only pure anaerobe using

LAS as the sole S sourceDenger and Cook1999

Nonionicsurfactant

Pelobacter propionicus &A. sp. (D): LAEOsfermented to CH4 and CO2

Wagener and Schink1988

Cationic

surfactant

Unable to isolate a single bacterium using cationic

surfactant as the sole C sourceMadsen et al. 2001

aBacteria:AAcetobacterium, CClostridium, D Desulfobacterium, G Geobacter, KKlebsiella, MMorganella,NNocardia, PPseudomonasbSource: A Aquifer materials, Ccompost; D sludge; Mmanure; Nnot specified; Ppetroleum related sites; Ssoil or sedimentcChemicals: CPchlorophenol, Dinoseb 2-sec-butyl-4,6-dinitrophenol

Pure cultures summarized in Table 1 have been isolated under strict anaerobic conditions (sulfate-

reducing and methanogenic). Example bacteria in this category include Clostridia,Desulfobacterium,Desulfovibrio,Methanococcus,Methanosarcina, and most of the newly isolated

dehalogenating bacteria (e.g.,Dehalococcoides). For practical purposes, some of the facultativedenitrifying microorganisms are also included in the table such asFlavobacterium andKlebsiella

to illustrate their potential role in these environmental communities. Anaerobic bacteria isolated

from environmental compartments and bioreactors are preferentially illustrated over anaerobes of

pathological origin.

Attention has focused on the isolation of anaerobic bacteria that play a role in the degradation of

two types of compounds due to their widespread environmental problems: the petroleum

hydrocarbons [benzene-toluene-ethylbenzene-xylene (BTEX); polycyclic aromatic hydrocarbons

(PAHs)] and chlorinated compounds including the pesticide DDT [1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane]. In particular, extensive efforts have focused on the latter, partly because

halogenated organic compounds probably cause about half of the environmental problems

attributable to organic pollution in the world today (Tiedje et al. 1993), and partly becauseanaerobic biodegradation is a preferred strategy. Following the discovery of the insecticidal

properties of DDT in the late 1930s, its subsequent use and the awareness of its environmental

persistence, more than 300 bacterial strains have been shown to convert DDT into DDD [1,1-dichloro-2,2-bis(p-chlorophenyl)ethane] (Cookson1995) and several novel dechlorinating strains
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have been reported (Chacko et al. 1966; Guenzi and Beard1967; Matsumura and Boush 1971;

Wedemeyer1966) from the late 1960s to the 1970s. Research on the biodegradation of DDT

declined drastically after it was banned in the 1970s (Quensen et al. 2001) and the focus during thelast 10 years has been directed toward chlorinated aliphatic hydrocarbons due to their worldwide

prevalence. A pure culture ofDehalococcoides ethenogenes was able to completely dechlorinate

tetrachloroethylene (PCE) into innocuous ethene (Magnuson et al.2000; Maymo-Gatell et al.1997; McCarty1997), andDesulfuromonas michiganensis can even grow on free-phase PCE

(Sung et al. 2003). Most PCE-dechlorinating bacteria convert PCE into trichloroethene (TCE) or

further into cis-dichloroethene (DCE) (Bagley and Gossett1990), while for others the more toxicvinyl chloride (VC) is produced as the end-product. Several recent efforts have therefore been

made to isolate VC-transforming bacteria.Dehalococcoides sp., which can grow on VC and

transform it into ethene in the presence of lactate and pyruvate as electron donors (He et al. 2003),

is one such isolate.

Anaerobic degradation of the monoaromatic BTEX hydrocarbons was considered to be negligible

prior to the 1980s, partially due to the favorable energetics of aerobic bacteria (Chakraborty and

Coates 2004). These compounds have been shown to serve as carbon and energy sources fordiverse anaerobic bacteria under nitrate-reducing, Fe(III)-reducing, sulfate-reducing and

methanogenic conditions. Except forp-xylene, isolation of pure bacterial cultures degrading all

other BTEX compounds has been successful (Table1). Like BTEX, 2- to 4-ring PAHs are quite

readily biodegradable aerobically (Cerniglia 1992), and anaerobic degradation of PAHs wasformerly thought impossible. However, naphthalene biodegradation through denitrification has

been documented (Eriksson et al.2003; Mihelcic and Luthy 1988), and phenanthrene

biodegradation through similar conditions was also reported (Rockne and Strand 1998). A fewPAH-degrading bacterial strains have been successfully isolated but none were able to produce

complete mineralization. As a concurrent contaminant with BTEX and PAHs in many petroleum-

contaminated sites, methyl tert-butyl ether (MTBE) is mainly susceptible to aerobic degradation;

however, anaerobic metabolism of MTBE has been reported (Finneran and Lovley 2001;Kolhatkar et al. 2002; Somsamak et al. 2001; Stocking et al. 2000).

Anaerobic degradation of halogenated phenol, particularly pentachlorophenol (PCP), has been the

subject of several studies due to its wide use as a wood preservative. Pure cultures able todechlorinate PCP into 3-chlorophenol have been isolated; some bacteria preferentially remove Cl

at the ortho and meta positions (Beaudet et al.1998; Tartakovsky et al. 1999). However, no single

bacterial culture with an ability for complete dechlorination and mineralization has yet beenisolated. For polychlorinated biphenyls (PCBs), although reductive dechlorination has been

observed frequently in many contaminated sediments and aquifers with an array of

microorganisms (Quensen et al.1988), only recently have pure cultures been characterized (Wu et

al.2002a, b). A strain was isolated that could dechlorinate hydroxylated PCBs (Wiegel et al.1999). A pure culture that could use dioxin as the sole electron acceptor was isolated (Bunge et al.

2003). The isolation of dioxin-degrading bacteria is a good example of how bacteria have evolved

to metabolize toxic xenobiotic compounds.

The biodegradation of nitroaromatic explosives [trinitrotoluene (TNT); dinitrotoluene (DNT)] hasbeen studied for more than two decades. Clostridium andDesulfovibrio spp. have been extensively

studied for their pathways transforming these compounds into amino- and hydroxyamino-
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derivatives under anaerobic conditions. Unlike aerobic mineralization pathways (e.g., DNT

mineralization can be readily demonstrated under aerobic conditions, Zhang et al. 2000a, b),

significant mineralization of TNT and DNT under anaerobic conditions has never been achievedand anaerobic mineralizing bacteria never isolated. On the other hand, for non-aromatic explosives

such as RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine) and HMX (octahyrdo-1,3,5,7-tetranitro-

1,3,5,7-tetrazocine), pure bacterial cultures able to transform both agents have been isolated(Boopathy et al.1998; Kitts et al. 1994; Young et al. 1997; Zhao et al. 2002).

With a significant number of pesticides in use, dissimilar chemical structures and limited pure

bacterial isolates, generalizations regarding pesticide-degrading microorganisms are difficult to

make. For instance, in the United States alone, over 125 herbicides, 300 insecticides and 325fungicides are registered (Cookson 1995). The most extensively studied pesticide has been DDT

due to its persistent nature in the environment. The biodegradability of many other new synthetic

pesticides are of less concern due to the shorter half-life associated with biotic and abioticprocesses. Furthermore, studies on the biodegradation of pesticides appear to be focused mostly on

aerobic bacteria, despite some limited studies on the isolation of anoxic bacterial cultures (e.g.,

Ruppe et al.2003, 2004).

Synthetic surfactants have created environmental problems due to the use of alkyl benzenesulfonate (ABS) detergents that were later replaced by linear alkylbenzene sulfonate (LAS) in the

late 1960s. A common misconception is that surfactants are readily removed through aerobic

processes in municipal wastewater treatment plants due to sorption and aerobic biodegradation.This is also why biodegradability data of surfactants are predominantly aerobic (Swisher1987). A

significant percentage of surfactants escape aerobic processes and accumulate in anaerobic sludge

digesters. A conservative estimate shows that approximately 20% of surfactants reached the

anaerobic compartment (AISE and CESIO1999). In addition, renewed interest in surfactantbiodegradation is based on the recent finding that many alkyl phenol polyethoxylates show toxicity

to fish and are suspected of being endocrine disrupters. While the importance of anaerobicpathways is still in debate, research efforts to isolate anaerobic surfactant degrading bacteria(Table 1) are limited.

Biochemistry of xenobiotic biodegradationHydrocarbons and fuel additives

The anaerobic biochemical pathways of petroleum hydrocarbons and related fuel additives have

been the subjects of many investigations during the last two decades. For hydrocarbons, the

elucidation of anaerobic BTEX (particularly toluene) degradation pathways is probably the most

advanced (Boll et al. 2002). This is not surprising since saturated alkanes are less of a healthconcern, although quantitatively they are more important than BTEX (Gieg and Suflita 2002).

Saturated alkanes are more susceptible to aerobic bacterial attack than unsaturated aliphatichydrocarbons (i.e., alkene, alkyne). It is also well established that alkanes with long carbon chains

and straight structures are more prone to aerobic biodegradation and the same is likely to be the

case for anaerobes. The most common aerobic pathway for alkane degradation is oxidation of the

terminal methyl group into a carboxylic acid through an alcohol intermediate, and eventually
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complete mineralization through -oxidation (Cookson 1995; Leahy and Colwell 1990). Several

physiologically and phylogenetically distinct anaerobes have been shown to degrade alkanes

(Aeckersberg et al. 1991; Ehrenreich et al. 2000; Rabus et al. 2001; Rueter et al. 1994). Methane

can also be formed from alkanes by anaerobic organisms (Zengler et al. 1999). Recent data with ann-hexane-utilizing denitrifying isolate pointed to a pathway involving initial enzymatic attack by

fumarate (OOCCH=CHCOO) addition in a manner similar to that for toluene as discussed below

(Krieger et al.2001; Rabus et al.2001; Wilkes et al.2002). Another pathway reported in a sulfate-reducing bacterium, Hxd3 (Aeckersberg et al. 1991), involves carboxylation followed by removal

of a terminal two-carbon unit to reduce the original alkane length by one carbon as the fatty acid is

formed (So et al. 2003). Observations of a carbon addition reaction internal to the chain were alsomade in studies of strain SK-01 (So and Young 1999a, b).

Similarly, anaerobic MTBE metabolism is not as well understood as aerobic pathways. In the

presence of oxygen, aerobes attack MTBE with a monooxygenase. The biochemical mechanisms

of the recalcitrant ether bond cleavage have been explained in a review by Fayolle et al. ( 2001).With anaerobic bacteria, the cleavage involves methyl transferases and tetrahydrofolate for the

degradation of lignin (a naturally occurring ether compound) and hydroxyl group addition during

fermentation of polyethylene glycols (-O-CH2-CH2OH). Anaerobic degradation of MTBE has beendemonstrated using compound-specific carbon isotope analyses in a groundwater site (Kolhatkar et

al.2002), and transformation of MTBE has been observed under sulfate-reducing conditions

(Somsamak et al. 2001).

Figure 1 delineates the major enzymes and intermediates involved in anaerobic degradation ofBETX compounds. Variations in pathways exist since various bacteria depend on different electron

acceptors corresponding to differing redox conditions (Chakraborty and Coates 2004). Complete

mineralization has been reported for all BTEX compounds exceptp-xylene, and research haselucidated the initial enzymatic reactions shown in Fig. 1. A difference from aerobic mechanisms,

which involve molecular oxygen, is the introduction of oxygen through H2O to form oxygenated

monoaromatic compounds that are susceptible to further ring cleavage. In some cases, for example

in the anaerobic degradation ofp-cresol, oxidation of the methyl group via addition of oxygenderived from water occurs (Bossert et al. 1989; Bossert and Young1986). Also shown in Fig.1is

benzoyl coenzyme A (benzoyl-CoA), a common intermediate for BTEX compounds. Benzoyl-

CoA is formed through the addition of fumarate to the BTEX compounds through the enzymaticaction of benzylsuccinate synthase (BSS) (for toluene) or methylbenzylsuccinate synthase (foro-

and m-xylene) (Biegert et al. 1996). Studies on the mechanism have demonstrated that these are

glycyl radical enzymes (Beller and Spormann 1998; Krieger et al.2001; Leuthner et al.1998).After formation of benzylsuccinate, it is converted to the CoA derivative benzylsuccinyl-CoA by a

CoA transferase and then oxidized to benzoyl-CoA and succinyl-CoA for further metabolism

(Leutwein and Heider1999). The genes encoding the benzyl succinate synthase have been isolated(Hermuth et al.2002) and, in strain EbN1, are near another operon encoding enzymes required forconversion of benzyl succinate to benzoyl-CoA (Kube et al. 2004). The enzyme benzylsuccinyl-

CoA dehydrogenase is encoded by bbsG in Thauera aromatica (Leutwein and Heider2002).

Benzoyl-CoA is transformed to 1,5-diene-1-carboxyl-CoA through the key enzyme, benzoyl-CoAreductase. After a series of hydration and dehydrogenation steps, 3 mol acetyl-CoA and 1 mol CO 2is formed from each mole of BTEX compound (Mogensen et al. 2003a).
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Fig. 1 Anaerobic pathways for the biodegradation of petroleum hydrocarbons [benzene-toluene-ethylbenzene-xylene (BTEX); adapted from Chakraborty and Coates 2004; Mogensen et al.2003].

A Fumarate (HOOCCH=CHCOOH), E1 benzylsuccinate synthase (BSS), E2 ethylbenzylsuccinate

synthase, E3 ethylbenzene dehydrogenase, E4 ethylbenzylsuccinate synthase, E5 benzoyl-CoAreductase

The anaerobic biochemical pathways for PAHs have been studied only in the last few years, with afocus on naphthalene and phenanthrene. Pure cultures of sulfate-reducing (Galushko et al.1999)and nitrate-reducing (Rockne et al.2000) bacteria that degrade naphthalene have been isolated.

Like monoaromatic hydrocarbons, research has focused on the rate-limiting step of the initial

enzymatic attack. In contrast to earlier work that supported phenol as the major intermediate in thefermentation of naphthalene [D. Grbic-Galic (1990) Microbial degradation of homocyclic and

heterocyclic aromatic hydrocarbons under anaerobic conditions. Unpublished report, Department

of Civil Engineering, Stanford University], recent work by several research groups has identified
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2-naphthoic acid (2-NA) as a common intermediate (Fig.2) (Zhang et al. 2000a,b). This acid is

formed through carboxylation with the addition of a C1 unit (Zhang and Young 1997) or fumarate,

catalyzed by naphthyl-2-methyl-succinate synthase in the case of a substituted 2-methylnaphthalene (Sullivan et al. 2001). The latter is analogous to the benzoyl-CoA pathway of

monoaromatic BTEX degradation. Researchers have identified several intermediates including two

ring-cleaved products (Annweiler et al.2000, 2002; Meckenstock et al. 2000, Fig. 2).

Fig. 2 Anaerobic pathways for the biodegradation of polycyclic aromatic hydrocarbons (PAHs)

(adapted from Annweiler et al.2000, 2002). A Fumarate (HOOCCH=CHCOOH), E1 naphthyl-2-

methyl-succinate synthase

Nitroaromatic compounds and explosivesThe metabolic scheme in Fig.3illustrates major intermediates and end-products representative of

several anaerobic TNT pathways reported to date (Esteve-Nuez et al.2001). TNT has three highlyoxidized NO2 groups at the 2,4,6-positions. Because of their electrophilic nature, these external

NO2 groups are amenable to enzymatic reduction. In the meantime, since -electrons in the

benzene ring are shielded by four functional groups (3NO2 and 1CH3) due to steric hindrance, thearomatic structure is very stable, preventing enzymatic attack that could lead to ring cleavage. This

unique chemical structure explains, to a large extent, why biotransformation of TNT occurs rapidlybut appreciable mineralization has never been achieved in either aerobic or anaerobic systems even

with more than two decades of intensive research effort (Hawari et al.2000).
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Fig. 3 Anaerobic pathways for the biodegradation of nitroaromatic explosives [trinitrotoluene

(TNT)] (adapted from Esteve-Nuez et al.2001). A Bamberger rearrangement,E1 carbon

monoxide dehydrogenase (CODH),E2 nitrite reductase,E3 the combination of enzymes including

hydrogenase, pyruvate-ferredoxin oxidoreductase, or CODH for the first step and sulfite reductasefor the final step of the reaction process (Preuss et al. 1993)

An advantage of anaerobic TNT biotransformation at low redox potential is to minimize oxidative

polymerization and the toxic azoxy compounds that can be readily formed in the presence of

oxygen. Among an array of end-products proposed or identified (Fig. 3), the amino (NH2) andhydroxylamino (NHOH) derivatives from the reduction of NO2 groups are frequently reported.

Results have also shown the removal of NO2 groups as nitrite (NO2), and the oxidation of CH3 into
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benzoic acids (Esteve-Nuez and Ramos 1998; Esteve-Nuez et al.2000). Boopathy and Kulpa

(1992) even noted the formation of NH4+ from the reductive elimination of NH2 and proposed a

pathway that included toluene as the transformation end-product. The role of triaminotoluene(TAT), hydroxylamino intermediates, and the resulting compounds from subsequent hydroxyl

additionpara to NHOH (through Bamberger rearrangement) are incompletely known under

environmental conditions but have been studied in laboratory experiments (Hughes et al.1998;1999). TAT is considered to be a dead-end product that precludes further mineralization (Hawari et

al.2000). While hydroxylamino intermediates are not stable, their transient toxicity could be an

issue in remediation systems (Tadros et al. 2000). The good news, however, is that bothcompounds are strongly, or even irreversibly, adsorbed to soilsa mechanism that may hold

promise for remediation (Daun et al. 1998; Xue et al. 1995), and the chemically unstable nature of

these compounds reduces long-term toxicity risks (Padda et al. 2000,2003). The use of

cyclodextrins for desorption of TNT-related compounds has been studied with various soils;however, the suitability of this practice over the long term is unclear (Sheremata and Hawari

2000).

The enzymes involved in anaerobic TNT transformation have not been fully characterized,although several key proteins have been implicated, including ferredoxins, hydrogenases, carbon

monoxide dehydrogenase (CODH), pyruvate-ferredoxin oxidoreductases, and sulfite reductase

(Huang et al. 2000; Preuss et al. 1993). Perhaps more important to revitalize future research efforts

is the search for new microorganisms capable of TNT ring cleavage and mineralization (Hawari etal.2000).

Unlike nitroaromatic TNT, the nonaromatic cyclic nitroamines (RDX and HMX) have weak CN

bonds. Initial enzymatic attack able to change NNO2 or CH bonds can readily destabilize the

cyclic structure and cause further molecular fragmentation. RDX is generally recalcitrant underaerobic conditions, therefore anaerobic metabolism has been the subject of investigation.

Unfortunately, our understanding of RDX biodegradation has been limited since an early pathwaystudy by McCormick et al. (1981). In several recent studies on the examination of approximately

24 hypothetical metabolites proposed in McCormick s pathways, only a few were confirmed,

several intermediates were excluded, and many other new metabolites were identified (Adrian andChow 2001; Hawari et al. 2000; Zhang and Hughes 2003). The full product analysis of RDX

biodegradation is particularly challenging because it involves gas-phase mineralization products,

unstable nitroso- and hydroxyamino intermediates, as well as small molecules such as

formaldehyde and methanol. At the present time, enzymatic analysis is even more speculativedespite the recent characterization of one enzyme (nitrate oxidoreductase) involved in RDX

biotransformation (Bhushan et al. 2002).

Chlorinated aliphatic and aromatic hydrocarbonsThe general features of anaerobic biodegradation of chlorinated compounds has been reviewed(Haggblom et al.2000, 2003). The pathways for degradation of chlorinated aliphatic hydrocarbons

(CAHs) such as PCE are well established (Fig. 4). Much remains to be understood about thebiochemical mechanisms, including the enzymes and the associated genes encoding these

metabolic enzymes in bacteria with various dechlorinating activities. A strain that has activity on

PCE and a variety of diverse halogen compounds isDehalococcoides ethenogenes 195 (Fennell et

al.2004; Maymo-Gatell et al. 1997). Related Dehalococcoides-like organisms have been studied
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