Bioremedacion

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Anaerobic biodegradation of saturated and aromatic hydrocarbons
Friedrich Widdel* and Ralf Rabus†
Saturated and aromatic hydrocarbons are wide-spread in our
environment. These compounds exhibit low chemical reactivity and
for many decades were thought to undergo biodegradation only in
the presence of free oxygen. During the past decade, however, an
increasing number of microorganismshave been detected that
degrade hydrocarbons under strictly anoxic conditions.
Addresses
Max-Planck-Institut für Marine Mikrobiologie, Celsiusstrasse 1,
D-28359 Bremen, Germany
*e-mail: fwiddel@mpi-bremen.de
† e-mail: rrabus@mpi-bremen.de
Current Opinion in Biotechnology 2001, 12:259–276
0958-1669/01/$ — see front matter
© 2001 Elsevier Science Ltd. All rights reserved.
AbbreviationsEPR
electron-paramagnetic resonance
PFL
pyruvate formate lyase
RNR
ribonucleotide reductase
SRB
sulphate-reducing bacteria

Introduction
Compounds that consist exclusively of carbon and hydrogen are termed hydrocarbons. Because of the lack of
functional groups, hydrocarbons are largely apolar and
exhibit low chemical reactivity at room temperature.
Differences in their reactivities areprimarily determined
by the occurrence, type and arrangement of unsaturated
bonds (π-bonds). It is, therefore, common to classify
hydrocarbons according to their bonding features into four
groups: the alkanes (saturated hydrocarbons), alkenes,
alkynes, and aromatic hydrocarbons. Within each of the
groups of non-aromatic (aliphatic) hydrocarbons we can
further distinguish betweenstraight-chain (e.g., n-alkanes), branched-chain and cyclic (alicyclic) compounds.
Aromatic hydrocarbons may be mono- or polycyclic, and
many important compounds in this class also contain
aliphatic hydrocarbon chains (e.g., alkylbenzenes).
The availability of hydrocarbons as fuels and starting compounds for a vast range of chemical syntheses is of
fundamental importance for our industrialisedcivilisation.
Nearly all of these hydrocarbons are of natural origin or
chemically synthesised directly from natural hydrocarbons.
Hydrocarbons are naturally formed by long-term geochemical reactions of buried biomass or as metabolites in living
organisms. Hence, anthropogenic activity has not introduced hydrocarbons as a novel class of compounds into the
environment, but rather led to an increase intheir accumulation; this can be deleterious as in the case of oil spills [1,2].
The presence of hydrocarbons in the biosphere throughout
the history of life may explain why many microorganisms
have acquired pathways to make use of these compounds

as growth substrates (Figure 1). The study of such
microorganisms is of basic scientific (e.g., biochemical),
environmental and biotechnologicalinterest.
Bacteria and fungi that utilise hydrocarbons in the presence of oxygen have been known since the beginning of
the 20th century. The fact that oxygen is not available in
all environments where hydrocarbons occur (e.g., in deep
sediments and in oil reservoirs) has repeatedly evoked the
question as to whether or not the biodegradation of hydrocarbons is possible under anoxicconditions, and if so to
what extent. It was not until the late 1980s that novel types
of microorganisms were definitively shown to degrade
hydrocarbons under strictly anoxic conditions (Figure 1).
So far, studies have shown that these microorganisms activate hydrocarbons by unprecedented biochemical
mechanisms that differ completely from those employed
in aerobic hydrocarbon metabolism. The presentarticle
focuses on saturated and aromatic hydrocarbons, which are
the main constituents of gas and oil [3]. For information
about anaerobic degradation of alkenes and alkynes, the
reader is referred to other overview articles [4,5].

Formation of aliphatic and aromatic hydrocarbons
Biological formation

Among hydrocarbons formed by living organisms, one may
distinguish between...