10.7 How to Degrade Diverse Hydrocarbons in the Absence
of Molecular Oxygen?
In oxic environments, the activation of C–H bonds in hydrocarbons is mainly
performed by O 2 -dependent reactions. As for the oxidation of methane, the classical
way of activating larger hydrocarbons is through the action of oxygenases that
convert the hydrocarbons to the corresponding alcohols. However, there are many
reports of anaerobic degradation of hydrocarbons. Thus, from oil wells and oil
production fluids, moderately thermophilic sulfate-reducing bacteria could be
enriched that utilize n-alkanes and alkylbenzenes under strictly anoxic conditions
(Rueter et al. 1994). There are even observations that alkanes might be consumed in
anoxic sediments below the zone of sulfate reduction where they are converted to
methane under strictly anoxic conditions (Zengler et al. 1999).
Starting in the early 1990s, highly diverse ways were discovered in which
bacteria, in most cases members of the Betaproteobacteria and the
Deltaproteobacteria, degrade both aliphatic and aromatic hydrocarbons under
anaerobic conditions (Rabus et al. 2016). Thus, a sulfate-reducing bacterium, designated strain Hxd3—“Desulfococcus oleovorans” (a name without standing in the
nomenclature)—can grow anaerobically on hexadecane as sole carbon source
(Aeckersberg et al. 1991). Alkanes are activated by carboxylation at the third carbon
with subsequent elimination of the terminal and subterminal carbons, yielding a fatty
acid that is one carbon shorter than the parent alkane (So et al. 2003).
Toluene can be anaerobically activated by the addition of fumarate to form
benzylsuccinate that is further converted to benzoyl-CoA. This reaction is found in
the denitrifying Betaproteobacteria Thauera aromatica and Azoarcus spp. (Biegert
et al. 1996). Ethylbenzene can be anaerobically converted to phenylethanol by
Azoarcus strain EbN1 (also known as “Aromatoleum aromaticum”) by the action
of ethylbenzene hydrolase (Kniemeyer and Heider 2001). Degradation of aromatic
hydrocarbons such as benzene and polycyclic aromatic hydrocarbons in the absence
of molecular oxygen is very slow. Bacterial cultures with doubling times of around
2 weeks were obtained that anaerobically degrade benzene, naphthalene, methylnaphthalene, and even phenanthrene, the largest polyaromatic hydrocarbon currently
known to be degradable under anoxic conditions. Degradation of benzene and
naphthalene is probably initiated by carboxylation to benzoate and 2-naphthoate,
respectively (Merckenstock et al. 2016).
170
A. Oren
of Molecular Oxygen?
In oxic environments, the activation of C–H bonds in hydrocarbons is mainly
performed by O 2 -dependent reactions. As for the oxidation of methane, the classical
way of activating larger hydrocarbons is through the action of oxygenases that
convert the hydrocarbons to the corresponding alcohols. However, there are many
reports of anaerobic degradation of hydrocarbons. Thus, from oil wells and oil
production fluids, moderately thermophilic sulfate-reducing bacteria could be
enriched that utilize n-alkanes and alkylbenzenes under strictly anoxic conditions
(Rueter et al. 1994). There are even observations that alkanes might be consumed in
anoxic sediments below the zone of sulfate reduction where they are converted to
methane under strictly anoxic conditions (Zengler et al. 1999).
Starting in the early 1990s, highly diverse ways were discovered in which
bacteria, in most cases members of the Betaproteobacteria and the
Deltaproteobacteria, degrade both aliphatic and aromatic hydrocarbons under
anaerobic conditions (Rabus et al. 2016). Thus, a sulfate-reducing bacterium, designated strain Hxd3—“Desulfococcus oleovorans” (a name without standing in the
nomenclature)—can grow anaerobically on hexadecane as sole carbon source
(Aeckersberg et al. 1991). Alkanes are activated by carboxylation at the third carbon
with subsequent elimination of the terminal and subterminal carbons, yielding a fatty
acid that is one carbon shorter than the parent alkane (So et al. 2003).
Toluene can be anaerobically activated by the addition of fumarate to form
benzylsuccinate that is further converted to benzoyl-CoA. This reaction is found in
the denitrifying Betaproteobacteria Thauera aromatica and Azoarcus spp. (Biegert
et al. 1996). Ethylbenzene can be anaerobically converted to phenylethanol by
Azoarcus strain EbN1 (also known as “Aromatoleum aromaticum”) by the action
of ethylbenzene hydrolase (Kniemeyer and Heider 2001). Degradation of aromatic
hydrocarbons such as benzene and polycyclic aromatic hydrocarbons in the absence
of molecular oxygen is very slow. Bacterial cultures with doubling times of around
2 weeks were obtained that anaerobically degrade benzene, naphthalene, methylnaphthalene, and even phenanthrene, the largest polyaromatic hydrocarbon currently
known to be degradable under anoxic conditions. Degradation of benzene and
naphthalene is probably initiated by carboxylation to benzoate and 2-naphthoate,
respectively (Merckenstock et al. 2016).
170
A. Oren