Alkane oxygenases exhibit structural differences, related for example to the
active site of the enzyme, which can contain iron–sulfur, di-iron, heme, or copper
(Fuentes et al. 2014). Monooxygenases are responsible for the oxidation of linear or
branched alkanes. The hydroxylation of alkanes leads to the creation of alcohol,
which is further oxidized to aldehyde by alcohol dehydrogenase. Aldehyde is
subsequently oxidized by aldehyde dehydrogenase to carboxylic acid, which serves
a substrate for acetyl coenzyme A (acyl-CoA) synthesis which is further funneled
into β-oxidation pathway (Fuentes et al. 2014). Substrate range of monooxygenases
is one of the factors determining the variety of alkanes, which can be degraded by a
particular strain.
Microorganisms degrading alkanes can be found within eubacteria, yeasts, fungi,
and even algae (van Beilen and Funhoff 2007). Bacteria, which possess the ability to
degrade alkanes, occur within phyla such as Proteobacteria, Actinomycetales,
Firmicutes (Bacillus, Geobacillus), Deinococcus–Thermus (Thermus), and
Bacteroidetes–Chlorobi (Flavobacteriia, Sphingobacteria) (van Beilen and Funhoff
2007). Alkane degraders occur not only in petroleum-contaminated sites; their
presence has also been noted in non-polluted ecosystems (van Beilen and Funhoff
2007).
Aliphatic hydrocarbons are often mineralized into H 2 O and CO 2 ; however,
production of intermediate metabolites may affect the biodegradation process.
Aldehydes are one of the intermediate products on the degradation pathway of
alkanes. Water solubility of aldehydes is higher than alkanes, thus temporary
accumulation may lead to elevated soil toxicity. This phenomena may be observed
often in the initial phase of bioremediation treatment as microbial activity is stimulated by nutrient addition (Xu and Lu 2010; Qin et al. 2013).
5.2.3 Biodegradation of Aromatic Hydrocarbons
To cleave the aromatic ring, two hydroxyl groups are incorporated by ring hydroxylating dioxygenase (RHD) (Fig. 5.3). However, di-oxygenation is not always the
primary reaction in the transformation of aromatic compounds. In case of toluene,
firstly the methyl group is oxidized while dioxygenation by ring hydroxylating
dioxygenase (RHD) enzymes occurs in further transformation steps (Fuentes et al.
2014).
Aromatic compounds with complex structure are often transformed into
monoaromatic hydrocarbons, which further enter the central metabolic pathways
(Cerniglia 1992; Moody et al. 2001; van Herwijnen et al. 2003). PAHs, such as
pyrene or phenanthrene, are mainly transformed into protocatechuate, gentisate, or
catechol. Naphthalene can be transformed into gentisate or catechol while biphenyl
and toluene are transformed into catechol (Fuentes et al. 2014).
Degradation of aromatic compounds may be conducted by various metabolic
pathways. For example, for toluene degradation, five aerobic pathways have been
detected in different bacterial strains. Some bacterial strains, such as Mycobacterium
5 Potential Use of Waste-to-Bioenergy By-Products in Bioremediation of Total. . .
255
active site of the enzyme, which can contain iron–sulfur, di-iron, heme, or copper
(Fuentes et al. 2014). Monooxygenases are responsible for the oxidation of linear or
branched alkanes. The hydroxylation of alkanes leads to the creation of alcohol,
which is further oxidized to aldehyde by alcohol dehydrogenase. Aldehyde is
subsequently oxidized by aldehyde dehydrogenase to carboxylic acid, which serves
a substrate for acetyl coenzyme A (acyl-CoA) synthesis which is further funneled
into β-oxidation pathway (Fuentes et al. 2014). Substrate range of monooxygenases
is one of the factors determining the variety of alkanes, which can be degraded by a
particular strain.
Microorganisms degrading alkanes can be found within eubacteria, yeasts, fungi,
and even algae (van Beilen and Funhoff 2007). Bacteria, which possess the ability to
degrade alkanes, occur within phyla such as Proteobacteria, Actinomycetales,
Firmicutes (Bacillus, Geobacillus), Deinococcus–Thermus (Thermus), and
Bacteroidetes–Chlorobi (Flavobacteriia, Sphingobacteria) (van Beilen and Funhoff
2007). Alkane degraders occur not only in petroleum-contaminated sites; their
presence has also been noted in non-polluted ecosystems (van Beilen and Funhoff
2007).
Aliphatic hydrocarbons are often mineralized into H 2 O and CO 2 ; however,
production of intermediate metabolites may affect the biodegradation process.
Aldehydes are one of the intermediate products on the degradation pathway of
alkanes. Water solubility of aldehydes is higher than alkanes, thus temporary
accumulation may lead to elevated soil toxicity. This phenomena may be observed
often in the initial phase of bioremediation treatment as microbial activity is stimulated by nutrient addition (Xu and Lu 2010; Qin et al. 2013).
5.2.3 Biodegradation of Aromatic Hydrocarbons
To cleave the aromatic ring, two hydroxyl groups are incorporated by ring hydroxylating dioxygenase (RHD) (Fig. 5.3). However, di-oxygenation is not always the
primary reaction in the transformation of aromatic compounds. In case of toluene,
firstly the methyl group is oxidized while dioxygenation by ring hydroxylating
dioxygenase (RHD) enzymes occurs in further transformation steps (Fuentes et al.
2014).
Aromatic compounds with complex structure are often transformed into
monoaromatic hydrocarbons, which further enter the central metabolic pathways
(Cerniglia 1992; Moody et al. 2001; van Herwijnen et al. 2003). PAHs, such as
pyrene or phenanthrene, are mainly transformed into protocatechuate, gentisate, or
catechol. Naphthalene can be transformed into gentisate or catechol while biphenyl
and toluene are transformed into catechol (Fuentes et al. 2014).
Degradation of aromatic compounds may be conducted by various metabolic
pathways. For example, for toluene degradation, five aerobic pathways have been
detected in different bacterial strains. Some bacterial strains, such as Mycobacterium
5 Potential Use of Waste-to-Bioenergy By-Products in Bioremediation of Total. . .
255