Microbial Degradation of Alkanes
Abstract
Petroleum hydrocarbons are introduced into the environment through excessive use of fuels and accidental spills during transportation and storage. Alkanes, a major fraction of crude oil, are saturated hydrocarbons and hence are chemically inert as non-polar molecules. However, a number of bacterial and fungal genera have been reported to degrade even high molecular weight alkanes in both aerobic and anaerobic conditions. The degradation process mainly involved enzymes such as methane monooxygenase (MMO), alkane hydroxylase and cytochrome P450 monooxygenase. Besides, a number of environmental factors affect the degradation of alkanes in soils.
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Chapter 17
Microbial Degradation of Alkanes
S. N. Singh, B. Kumari and Shweta Mishra
17.1 Introduction
Petroleum hydrocarbons are introduced into the environment due to their extensive
use as fuels and chemicals. Besides, leaks and accidental spills occur often during
exploration, production, refining, transport and storage of petroleum and petroleum
products which used to add an additional burden of hydrocarbons to soils and
water systems. The technologies commonly used for soil remediation of petroleum
hydrocarbons include mechanical burying, evaporation, dispersion and washing.
These remedial measures are not only cost intensive and time consuming, but
also not very effective. On the other hand, bioremediation leads to complete
mineralization of organic compounds into CO
2
and water by indigenous micro-
organisms and hence a preferred choice also being eco-friendly and cost-effective.
Anthropogenic hydrocarbon contamination of soil is a global issue throughout
the industrialised world (Macleod et al. 2001; Brassington et al. 2007). In England
and Wales alone, 12% of all serious contamination incidents in 2007 were
hydrocarbon related. Soil acts as a repository for many hydrocarbons, which is a
serious concern due to their adverse impact on human health and environmental
persistence for a long time (Jones et al. 1996; Semple et al. 2001).
Alkanes are a major fraction ([50%) of the crude oil depending upon the
oil source. Alkanes are saturated hydrocarbons and chemically very inert as
apolar molecules (Labinger and Bercaw 2002). They may be classified as linear
(n-alkanes), cyclic (cyclo-alkanes) or branched (iso-alkanes) and found in three
states: gaseous (C1–C4), liquid (C5–C16) and solid ([C17) (Fig. 17.1). Although
S. N. Singh (&)B. Kumari S. Mishra
Environmental Sciences Division, CSIR-National Botanical
Research Institute, Lucknow 226 001, India
e-mail: drsn06@gmail.com
S. N. Singh (ed.), Microbial Degradation of Xenobiotics,
Environmental Science and Engineering, DOI: 10.1007/978-3-642-23789-8_17,
ÓSpringer-Verlag Berlin Heidelberg 2012
439
highly inflammable, alkanes are less reactive as organic compounds. They are
highly essential for modern life, but their inertness poses serious ecological
problems when released to the environment. However, microbes have developed
effective strategies involving specific enzymes and metabolic pathways to use
n-alkanes as a carbon source. Thus, microbes have the capability to degrade
alkanes and convert them to easily metabolizable substrates.
17.2 Microbial Degradation of Alkanes
Due to lack of functional groups as well as very low water solubility, aliphatic
hydrocarbons exhibit both, low chemical reactivity and bioavailability for
microorganisms. However, some microorganisms possess the metabolic capacity
to use these compounds as carbon and energy sources for their growth
(Berthe-Corti and Fetzner 2002).
A number of microbes including bacteria, fungi and yeasts have been reported
to degrade alkanes using them as the source of carbon and energy (van Beilen et al.
2003; Wentzel et al. 2007). Bacteria with alkane degradation ability have
also versatile metabolism to use other compounds in addition to alkanes as
source of carbon (Margesin et al. 2003; Haryama et al. 2004). Use of bacteria
in the degradation of alkane compounds has been extensively studied by
Haryama et al. (2004). Many microbes have been reported for the degradation of
aliphatic compounds, such as Arthrobacter sp., Acinetobacter sp., Candida sp.,
Pseudomonas sp., Rhodococcus sp., Streptomyces sp., Bacillus sp., Aspergillus
japonicus,Arthrobacter sp., Acinetobacter sp., etc.In addition, some bacterial
species are reported as highly specialized in degrading hydrocarbons and hence
called hydrocarbonoclastic bacteria. They play a key role in the removal of
hydrocarbons from the polluted environments (Head et al. 2006; Yakimov
et al. 2007). Schneiker et al. (2006) found a marine bacterium (Alcanivorax
borkumensis) capable of assimilating both linear or branched alkanes, but unable
to metabolize aromatic hydrocarbons. Alcanivorax dieselolei,ag-proteobacterium,
is also a member of the hydrocarbonoclastic bacteria and cannot assimilate sugars
Fig. 17.1 Examples
of linear; n-Hexane
(a) branched; Iso-hexane
(b) and cyclic alkanes;
Cyclopentane (c)
440 S. N. Singh et al.
or amino acids as sources of energy and carbon. But it can utilize some organic
acids and alkanes. Notably, the spectrum of alkanes utilized by A. dieselolei
(C5–C36) (Liu and Shao 2005) is substantially broader than those of most other
previously described alkane degraders (van Beilen and Funhoff 2007). Other
alkane degrading bacterial genera are Thalassolitus (Yakimov et al. 2004), Ole-
iphilus (Golyshin et al. 2002), Bacillus,Geobacillus (Marchant et al. 2006),
Thermus (Meintanis et al. 2006) and Oleispira (Yakimov et al. 2003).
Acinetobacter sp. was found to be capable of utilizing n-alkanes of chain length
C10–C40 as a sole source of carbon (Throne-Holst et al. 2007). Other bacterial
genera, namely, Gordonia,Brevibacterium,Aeromicrobium,Dietzia,Burkholderia
and Mycobacterium isolated from petroleum contaminated soil were proven to be
potential degraders of hydrocarbons (Chaillan et al. 2004). Hexadecane degrada-
tion was observed by the bacteria, such as Pseudomonas putida,Rhodococcus
erythroplotis and Bacillus thermoleovorans (Abdel-Megeed et al. 2010) and two
bacterial strains; Flavobacterium sp. ATCC39723 and Arthrobacter sp. (Steiert
et al. 1987). Hexadecane (HXD) is present in the aliphatic fraction of crude oil and
is one of the major components of diesel (Chenier et al. 2003). Volke-Sepulveda
et al. (2003) demonstrated that HXD biodegradation by Aspergillus niger was
considerably higher in SSF (Solid state fermentation) than in submerged fer-
mentation. Complete HXD conversion was achieved at a C/N ratio of 29
under SSF conditions (Stroud et al. 2008). Desulfatibacillum alkenivorans AK-01
is a mesophilic sulfate-reducer isolated from estuarine sediment which utilizes
C13–C18 alkanes, 1-alkenes (C15 and C16) and 1-alkanols (C15 and C16) as
growth substrates.
Thermophilic alkane degrading bacterium, Goebacillus thermoleovorans (pre-
viously Bacillus thermoleovorans) B23 was reported from a deep-subsurface oil
reservoir in Japan (Kato et al. 2001). This strain effectively degraded alkanes at
70°C with the carbon chain longer than dodecane (C12). Since tetradecanoate and
hexadecanoate or pentadecanoate and heptadecanoate were accumulated as deg-
radation intermediates of hexadecane or heptadecane degradation, respectively, it
indicated that the strain B23 degraded alkanes by a terminal oxidation pathway,
followed by b-oxidation pathway. Recently, another long chain alkane degrading
Geobacillus thermodenitrificans NG80-2 was also isolated from a deep sub
-surface oil reservoir and its complete genome sequence was determined (Feng
et al. 2007).
Some organisms adapted to cold environment are capable of degrading high
molecular weight petroleum hydrocarbons. Whyte et al. (1998) reported that
Rhodococcus sp. strain Q15 was able to degrade alkanes up to n-C21 as well as
some branched alkanes in diesel, and could also grow on dotriacontane (n-C32).
Rhodococcus strains capable of growing on eicosane (n-C20) have been reported by
Bej et al. (2000). Studies on petroleum biodegradation in soils from cold regions
have reported that lower-molecular weight n-alkanes and unsubstituted aromatic
hydrocarbons are biodegraded preferentially over the relatively higher-molecular
weight n-alkane compounds, isoalkanes, alkylated aromatic hydrocarbons, isopre-
noids and the branched and cyclic hydrocarbons (Sanscartier et al. 2009).
17 Microbial Degradation of Alkanes 441
Besides, many yeasts and fungi, are also known to thrive on alkanes
(van Beilen et al. 2003). Among fungal genera, Amorphoteca,Neosartorya,
Talaromyces and Graphium and yeast genera, Candida,Yarrowia and Pichia,
isolated from oil-contaminated soil were found potential degraders of petroleum
of petroleum hydrocarbons (Chaillan et al. 2004). Singh (2006) has reported a
group of fungi, namely Aspergillus,Cephalosporium and Pencillium to be high
degraders of crude oil hydrocarbons. Among yeast species, Candida lipolytica,
Rhodotrula mucilaginosa,Geotrichum sp. and Trichosporam mucoides isolated
from contaminated water were capable to degrade petroleum compounds effec-
tively (Boguslawska-Was and Dabrowski 2001). New genera containing alkane
degraders are constantly being identified, leading to a better understanding of
ecosystems.
17.2.1 Uptake of n-Alkanes
Alkanes are insoluble in water. The solubility of alkanes depends largely on the
molecular weight. With the increase in molecular weight, the solubility decreases
in water (Eastcott et al. 1988). Hydrocarbons with a chain length C12 and above
are virtually water insoluble. It is still not very clear how alkanes enter the cells of
bacteria. The uptake mechanism depends on the bacterial species, the molecular
weight of alkane and physico-chemical environment (Wentzel et al. 2007). Low
molecular weight alkanes are sparingily soluble in water to ensure a sufficient mass
transfer to bacterial cell, while high molecular weight (medium and long chain
n-alkanes) alkanes find their accessibility to cell either by adherence or by a
surfactant-mediated process. This is the reason that alkane degrading bacteria
produce diverse surfactants which facilitate the emulsification of hydrocarbons
(Ron and Rosenberg 2002). Noordman and Janssen (2002) have reported an
increase in the uptake of alkanes in presence of biosurfactants, such as hexadecane
in cultures, however, their role in soils and other environments is still not very
evident (Holden et al. 2002).
In addition, biosurfactants may also facilitate cell mobility and adhesion to
surfaces or biofilms (Boles et al. 2005). They also shield bacterial cells from direct
exposure to toxic substances (Kang and Park 2009). Depending on the solubility,
the alkanes may be arranged as follow: linear alkanes [branched alka-
nes [cyclic alkanes with regard to their susceptibility to microbial degradation.
17.2.2 Aerobic Degradation of Alkanes
Aerobic alkane degraders activate alkane molecules using O
2
as a reactant. The
alkane-activating monooxygenase overcomes the low reactivity of the hydrocarbon
by producing reactive oxygen species. Oxidation of methane leads to formation of
442 S. N. Singh et al.
methanol which is subsequently transformed to formaldehyde and then to formic
acid (Fig. 17.2). This compound either gets converted to CO
2
or assimilated for
biosynthesis of other organic compounds either by the ribulose monophosphate
pathway or by the serine pathway depending upon the organism (Lieberman and
Rosenzweig 2004). The complete degradation of hydrocarbons mainly occurs
under aerobic conditions (Riser-Robert 1998). This process involves several
steps as illustrated in Fig. 17.3: (1) Accessibility of chemicals to microbes
having degradation ability. Since hydrocarbons are insoluble in water, their
degradation essentially requires biosurfactants which are produced by bacteria.
(2) Activation and incorporation of oxygen is the vital reaction catalysed by
oxygenase and peroxidase. (3) Peripheral degradation pathways which
convert hydrocarbons into intermediates of the tricarboxylic acid cycle (TCA)
and (4) Biosynthesis of cell biomass from the central precursor metabolites
i.e. acetyl-CoA, succinate and pyruvate, sugars are required for various bio-
synthesis and gluconeogenesis for growth.
Degradation of n-alkanes is initiated by the oxidation of a terminal methyl
group to render a primary alcohol, which gets further oxidized to the corre-
sponding aldehyde, and finally converted into a fatty acid. Fatty acids are con-
jugated to CoA and further processed by b-oxidation to generate acetyl-CoA
(Wentzel et al. 2007) (Fig. 17.4). However, in some cases, both ends of the
alkane molecule are oxidized through x-hydroxylation of fatty acids at the ter-
minal methyl group (xposition), rendering an x-hydroxy fatty acid that is
further converted into a dicarboxylic acid and processed by boxidation (Coon
2005). Sub-terminal oxidation of n-alkanes has also been reported (Kotani et al.
2007). The product generated a secondary alcohol which is converted to the
corresponding ketone, and then oxidized by a Baeyer–Villiger monooxygenase to
render an ester. The ester is hydrolysed by an esterase, generating an alcohol and
a fatty acid. Both terminal and sub-terminal oxidation can co-exist in some
microorganisms.
Fig. 17.2 Aerobic pathways of methane oxidation (after Rojo 2009)
17 Microbial Degradation of Alkanes 443
Some strains of Pseudomonas are able to utilize alkanes as the sole carbon and
energy source (Stanier et al. 1966). The initial pathway of alkane oxidation is the
following:
RCH3!RCH2OH !RCHO !RCOOH
This pathway has been established by simultaneous adaptation experiments
(Heringa et al. 1961) and chromatographic analysis of the products of alkane
oxidation (Thijsse and van der Linden 1963). Acinetobacter spp. can split a
hydrocarbon at the number of ten position, forming hydroxyl acids. The initial
steps appear to involve terminal attack to form carboxylic acid, sub-terminal
dehydrogenation at the number ten position to form an unsaturated acid, and
splitting of carbon chain to form a hydroxyl acid and alcohol. Highly branched
isoprenoid alkanes, such as Pristane, have been found to undergo x-oxidation with
the formation of dicarboxylic acids as the major degradative pathway.
Fig. 17.3 Process of microbial aerobic degradation of hydrocarbons associated with growth
process (after Fritsche and Hofrichter 2000)
444 S. N. Singh et al.
Methyl branching increases the resistance of hydrocarbons to microbial
attack. Methyl branching at b-oxidation requires an additional strategy, such as
a-oxidation, x-oxidation or balkyl group removal (Atlas 1981). Acremonium spp.
oxidize ethane to ethanol by NADPH dependent monooxygenase, which is sub-
sequently oxidized to acetaldehyde and acetic acid. Acetate, thus formed, is
assimilated into cellular carbon via reverse tricarboxylic acid cycle and glyoxalate
Fig. 17.4 Aerobic pathways of n-alkane degradation (after Fritsche and Hofrichter 2000)
17 Microbial Degradation of Alkanes 445
bypass. Similarly, a number of propane and butane utilizers have been reported
that are also capable of growth on long chain alkanes, such as n-dodecane and
n-hexadecane.
Long chain hydrocarbons (C10–C18) can be used rapidly by many high G ?C
Gram-positive bacteria, but only a few bacteria can oxidize C2–C8 hydrocarbons.
Degradation of n-alkanes requires activation of the inert substrates by molecular
oxygen with the help of oxygenases by three possible ways that are associated with
membranes:
1. Monooxygenase attacks at the end producing alkan-1-ol:
RCH3þO2þNAD PðÞHþHþ!RCH2OH þNAD PðÞ
þþH2
2. Dioxygenase attack produces hydroperoxides, which are reduced to yield also
alkan-1-ol:
RCH3þO2!RCH2OOH þNAD PðÞHþHþ
!RCH2OH þNAD PðÞ
þþH2O
3. Rarely, subterminal oxidation at C
2
by monooxygenase yields secondary
alcohols.
Brevibacterium erythrogenes can use 2-methylundecane as substrate for growth
by a combination of x- and b-oxidation. Arthrobacter sp. has been reported to
metabolize squalene (C30-multiple, methyl branched compound) to geranylace-
tone, which is accumulated in the medium as it cannot be further metabolized.
Similarly, Corynebacterium sp. and B. erythrogenes have been shown to degrade
pristane (2,6,10,14-tetramethyl pentadecane) involving x-oxidation, followed by
b-oxidation, yielding propionyl-CoA and acetyl-CoA units alternately.
17.2.3 Anaerobic Degradation of n-Alkanes
Apart from aerobic oxidation, anaerobic degradation also plays an important role
in the recycling of hydrocarbons in the environment. Alkanes are also degraded
through anaerobic process as reported by various workers (Callaghan et al. 2009;
Higashioka et al. 2009). There are two known pathways of anaerobic n-alkanes
degradation (Fig. 17.5). First pathway is the alkane addition to fumarate, and
second is through putative pathways (So et al. 2003). Fumarate addition proceeds
via terminal or sub-terminal addition of the alkanes to the double bond of fumarate,
resulting in the formation of alkyl succinate which is further degraded via
carbon skeleton rearrangement and b-oxidation. Alkane addition to fumarate has
been documented for denitrifying bacteria (Wilkes et al. 2002), sulphate reducing
consortia (Kniemeyer et al. 2007) and sulphate reducing bacteria (Callaghan
et al. 2006; Kniemeyer et al. 2007). Azoarcus sp. HxN1, a denitrifying bacterium,
uses C6–C8 alkanes, while Desulfobacterium Hdx3 metabolizes C12–C20 alkanes
(reviewed in Widdel and Rabus 2001).
446 S. N. Singh et al.
Zedelius et al. (2011) studied alkane degradation under anaerobic conditions by
a nitrate reducing bacterium to find out involvement of electron acceptor in sub-
strate activation. Three bacterial isolates (HXN1, OcN1, HdN1) which were able
to grow under aerobic conditions by coupling alkane oxidation to CO
2
with NO
3
-
reduction to N
2
, were compared for alkane metabolism (Fig. 17.6). Out of which,
Fig. 17.5 Anaerobic activation of short chain alkanes by furarate addition. The formed meth-
ylalkylsuccinates are activated by binding with acetyl-coenzyme A (CoA), which yields a thioester
that undergoes C-skeleton rearrangement, followed by decarboxylation and b-oxidation. aActivation
of the secondary carbon in propane. bActivation of the primary carbon in propane, which requires
more energy. * indicates the position of the radical carbon (after Kniemeyer et al. 2007)
17 Microbial Degradation of Alkanes 447
two strains HXN1 and OcN1 (both Betaproteobacteria) metabolized C6–C8 and
C8–C12 alkanes, respectively. Both of them activated alkanes anaerobically in a
fumarate-dependent reaction yielding alkylsuccinates as evidenced by metabolite
and gene analyses. However, strain HdN1 was unique. It belonged to Gamma-
proteobacteria and utilized alkanes in the range of C6–C30. It also did not indicate
fumarate-dependent alkane activation. While HXN1 and OcN1 grew on alkanes
and NO
3
-
,NO
2
-
or N
2
O added to medium, strain HDN1 oxidized alkanes only with
NO
3
-
or NO
2
-
but not with N
2
O. Since N–O species are the strong oxidants, these
Fig. 17.6 Hypothetical involvement of denitrification intermediates in alkane activation. A small
proportion of NO
2
-
or NO is deviated from the respiratory chain for alkane activation. They may
be used for activation indirectly (by yielding O
2
that is used by alkane monooxygenase; or by
giving rise to another reactive factor or enzyme centre) or directly (as co-reactants introducing a
polar group). The alkyl residue R0may or may not be identical with the original residue R
(depending on the activation mechanism and alkane C-atom being attacked). FA, fatty acid; TCA,
tricarboxylic acid cycle (after Zedelius et al. 2011)
448 S. N. Singh et al.
strains may not activate alkane under the conditions of sulphate reduction or
methanogenesis and allow a special mode of alkane activation.
Squalane (2,6,10,15,19,23-hexamethyltetracosane) is susceptible to microbial
degradation and Actinomycetes, in particular, and those belonging to the genus
Mycobacterium, are potent degraders of this multibranched saturated hydrocarbons.
The putative pathway demonstrated that after the conversion of squalane to a dioic
acid as one of the first intermediates, two propionyl- coA and acetyl-CoA molecules
are oxidatively removed by b-oxidation route to form 3,7,11-trimethyldodecandioic
acid as intermediate by a pathway analogous to that for degradation of the
multiple branched alkane pristane (2,3,10,14-tetramethylpentadecane) (Berekaa
and Steinbüchel 2000).
17.2.4 Non-Conventional Dissimilation Pathway
Sakai et al. (1996) observed a non-conventionaldissimilationpathway in Acinetobacter
sp. M1 in which n-alkanes are postulated to be converted to acid:
RCH3!RCH2OOH !RCO(O)OH !RCHO !RCOOH (Finnerty 1988Þ:
However, there is little information available on the enzymes involved in the
postulated pathway, particularly at the first step. They identified an enzyme—a
flavoprotein which needed O
2
and Cu
2+
for expression of its activity, but did not
require NAD(P)H as a coenzyme. The enzyme reaction yielded hydroperoxide
and the enzyme involved in n-alkane oxidation is likely to be a dioxygenase.
Further, the postulated pathway is supported by the following observations: (1)
n-alkane monooxygenase activity not detected, (2) low activity of fatty alcohol
dehydrogenase, (3) induction of NAD(P)H-dependent long chain fatty aldehyde
dehydrogenase in n-alkane grown cells.
Meng et al. (1996) isolated three kinds of enzymes designated A, B and C found in
the cytoplasm of n-alkane grown Acinetobacter sp. M1, that catalyzed dioxygenation
of n-alkanes to the corresponding n-alkyl hydroperoxides. Purified enzyme A con-
sisted of four identical subunits having a molecular mass and strongly inhibited by
several iron-chelating agents. Enzymes B and C were more active towards relatively
short n-alkanes (C12–C16) where as enzyme A oxidized solid n-alkanes with the
most preferable substrate being Tetracosane C24.
17.3 Oil Alkanes
Alkanes are the most important fraction of crude oil. The anaerobic degradation of
alkanes is today of great significance for the oil industry. It is well established that
microbial activities associated with oil reservoirs led to the decrease of oil quality,
making refining more costly and recovery more difficult (Head et al. 2003).
17 Microbial Degradation of Alkanes 449
Because of presence of microbial communities mainly dominated by anaerobes,
the oil reserves are referred as ‘geo bioreactors’, in which fermentative, syn-
trophic, suthdogenic and methanogens are responsible for removal of alkanes from
the saturated hydrocarbon fraction (Jones et al. 2008; Wang et al. 2010). More-
over, biogenic CH
4
production is the result of microbial degradation of oil alkanes.
Since world demand for methane is likely to increase many folds in coming
decades, the methanogenic conversion of oil alkanes to CH
4
is seen as a future
solution for world increasing demand of energy (Fig. 17.7).
17.4 Enzymes Involved in Alkane Degradation
Ayala and Torres (2004) have indicated the involvement of three major
enzymes in the degradation of alkanes; Methane monooxygenase (MMO), Alkane
hydroxylase (Alk) and Cytochrome P450 monooxygenase (Fig. 17.8).
17.4.1 Methane Monooxygenase
Methane monooxygenase is expressed in microorganisms to use CH
4
as energy
source and found in methanotrophs in two forms pMMO (particulate Methane
Fig. 17.7 Presumptive methanogenic degradation of oil alkanes (after Mbadinga et al. 2011)
450 S. N. Singh et al.
monooxygenase) and sMMO (soluble Methane monooxygenase). While pMMO is
a membrane-bound protein produced by all methanotrophs, sMMO is expressed by
a subset of methanotrophs. pMMO is an iron–copper protein, produced under
conditions of copper sufficiency (Nguyen et al. 1994) where as sMMO is an iron-
containing enzyme produced only under Cu-depleted sites (Murrell et al. 2000b).
sMMO is comprised of three components; an oxygenase, a reductase and a cou-
pling protein (Fox et al. 1989). The NADH-dependent oxidation reaction catalysed
by sMMO is reflected in Fig. 17.9. Both sMMO and Alk are characterized by the
presence of diiron cluster in the hydroxylase component. The metallic center
activates dioxygen during the oxidation of substrates. However, in sMMO, the
diiron cluster is bridged by carboxylic residues, similar to the diiron centers of
proteins, such as ribonucleotide reductase R2, stearoyl-ACP-9 desaturase and other
monooxygenases, such as alkene monooxygenases, phenol monooxygenases and
toluene monooxygenases (Leahy et al. 2003). sMMO shows a wide range of
substrate specificity, including alkenes, aromatic, alicyclic and hetrocyclic com-
pounds where as pMMO mediates the oxidation of a small group of alkanes (Murell
et al. 2000a). Four different reaction mechanisms of sMMO for hydrocarbon
hydroxylation have been suggested: (1) hydrogen atom abstraction from the sub-
strate followed by radical recombination (Fox et al. 1990), (2) cation formation by
electron abstraction from the substrate radical intermediate generated in first step
followed by reaction with metal bound hydroxide (Jin et al. 2001), (3) direct insertion
of the oxygen atom into the C–H bond (Valentine et al. 1997) and (4) cation for-
mation on the substrate by transfer of a protonated oxygen from a hydroperoxy
intermediate (derived from O
2
), followed by loss of water (Choi et al. 1999).
Similar to sMMO, butane monooxygenase (BMO) is a non-heme iron
monooxygenase and it can hydroxylate C2–C9 alkanes (Dubbels et al. 2007).
Fig. 17.8 a Methane to methanol by Methane Monooxygenase (MMO), bButane to 1- butanol
by Butane Monooxygenase (BMO), cOctane to 1-Octanol, dOctane to 2-Octanol, eFarnesol to
1-hydroxyfarnel (after van Beilen and Funhoff 2005)
17 Microbial Degradation of Alkanes 451
Chaperonin-like protein, BmoG is required for proper assembly of BMO (Kurth
et al. 2008). However, its specificity is towards producing the terminal alcohols,
unlike sMMO that produces sub-terminal alcohols.
Recently, a unique alkane monooxygenase that belongs to luciferase family
was reported for G. thermodenitrificans (Li et al. 2008). Kato et al. (2009) reported
that three novel membrane proteins, superoxide dismutase, catalase, and acyl-CoA
oxidase in G. thermoleovorans B23 which were previously reported only in yeast,
such as C. tropicalis (Shimizu et al. 1979), Activities of these enzymes were
dramatically increased in the cells of G. thermoleovorans B23 when they were
grown on alkanes.
17.4.2 Alkane Hydroxylase
This enzyme is three component monooxygenase, comprising a hydroxylase, a
rubredoxin and rubredoxin reductase (Shanklin et al. 1997). The hydroxylase
component is membrane-bound, while both rubredoxin and rubredoxin reductase
components are soluble and cytoplasmic proteins. This enzymatic complex is able
to oxidize medium and long chain linear alkanes using reducing equivalents from
NADH or NADPH.
AlkB, an integral membrane protein, carries out a terminal hydroxylation of
n-alkane (Kok et al. 1989). The electrons needed to carry out this step are
delivered to AlkB via a rubredoxin reductase (AlkT) and two rubredoxins (AlkF
and AlkG) (van Beilen et al. 2002). The resulting alcohol is further converted to a
fatty acid via a pathway involving an alcohol dehydrogenase (AlkJ), an aldehyde
dehydrogenase (AlkH) and an acyl-CoA synthetase (AlkK), that enters the b
oxidation pathway (van Beilen et al. 2001). The histidine residues are required
for activity in the members of this family (Shanklin et al. 1994). There is a
Fig. 17.9 Steps involved in the oxidation reaction catalysed by alkane hydroxylase (a) and
methane monooxygenase and cytochrome P450 monooxygenase (b) (after Ayala and Torres
2004)
452 S. N. Singh et al.
conserved NYXEHYG(L/M) motif in all identified alkane hydroxylases (Smits
et al. 2002). This motif has been proposed as a signature for membrane-bound
alkane hydroxylases (Smits et al. 2002).
Although crystal structure of Alk is not known, it is believed to have six
transmembrane segments and a catalytic site that faces the cytoplasm. The active
site includes four His-containing sequence motives that are conserved in other
hydrocarbon monooxygenases which chelate two iron atoms (Shanklin et al.
1994). The diiron cluster allows the O
2
-dependent activation of the alkane
through a substrate radical intermediate (Shanklin et al. 1997; Bertrand et al.
2005). One of the O
2
atoms is transferred to the terminal methyl group of the
alkane, rendering an alcohol, while the other one is reduced to H
2
O by electrons
transferred by the rubredoxin. Oxidation is regio- and stereospecific (van Beilen
et al. 1995).
Baptist et al. (1963) have identified an enzyme system from Pseudomonas
putida PpG6 grown on alkanes which is capable of oxidizing octane to octanoic
acid, and the properties of the enzyme complex, which catalyzes the initial
hydroxylation reaction, have been extensively studied (Mckenna and Coon
1970). In vitro, this hydroxylase complex is also capable of omega-oxidizing
fatty acids (Mckenna and Coon 1970). This suggests that the oxidation of alkane
and fatty acid chains might occur from both ends in strains with a functional
hydroxylase.
The AlkB protein from Pseudomonas putida GPo1 is presently the best charac-
terized Alk (van Beilen et al. 1994). It catalyses the first step of alkane degradation
with the help of two electron transfer proteins, rubredoxin (AlkG) and rubredoxin
reductase (AlkT) (van Beilen et al. 1994). Over the past decade, alkB-like hydrox-
ylase genes have been detected in a wide range of alkane degrading bacteria,
including a-, b- and g-proteobacteria; as well as in some high G ?C content Gram-
positive bacteria (Smits et al. 2002). Many of these contain more than one alkB
homologue, such as Pseudomonas aeruginosa PAO1 (alkB1 and alkB2),
Rhodococcus erythropolis Q15 (alkB1-4) and Acinetobacter sp. M1 (alkMa and
alkMb).
The enzymes, that oxidize alkanes larger than C20, seem to be totally different.
For example, Acinetobacter sp. M1, which can grow on C13–C44 alkanes, con-
tains a soluble, Cu
2+
-dependent Alk that is active on C10–C30 alkanes. It has been
proposed to be a dioxygenase that generates n-alkyl hydroperoxides to render the
corresponding aldehydes (Tani et al. 2001). A different Acinetobacter strain, DSM
17874, has been found to contain a flavin-binding monooxygenase, named AlmA,
which oxidizes C20 to [C32 alkanes (Throne-Holst et al. 2007). Genes homolo-
gous to almA have been identified in several other long chain n-alkane degrading
strains, including Acinetobacter sp. M1 and A. borkumensis SK2. A different long
chain alkane hydroxylase, named LadA, has been characterized in Geobacillus
thermodenitrificans NG80-2 (Feng et al. 2007). It oxidizes C15–C36 alkanes,
generating primary alcohols. Its crystal structure has shown that it is a two-com-
ponent flavin-dependent oxygenase belonging to the bacterial luciferase family of
proteins (Li et al. 2008).
17 Microbial Degradation of Alkanes 453
17.4.3 Cytochrome P450 Monooxygenase
These enzymes are heme proteins and catalyze the hydrocarbons using NAD(P)H
as cofactor. They usually consist of two components; hydroxylase and reductase
(Sono et al. 1996). These enzymes are usually membrane-bound and have a multi-
component nature (Ayala and Torres 2004).
The molecular mechanisms of oxygen activation for some metalloenzymes are
well investigated. Heme-oxygenases, such as CYP, hydroxylate inert hydrocarbon
substrates by using a high-valent oxoiron(IV) porphyrin p-cation-radical
intermediate similar to peroxidase compound I (Groves 2005). The consensus
mechanism for oxygen activation and transfer involves a hydrogen atom
abstraction-oxygen rebound pathway (Groves 2003,2005). Hydroxylation of the
very unreactive C–H bond of methane by non-heme diiron enzyme sMMO has
many similarities to the P450 mechanism (Kopp and Lippard 2002; Newcomb
et al. 2002).
Cytochrome P450 monooxygenase (CYP), present in certain strains of yeast
Candida, is able to convert[C12 alkane by a,x-oxidation to the corresponding
dicarboxylic acids. The x-oxidation of the alkane to alcohol is first reaction to
be catalyzed by a hydroxylase complex composed of a CYP monooxygenase
and NADPH and CYP oxireductase. Further oxidation to the acid is catalysed
by fatty alcohol oxidase and a fatty aldehyde dehydrogenase (Gallo et al. 1973).
Vatsyayan et al. (2008) studied the cytochrome P450 monooxygenase activity
in the cells of Aspergillus terreus MTCC6324 and found that CYP catalysis of
n-Hexadecane had followed both terminal and sub-terminal oxidations. The
activity was localized in cytosol of n-hexadecane grown cells. CYP activity was
obtained only when NADH was used as co-factor. No other compounds
checked, such as NAD, NADP, NADPH, FMN, FAD and FADH
2
, could serve
as co-factor of the enzyme. Size of isolated enzyme was closer to that reported
for Fusamarium oxysporum i.e. 118 kDa (Nakayama et al. 1996). The presence
of secondary alcohol oxidase in mitochondrial fraction indirectly supports
the existence of n-alkanes sub-terminal oxidation. van Beilin and Funhoff
(2005) reported the sub-terminal oxidation of long chain alkane by bacteria
and yeast.
In addition to these enzymes, other catabolic enzymes are also reported from
the different microorganism as shown in Table 17.1.
17.5 Recombinant Bacteria for Alkane Degradation
Due to multi-component nature, recombinant production of CYP450 is difficult,
but CYP BM-3 is readily expressed in E. coli (Peter et al. 2003).
Rothen et al. (1998) constructed a plasmid with gene coding for the three
enzymes; alkane hydroxylase, alcohol dehydrogenase and aldehyde dehydroge-
nase simultaneously. The plasmid was inserted into an E. coli strain unable to
454 S. N. Singh et al.
metabolize fatty acids. The recombinant bacteria were able to oxidize octane to its
corresponding carboxylic acid.
Glieder et al. (2002) produced a mutant 139-3 that was capable to catalyze the
oxidation of medium chain alkanes. This mutant has the fastest known enzyme for
alkane hydroxylation (more than 17 times faster than the MMO or Alk enzymatic
systems).
A plasmid having three components of Alk system was introduced to a
Pseudomonas lacking the alcohol dehydrogenase. Now the recombinant bacteria
were able to transform C7–C11 alkanes to their corresponding alcohols (Bosetti
et al. 1992).
Throne-Holst et al. (2007) constructed alkMa, alkMb and alkMa/alkMb dist-
ruption mutants of Acinetobacter venetianus 642. Single and double mutants were
able to grow on n-alkanes ([C20).
Table 17.1 Different enzymes involved in alkane degradation (van Beilen et al. 2003)
Enzymes Microrganism Substrate Reference
sMMO Methylococcus C1–C10 Baik et al. (2003)
capsulatus
Methylisinus
trichosporum
OB3b
pMMO All methanotrophs C1–C5 Leieberman and
Rosenzweig
(2004)
Propane monooxygenase Pseudomonas C2–C8 Kotani et al. (2003)
butanovora
(ATCC 43655)
Butane monooxygenase Gordonia sp. TYP C3 and C13–C22 Sluis et al. (2002)
AlkB Acinetobacter, C5–C16 Smits et al. (2002)
Alcanivorax,
Burkholderia,
Mycobacterium,
Pseudomonas,
Rhodococcus etc.
Cytochrome P450 (CYP153)
monooxygenase
Sphingomonas sp. C4–C16 Maier et al. (2001)
HXN-200,
Mycobacterium sp.
HXN1500
Acinetobacter sp.
EB104
Cytochrome P450 (CYP52)
monooxygenase
Candida maltosa, C10–C16 Craft et al. (2003)
Candida tropicalls,
Yarrowia lipolytica
17 Microbial Degradation of Alkanes 455
17.6 Genes Involved in Alkane Degradation
The organization of the genes involved in alkane oxidation differs significantly
among alkane degrading bacteria (van Beilen et al. 2003). The alkane degradation
genes encoded by the OCT plasmid of P. putida GPo1 are clustered in two
operons, and this pathway has clearly been transferred horizontally to many
bacteria (van Beilen et al. 2001). When several alkane hydroxylases coexist in a
single strain, they are normally located at different sites in the chromosome.
Moreover, the regulators that control the expression of alkane degradation genes
may or may not map adjacent to the genes they regulate. Therefore, the degree of
clustering of alkane degradation genes is variable among bacterial strains.
Expression of the genes involved in the initial oxidation of alkanes is tightly
controlled. A specific regulator assures that the pathway genes are expressed only in
the presence of the appropriate alkanes. In addition, superimposed to this specific
regulation, there are several mechanisms that modulate the induction of the pathway
genes according to cell needs. The known specific regulators, that induce alkane
degradation genes in response to alkanes, belong to different families, such as the
LuxR/MalT, the AraC/XylS, the GntR or other non-related families of regulators.
The A. borkumensis AlkS transcriptional regulator is believed to activate expression
of the gene coding for the AlkB1 Alk and of downstream genes in response to alkanes
(van Beilen et al. 2004; Schneiker et al. 2006). In a proteomic study, this regulator
appeared associated to the membrane fraction, rather than to the cytoplasmic fraction
(Sabirova et al. 2006). Some bacterial strains contain only one alkane hydroxylase, as
is the case for the well-characterized alkane degrader P. putida GPo1. However,
many other strains have several alkane degradation systems, each one being active on
alkanes of a certain chain length or being expressed under specific physiological
conditions. For example, Acinetobacter sp. strain M1 contains two AlkB related
alkane hydroxylases, named AlkMa and AlkMb, which are differentially regu-
lated depending on the alkane present in the medium. Expression of AlkMa,
which is controlled by the AlkRa regulator, is induced by alkanes having a very
long chain length ([C22), while that of AlkMb is induced by AlkRb in the
presence of C16–C22 alkanes (Tani et al. 2001). A. borkumensis has two AlkB
like alkane hydroxylases and three genes coding for cytochromes P450 believed
to be involved in alkane oxidation (van Beilen et al. 2004; Schneiker et al.
2006). In addition, A. borkumensis seems to have other uncharacterized genes
involved in the oxidation of branched alkanes and phytane (Schneiker et al.
2006). Finally, a gene similar to Acinetobacter sp.DSM 17874 almA, which
oxidizes alkanes of very long chain length, has been predicted in A. borkumensis
SK2 (Throne-Holst et al. 2007). Expression of all these alkane oxidation genes
should be differentially induced according to the substrate present under each
circumstance. The three A. borkumensis genes coding for similar cytochromes
P450 of the CYP153 family are believed to participate in alkane degradation
(Schneiker et al. 2006). Cytochrome P450-1 maps adjacent to other genes
involved in the oxidation of alkanes. Cytochrome P450-2 is identical to P450-1,
456 S. N. Singh et al.
and highly homologous to P450-3. Proteomic profiling analyses revealed that
P450-1 and/or P450-2, which cannot be differentiated with this technique, are
expressed in cells grown with either pyruvate or hexadecane as the carbon
source, although expression was higher in alkane-grown cells (Sabirova et al.
2006). As P450-1 is probably co-transcribed with other adjacent genes that
are upregulated by hexadecane, it is likely that expression of P450-1 is induced
by hexadecane but, not that of P450-2 and P450-3. A gene coding for a tran-
scriptional regulator of the AraC family maps close to P450-1.
Certain plasmids play an important role in adaptation of natural microbial pop-
ulations to oil and other hydrocarbons. Some of the microbial catabolic pathways
responsible for the degradation, including the alk (C5–C12 n-alkanes), nah (naph-
thalene) and xyl (toluene) pathways, have been extensively characterized and are
generally located on large catabolic plasmids (Gary et al. 1990), but many reports
describe and characterize microorganisms that can catabolize both aliphatic
and aromatic hydrocarbons (Foght et al. 1990). Several environmental isolates of
Acinetobacter sp. and Alcaligenes sp. (Lal and Khanna 1996), Arthrobacter sp.
(Efroymson and Alexander 1991) and two Rhodococcus strains (Malachowsky et al.
1994) have been found to degrade both alkanes and naphthalene.
Vomber and Klinner (2000) used gene probe derived from alkB gene of
Pseudomonas aleovorans ATCC 29347 to test the ability to assimilate short/
medium chain of 54 bacetrial strains belonging to 37 species. The derived amino
acid sequence of the alkB-amplificate of Pseudomonas aureofaciens showed high
homology (95%) with AlkB from P. oleovorans. AlkB gene disruptants were not
able to grow on decane.
17.7 Environmental Factors Regulating Biodegradation
of Alkanes
Additional factors that influence the degradation process included soil pH, mois-
ture and organic matter content and hydrocarbon aqueous solubility, octanol water
partitioning coefficient and structure (Leahy and Colwell 1990; Ramírez et al.
2008). Effective biodegradation is dependent upon optimal biological (microbial
functionality and biomass size), chemical (bioavailability and nutrients) and
physical (water holding capacity) parameters (Towell et al. 2011).
17.7.1 Structure and Physical State
n-alkanes of intermediate chain length (C10–C24) are degraded most rapidly.
Short chain alkanes (\C9) are toxic to many microorganisms, but being volatile,
they are generally lost rapidly to the atmosphere. Higher chain length alkanes are
generally resistant to biodegradation. Branching in alkanes generally reduces the
rate of biodegradation. The bioavailability of hydrocarbons, which is largely a
17 Microbial Degradation of Alkanes 457
function of concentration and physical state, hydrophobicity, sorption onto soil
particles, volatilization and solubility of hydrocarbons, greatly affects the extent of
biodegradation.
Water solubility of decane d10 is 0.052 mg/l, but the solubility of octadecane
is almost tenfold less (0.006 mg/l). The water solubility of butane (C4) is
61.4 mg/l, but very toxic to cells. Short chain alkanes are toxic to microorganisms,
because their increased water solubility results in increased uptake of the alkanes.
17.7.2 Temperature
Merin and Bucala (2007) reported that increase in temperature made the biological
membranes to have more fluid due to increased vibrational activity to the fatty acid
chains in the phospholipids bilayer. The increase in the rate of fluidity helps in
increasing the rate of substance uptake from a cell’s surrounding medium.
Biodegradation of hydrocarbon has been shown to occur over a wide range of
temperature from 0°C to as high as 70°C, though, in general optimum degradation
occurs in the mesophilic temperature range. It also affects the solubility of
hydrocarbon and enzyme activity. The stability of the enzyme CYP P450 mono-
oxygenase in Aspergillus terreus MTCC6324 ranges between 25–40°C, maximum
being at 25°C (Vatsayayan et al. 2008).
17.7.3 Nutrients
van Hamme et al. (2003) reported that nitrogen and phosphorus contents greatly
affect the microbial degradation of hydrocarbons. They further stated that
adjustment of the ratios of N and P by their addition in the form of slow releasing
fertilizers stimulated the biodegradation of hydrocarbons. Östeberg et al. (2006)
found accelerated biodegradation of n-alkanes in aqueous solution by the addition
of fermented whey. Bulking agents, such as compost, will enhance metabolism of
organic contaminants because they provide extra nutrients, additional carbon
sources and assist in retaining moisture content of the pile.
The increase in C/N ratios reduced the hexadecane (HXD) biodegradation.
Limitation of microbial growth and metabolism in polluted soils can be related to the
low concentration of inorganic nutrients, such as nitrogen, phosphorous and potas-
sium, producing high C/N, C/P and C/K ratios (Volke-Sepulveda et al. 2006).
17.7.4 Oxygen
Hydrocarbons being highly reduced substrates, require an electron acceptor, with
molecular oxygen being the most common. Though most studies have shown
biodegradation of hydrocarbon to be an aerobic process, anaerobic biodegradation
458 S. N. Singh et al.
of hydrocarbons has also been reported. In the absence of molecular oxygen,
nitrate, iron, bicarbonate, nitrous oxide and sulfate, have been shown to act as an
alternate electron acceptor during anaerobic hydrocarbon degradation.
17.7.5 pH
pH is not of much significance in marine environments since it is well buffered at
about pH 8.5, but soil pH varies widely and pH between 7 and 8 has been found to
support optimum degradation of alkanes in soils/sediments.
17.7.6 Surfactants
Surfactants are amphiphilic compounds, that reduce surface and interfacial ten-
sions by accumulating at the interface of immiscible fluids or of a fluid and a solid,
increase the contact surface areas of insoluble compounds, leading to increased
mobility, bioavailability and subsequent biodegradation.
17.7.6.1 Synthetic Surfactants
The use of chemical surfactants has been extensively studied by various authors
(Suchanek et al. 2000; Stortini et al. 2009). Chrzanowski et al. (2006) performed
biodegradation studies of a model mixture of hydrocarbonyl dodecane and hexa-
decane (1:1 w/w) by applying different surfactants like: lecithin extracted from
soyabeans, rhamnolipids from Pseudomonas aeruginosa, saponin, lutensol GD 70,
Triton X-100 and Tween 20 with different concentration 150, 300 and 600 mg l
-1
for 7 days. Candida maltosa was found capable to degrade hydrocarbons by a
maximum of 92.7% in case of saponin (300 mg l
-1
), followed by 90.3% in case of
saponin (150 mg l
-1
) and 80.9% with rhamnolipid (150 mg l
-1
).
Surfactants have been also reported to increase the uptake and assimilation of
alkanes such as hexadecane in liquid cultures (Beal and Betts 2000; Noordman and
Janssen 2002), but their usefulness in soils and other situations is less evident
(Holden et al. 2002). Surfactants produced by microorganisms probably have other
roles as well, such as facilitating cell motility on solid surfaces (Caiazza et al.
2005), or the adhesion/detachment to surfaces or biofilms (Boles et al. 2005).
17.7.6.2 Biosurfactant
Microorganisms are grouped to endo- and exo-type ones, based on biosurfactant
accumulation. Endo-type biosurfactants are bound up with the wall surface of
17 Microbial Degradation of Alkanes 459
the microorganism cell and, as a rule, constitute components liposomally active
(Al Tahhan et al. 2000). Exo-type biosurfactants are excreted into medium by
cell to provide substrate access to cell surface, due to emulsion or suspension
production in liquid medium (Deziel et al. 1996). Biosurfactants are very
diverse in their chemical composition. They include glycolipids, lipopeptides,
lipoproteins, phospholipids, fatty acids and polymeric surfactants (Rosenberg
and Ron 1999).
Biosurfactants are organic molecules consisting of a hydrophilic moiety and act
as emulsifying agents by decreasing the surface tension and forming micelles. The
uptake mechanism of hydrocarbons and emulsification by rhamnolipid produced
by the bacteria has been demonstrated in Fig. 17.10.
Microbial surfactants have advantages over synthetic surfactants due to lower
toxicity, higher biodegradability and environmental compatibility (Cameotra and
Makkar 2004). It may be produced cost effectively under ex-situ conditions and
in-situ production may be stimulated at the site of contamination and can be
recovered and recycled (Moran et al. 2000).
Bushnell and Haas (1941) were among the first to demonstrate bacterial
production of biosurfactants. Based on molecular weight, microbial surfactants are
classified in two groups (Hua et al. 2010). Glycolipids and lipopeptides are counted
under low molecular-weight surfactants, whereas emulsa, alasan, biodispersan
Fig. 17.10 Involvement of biosurfactants in the uptake of hydrocarbons and the emulsifying
effect of a rhamnolipid produced by Pseudomonas spp. within the oil–water interphase and the
formation of micelles (after Fritsche and Hofrichter 2000)
460 S. N. Singh et al.
and extracellular or cell membrane-bound bioemulsifiers (exopolysaccharide;
EPS) are high molecular weight compounds. Maximum study was done with
rhamnolipids produced by Pseudomonas aeruginosa (Rahman et al. 2002).
It was shown that rhamnolipid extracts lipopolysaccharides (LPS) from cells of
Pseudomonas, thereby increasing the hydrophobicity of the cell surface and
promoting attachment of the cells to hydrocarbon droplets (Al-Tahhan et al.
2000). It is suggested that greater attachment stimulates hexadecane degradation
(Al-Tahhan et al. 2000) while it was found that it inhibits octadecane degra-
dation due to flocculation of the cells. Christova et al. (2004) reported that
Renibacterium salomininarum 27BN also produced biosurfactant of glycolipid.
It secretes two rhamnolipids RLL and RRLL from Pseudomonas aeruginosa
when grown on hexadecane (2%) as sole source. At the end of 192 h, only
9.3 ±2.1% residual hexadecane was obtained in cultures incubated with the
whole cell.
Biosurfactant activities can be determined by measuring the changes in surface
and interfacial tensions, stabilization or destabilization of emulsions and hydro-
philic–lipophilic balance. The hydrophilic–lipophilic balance is directly related to
the length of the hydrocarbon chain of fatty acids (Desai and Banat 1997). They
are often good emulsifiers; the emulsions they form are more stable than the
emulsions obtained by synthetic surfactants (Desai and Desai 1993).
In addition to solubility enhancement, EPS shields bacterial cells from direct
exposure to toxic substances (Gutierrez et al. 2009). Ron and Rosenberg (2001)
found that EPS alters the hydrophobicity by exposure of hydrophobic phospho-
lipids tails of cells (Al-Tehhan et al. 2000). EPS, in case of biodegradation of
petroleum hydrocarbon, was first reported by Watanabe and Takahashi (1997)in
Pseudomonas sp. SLI and SLK. Halomonas spp. (Martinez-Checa et al. 2007) and
marine Enterobacter cloacae (Iyer et al. 2006) have also been reported for the
production of EPS. Iyer et al. (2006) found that emulsion of EPS produced by
Enterobacter cloacae (EPS 71a) with hexane was stable up to 10 days between pH
2 and 10 in presence of NaCl in the range of 5–50 mg ml
-1
at 35–37°C. Hua et al.
(2010) found that EPS, secreted by Enterobacter cloacae strain TU during
growing on n-hexadecane as the sole carbon source, composed of glucose and
galactose with molecular weight of 12.4 ±0.4 kDa. Kumar et al. (2007) observed
the reduction of interfacial tension by EPS produced by Planococcus maitriensis
Anita I for hexane and found that this EPS contained carbohydrate (12.06%),
protein (24.4%), uronic acid (11%) and sulphate (3.03%).
17.8 Conclusion
Researches carried out on microbial degradation have provided new insights into
the mechanism of alkane degradation. However, many aspects of degradation are
still not very clear, particularly incorporation of alkanes into the microbial cell.
A few new enzymes have been recently found which metabolize long chain
17 Microbial Degradation of Alkanes 461
alkanes. Although there are indications for existence of new alkane hydroxylases,
but they have been not yet characterized. We are still curious to know why there
are several alkane hydroxylases with similar substrate specificities. Regulation and
expression of genes coding for alkane degradation pathways are still not very
clear. Elucidation of these pathways is very important to design bioremediation
strategies for enhancing degradation of alkanes in the contaminated sites. Besides,
recombinant and functionally improved strains have to be developed to enhance
the process of biodegradation of oil hydrocarbons at contaminated sites.
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- "Different degradation intermediates that found in the treated sample are (a)10- chlorodecyl Formic acid ester; (b) Cyclohexylmethyl oxalic acid; (c) Nonahexacontanoic acid; (d) Carbonic acid, 2-biphenyl ester; (e) octanoic acid 2-pentadecyl ester; (f) Carbonic acid, octadecyl- 2-propyl ester; (g) 9-10-Dimethylanthracene; (h) 3,4-dihydroxyphenanthrene diol; (i) octacosanoic acid methyl ester; (j) Hexadecanoic acid, methyl ester; (k) Pentafluoro propionic acid, dodecyl ester; (l) Phthalic acid ester; (m) Propanedioic acid, dipropyl, dimethyl ester and (n) Oxalic acid, cyclohexyl-methyltridecyl ester. The appearance of these new peaks resulted either from the degradation of the compounds or the synthesis of new metabolites and intermediates in the fermentation process (Seo et al., 2009; Singh et al., 2012). "
[Show abstract] [Hide abstract] ABSTRACT: The intrinsic biodegradability of hydrocarbons and the distribution of proficient degrading microorganisms in the environment are very crucial for the implementation of bioremediation practices. Among others, one of the most favorable methods that can enhance the effectiveness of bioremediation of hydrocarbon-contaminated environment is the application of biosurfactant producing microbes. In the present study, the biodegradation capacities of native bacterial consortia toward total petroleum hydrocarbons (TPH) with special emphasis to poly aromatic hydrocarbons were determined. The purpose of the study was to isolate TPH degrading bacterial strains from various petroleum contaminated soil of Assam, India and develop a robust bacterial consortium for bioremediation of crude oil of this native land. From a total of 23 bacterial isolates obtained from three different hydrocarbons contaminated samples five isolates, namely KS2, PG1, PG5, R1, and R2 were selected as efficient crude oil degraders with respect to their growth on crude oil enriched samples. Isolates KS2, PG1, and R2 are biosurfactant producers and PG5, R1 are non-producers. Fourteen different consortia were designed involving both biosurfactant producing and non-producing isolates. Consortium 10, which comprises two Bacillus strains namely, Bacillus pumilus KS2 and B. cereus R2 (identified by 16s rRNA sequencing) has shown the best result in the desired degradation of crude oil. The consortium showed degradation up to 84.15% of TPH after 5 weeks of incubation, as revealed from gravimetric analysis. FTIR (Fourier transform infrared) and GCMS (Gas chromatography-mass spectrometer) analyses were correlated with gravimetric data which reveals that the consortium has removed a wide range of petroleum hydrocarbons in comparison with abiotic control including different aliphatic and aromatic hydrocarbons. -
- "The Σ odd-chain carbon number saturated n-alkanes to Σ even-chain carbon number saturated n-alkanes ratio was calculated in the range from nC19 to nC32, with the following formula: sum of the oddchain carbon number saturated n-alkanes / sum of the even-chain carbon number saturated n-alkanes. The odd-chain carbon number saturated n-alkanes had a biogenic origin whereas the even-chain carbon number saturated n-alkanes had an anthropogenic origin: mineral oils (Grundböck et al., 2010), paraffins (Grob et al., 2001), air pollution (Singh et al., 2012) and soil pollution (Lee et al., 2008). As a consequence, a low odd-to-even predominance, or a ratio equal or lower than 1 can be used as parameters to detect contamination in vegetable oils. "
[Show abstract] [Hide abstract] ABSTRACT: Three cultivars of tomato (Solanum lycopersicum L. cv 'Principe Borghese', 'Rebelion F1' and 'San Marzano') were separately grown in three greenhouses in Southern Italy to investigate the n-alkane (saturated linear hydrocarbons) composition of the tomato seed oil (TSO). The oil was obtained by Soxhlet-petroleum ether extraction. Oneway ANOVA and principal component analysis were applied to differentiate the three cultivars. 14 components were identified. Tomato seed oil contained mainly odd-chain carbon number n-alkanes (79.09% - 89.35%), and among them, n-C25, n-C21 and n-C23 were prevalent in all cultivars. The Σ odd chain carbon number / Σ even chain carbon number ratio was 8.39 in 'Principe Borghese', 3.78 in 'Rebelion F1' and 7.60 in 'San Marzano'. Relevant differences in the quantitative composition were observed among the oils 'San Marzano' contained the lowest n-alkane quantity (100.67 mg/kg) whereas 'Principe Borghese' contained 347.33 mg/kg. -
- "Terrigenous/aquatic ratio (TAR) which is the ratio of (C 27 + C 29 + C 31 ) over (C 17 + C 19 + C 21 ) can be used to differentiate between land based and marine based biogenic n-alkanes (Silliman et al., 1996 ). Generally, terrigenous origin n-alkanes are dominant over marine biogenic n-alkanes in the sediment due to rapid biodegradation of marine biogenic n-alkanes and less hydrocarbon quantity of marine biogenic sources (Singh et al., 2012). Average chain length (ACL) of n-alkanes is another indicator of hydrocarbon sources and shows average number of carbon atoms for n-alkanes having vascular plant origin (Boot et al., 2006). "
[Show abstract] [Hide abstract] ABSTRACT: Peninsular Malaysia has gone through fast development during recent decades resulting in the release of large amounts of petroleum and its products into the environment. Aliphatic hydrocarbons are one of the major components of petroleum. Surface sediment samples were collected from five rivers along the west coast of Peninsular Malaysia and analyzed for aliphatic hydrocarbons. The total concentrations of C10 to C36 n-alkanes ranged from 27,945 to 254,463ng·g(-1)dry weight (dw). Evaluation of various n-alkane indices such as carbon preference index (CPI; 0.35 to 3.10) and average chain length (ACL; 26.74 to 29.23) of C25 to C33 n-alkanes indicated a predominance of petrogenic source n-alkanes in the lower parts of the Rivers, while biogenic origin n-alkanes from vascular plants are more predominant in the upper parts, especially in less polluted areas. Petrogenic sources of n-alkanes are predominantly heavy and degraded oil versus fresh oil inputs. Copyright © 2015 Elsevier Ltd. All rights reserved.
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Sri Ramachandra University
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