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Biodegradation of Hydrophobic Polycyclic
Aromatic Hydrocarbons 6
Daniel Chikere Ali and Zhilong Wang
Abstract
Polycyclic aromatic hydrocarbons (PAHs) are hydrophobic, toxic, and carcinogenic
compounds which comprise of high molecular weight (HMW) and low molecular
weight (LMW) compounds and are classified based on different aromatic rings
present. PAHs have been reported on its negative implications by the US Center for
Children’s Environmental Health (CCEH) on consistent exposure to polluted envi-
ronment among the pregnant woman such as premature child delivery and delayed
in child development. In this review, biodegradation pathway of PAHs (naphtha-
lene, fluoranthene, and pyrene) and its proceeding enzymes involved for effective
degradation of PAHs are briefly discussed. However, the biodegradation efficiency
is limited because the compounds are highly lipophilic and therefore very insoluble
in an aqueous solution. The production of biosurfactants by microorganisms and its
contribution to ongoing degradation of PAHs are properly discussed.
Keywords
Biodegradation · Polycyclic aromatic hydrocarbons · Hydrophobic compound ·
Biosurfactants · Pseudomonas species
6.1 Introduction
PAHs are heteroaromatic hydrocarbons with carbon and hydrocarbon atoms. PAHs
can be substituted by sulfur, oxygen, and nitrogen (Kafilzadeh 2015; Abdel-Shafy
and Mansour 2016). They are among the complex mixture of potentially ranging to
D. C. Ali · Z. Wang (*)
State Key Laboratory of Microbial Metabolism, School of Pharmacy, Shanghai Jiao Tong
University, Shanghai, China
e-mail: zlwang@sjtu.edu.cn
#Springer Nature Singapore Pte Ltd. 2021
Inamuddin et al. (eds.), Microbial Biosurfactants, Environmental and Microbial
Biotechnology, https://doi.org/10.1007/978-981-15-6607-3_6
117
hundreds of different chemicals including saturated, unsaturated, and aromatic
groups (Wang et al. 2007). PAHs enventually accumulate in large quantity in the
soil and may be released into the atmosphere following the anthropogenic activities,
for example, combustion of fossil fuels (Bamforth and Singleton 2005; Dean-Ross
et al. 2002), volcano eruptions, wild fires, and erosions of ancient sediment (Boada
et al. 2015; Sousa et al. 2017). The release of these organic contaminants into
freshwater remains a big health concern across the globe (Reizer et al. 2019;Wu
et al. 2018), because they are toxic and carcinogenic compounds that occur in the
environment. These compounds are chemically stable and poorly degradable. Once
ingested, the compounds and their metabolites have the capacity to form DNA
adduct and induce mutations which can cause cancer. The carcinogenic
characteristics of PAH is a big health-related problem (Boada et al. 2015;Kafilzadeh
2015; Abramsson-Zetterberg and Maurer 2015; Seo et al. 2009).
Presently, fluorene, anthracene, acenaphthene, naphthalene, chrysene, benzo[b]
fluoranthene, pyrene, benzo[k]fluoranthene, acenaphthylene, benzo[ghi]perylene,
dibenzo[a,h]anthracene, indeno[1,2,3-cd]pyrene, benzo[a]pyrene, phenanthrene,
benzo[a]anthracene, and benzo[k]fluoranthene (Wang et al. 2007) had been
recognized among the 16 priority pollutants which are regarded as a threat to
human life by the US Environmental Protection Agency (US EPA) (Zhuo et al.
2017; Seo et al. 2009; Arun et al. 2011). Benzo[α]pyrene as a member of PAHs is
believed to undergo metabolism and therefore referred as carcinogenic, teratogenic,
and mutagenic (Guo et al. 2019; Jelena et al. 2015; Lily et al. 2009). In this regard,
the World Health Organization (WHO) has warned that continuous exposure to air
pollutants associated with PAHs has caused about seven million deaths, constituting
high environmental risk to human health across the globe (Sosa et al. 2017).
According to WHO in 2006, it was estimated that environmental pollution (air)
associated with PAHs represents 23% to 24% of the world’s morbidity/mortality rate
(Montaño-Soto et al. 2014).
PAH’s chemical and physical features are basically varying with the number of
rings and their molecular weight which show that its chemical reactivity, volatility,
and aqueous solubility decrease with an increase in molecular weight and thus
contribute to their distribution, transportation, and eco-biological effects (Seo et al.
2009). PAHs are made up of high molecular weight (HMW) and low molecular
weight (LMW) compounds, and they are classified based on different aromatic rings
present. PAHs that contain 4 to 6 aromatic rings are HMW, and their degradability is
less with native microbes, whereas PAHs that contain 2–4 aromatic rings are called
LMW and are less carcinogenic when compared to HMW (Nwaichi and Ntorgbo
2016). Those having five rings “benzo(α)anthracene, anthracene, and fluoranthene”
involve consortia in the soil and are effectively degraded. The ability to metabolize
two or more organic substrates at the same time with respect to their concentration
within suitable substrate level and bioavailable carbons contributes to biodegradable
efficiency (Toräng 2004). In this review, the toxic effect of PAHs, the PAH
pathways, and the use of biosurfactants produced by bacteria for application in
environmental degradations are exclusively reviewed.
118 D. C. Ali and Z. Wang
6.2 Health Related to PAHs
6.2.1 Consequences of Consistent of PAH Exposure by Human
PAHs can easily be absorbed from the gastrointestinal tract of a mammal because
they are highly lipid soluble with a devastating effect in bone marrow cells, e.g.,
non-Hodgkin lymphoma, leukemia, and multiple myeloma (Kafilzadeh 2015;
Montaño-Soto et al. 2014). Therefore, consistent exposure to high levels of PAH
pollutant conglomeration possesses several dangers to human health condition
including the eye, vomiting, irritation, diarrhea, and nausea. Nowadays, an increased
health risk of skin, bladder, gastrointestinal, and lung cancer among worker prior to
exposure to PAHs has become a public health concern. Naphthalene is among the
PAHs that causes skin irritants, whereas anthracene and benzo[α]pyrene causes
allergy to the skin in both animal and human (Rand and Petrocelli 1985).
According to epidemiological evaluation, there is a clear relationship between
PAHs associated with human lung and bladder cancer on exposure to the organic
compound at the workplace. Meanwhile, the relationship between PAH and cancer
is very crucial for determining occupational and environmental standards. Exposure
to PAH at the workplace is considered to be predominantly causing lung or bladder
cancer. The workplaces (sources) are rubber industries, through steel works and
diesel exhaust (Guo et al. 2019; Armstrong et al. 2004). And occupational exposure
in these types of industries encounters negative impact of PAH via inhalation and
has been considered a huge health threat such as smoking cigarettes from open
fireplaces because tobacco contains benzo[α]pyrene suspected as human carcinogen
(Kafilzadeh 2015; Guo et al. 2019).
The US Center for Children’s Environmental Health (CCEH) has warned that
pregnant woman consistently exposed to environment polluted with PAH may likely
experience complication/advert effect in birth such as premature delivery or delayed
child development and low birth weight. Benzo[a]pyrene when gets in contact with a
pregnant woman could cause congenital disorder. Prenatal exposure to PAH causes
low intelligence quotient (IQ) during the age of three (3) and an increased behavioral
problem at six (6) to eight (8) years of childhood (Kim et al. 2013).
6.2.2 Problems Associated with PAHs Via Cytochrome P450
Human population and its activities affect freshwater ecosystem, thereby posing
health threats to the society. The health threats are due to the adverse impacts on
aquatic organisms carried by human population or activities with some organic and
trace compounds. Organisms living in PAH-contaminated environments have enor-
mous potentials to alter their metabolism and their cellular components (Balcıoğlu
2016). The role of microsomal cytochrome system (cytochrome P450 and flavopro-
tein monooxygenases) has been established in invertebrates and fish which are
familiar with mammalian systems. It further shown that cytochrome P450 is induced
by chemical substrates in fish and mammals, though invertebrates possess lower rate
6 Biodegradation of Hydrophobic Polycyclic Aromatic Hydrocarbons 119
of PAH metabolism than fish. Studies reveal that after treating hepatic microsomes
of fish with aromatic hydrocarbon enhance the catalytic activities with selected
substrates. The induction of cytochrome P450 in fish increases the disposition of
hydrocarbon but also is capable of enhancing the formation of PAH’s derivatives
(Stegeman and Lech 1991; Gagnaire et al. 2010). The formation of stable DNA
adducts by PAHs and their derivatives has been reported (Luch and Baird 2010), and
this is because PAH derivatives are metabolized in the living organism leading to an
oxidatively induced DNA damage (Michel and Vincent-Hubert 2015). Additionally,
it is also reported that these oxygen species known as hydroxyl radical (OH) may be
activated via inflammation reactions enhancing nitric oxide production in the process
(Cadet et al. 2012). The generation of nitric oxide through OHenhances inter- and
intracellular signaling function which in turn can modify cellular biomolecules and
other accumulation associated with several diseases (Evans et al. 2004). The ability
of invertebrates to metabolize PAHs via the activities of cytochrome P450-
dependent oxidases varies. The reaction of OHcauses biological damage by
inducing significant molecules such as DNA, protein, and lipids which can lead to
genetic inflammation, instability, cell death, apoptosis, and angiogenesis. It can
result to mutation and a development leading to cancer and even death, if not
repaired (Park et al. 2005; Capó et al. 2015). Furthermore, the oxidatively induced
DNA damage can undergo repair in living cell via different mechanisms involved in
several DNA repair proteins (Dizdaroglu et al. 2015; Von 2006; Dizdaroglu and
Jaruga 2012) (Fig. 6.1), though the DNA damage repair using deletion in the Atp7b
gene present in some animals as a promising tool in studying the effect of oxidatively
induced DNA damage in the pathogenesis of transition metal-induced hepatic
Wilson’s diseases has been established (Evans et al. 2004; Dizdaroglu et al. 2015;
Wang et al. 2012; Loeb 2011; Friedberg Errol et al. 2006). DNA oxidatively
generated damage enhances a process leading to cancer as a result of mutagenesis
(Cadet et al. 2012; Dizdaroglu and Jaruga 2012).
Fig. 6.1 Oxidatively induced DNA damage via cytochrome P450
120 D. C. Ali and Z. Wang
6.3 Biodegradation of PAHs
6.3.1 Challenges of Limited Aqueous Solubility in Water
Most PAHs exist as hybrids enclosed with different structural components, e.g.,
benzo[α]pyrene (BαP). When there is an increase in the size of PAH’s molecule, it
will enhance and increase PAH’s hydrophobicity and electrochemical stability.
Thus, hydrophobicity and molecule’s stability of PAHs are the two primary factors
that determine whether HMW PAHs are capable of persisting in the environment
(Kanaly and Harayama 2000). Some basic characters of PAHs with HMW and
LMW are shown in Table 6.1. PAHs are found in a wide number of range of
molecular weight especially in vegetable oil which most of them are alkylated
compounds (Hossain and Salehuddin 2012). The solubility of PAHs in water
decreases with an increase in molecular weight which enables PAHs to settle out
of the water and accumulate in the lowest sediment. This implies that the PAH
concentration is high in the sediments of the polluted environment such as aquatic
organisms (Kafilzadeh 2015). The high molecular weight and low water solubility of
PAH have an impact in lowering the bioavailability, and this causes resistance to
microbial degradation (Bhattacharya et al. 2014). The bioavailability of a compound
determines the rate of mass transfer and soil biota intrinsic activities of a particle
compound such as PAHs (Snežana 2013). In addition, the high resonance energies of
HMW PAHs make them recalcitrant to degradation because of the dense cloud of
pi-electrons surrounding the aromatic rings (Ukiwe et al. 2013). LMW PAHs, such
Table 6.1 Characters of PAHs with HMW and LMW
No. HMW PAH LMW PAH Reference
1 It contains 4 to
6 aromatic rings
It contains 2–4 aromatic
ring
Abdel-Shafy and Mansour
(2016), Soberón-Chávez and
Maier (2011), Uzoigwe et al.
(1999) and Rosenberg and
Ron (1999)
2 It originated from
pyrolytic PAHs
Occurs from petrogenic
PAH
Kafilzadeh (2015)
3 Degradation of benzo[b]
fluoranthene and benzo
[a]pyrene is resistant to
bacteria
Phenanthrene,
naphthalene, and
fluoranthene are degraded
by the individual strains
Daugulis and McCracken
(2003) and Cui et al. (2011)
4 They are more
carcinogenic
They are less carcinogenic Nwaichi and Ntorgbo (2016),
Soberón-Chávez and Maier
(2011), Uzoigwe et al. (1999)
and Rosenberg and Ron
(1999)
5 Mineralization is higher
in HMW PAH-degrading
bacteria
Mineralization is lower in
LMW PAH-degrading
bacteria
Raquel et al. (2013)
6 They are less susceptible
to biodegradation
They are more susceptible
to biodegradation
Soberón-Chávez and Maier
(2011)
6 Biodegradation of Hydrophobic Polycyclic Aromatic Hydrocarbons 121
as phenanthrene, naphthalene, and fluoranthene biodegradability decrease, are deter-
mined by the number of rings (Daugulis and McCracken 2003). Generally, LMW is
more volatile and soluble in water, and they are widely found in all environment, and
it possibly helps to detect PAH-contaminated environment (Ghosal et al. 2016a).
High melting and boiling point, low vapor pressure, very low aqueous solubility, and
resistance to oxidation and reduction are the crucial features of HMW PAHs (Boada
et al. 2015). Benzene is an example of PAHs with high vapor pressure, but the
viability of vapor pressure in different PAH compound causes the distribution of
different concentration in the vapor by individual PAHs (Kafilzadeh 2015). There is
no doubt that when the molecular weight of PAHs is high, it may likely absorb more
to the soil organic matter (Ukiwe et al. 2013).
Interestingly, the effectiveness of PAH bioremediation is limited due to their
failure to effectively remove HMW PAHs and PAH resistance to microbial degra-
dation as a result of its hydrophobic features (Potin et al. 2004). Several evidence
has emerged that the microbial consortium is more effective in the degradation of
PAHs with 4 to 5 rings occurring at faster rate (Daugulis and McCracken 2003),
whereas PAHs that contain 2–3 rings are degraded at slow rate under anaerobically
(Vaidya et al. 2017). PAHs are stable when they are absorbed into sediments
because their non-pore structures can limit its dissolution in water. LMW PAHs
are not completely insoluble, although small amounts of PAHs are capable of
dissolving, and they become induced in the pore water. An increase in PAH
concentration above aqueous solubility is enhanced by the presence of pore water
organic colloids. This is because PAHs will be water-ice onto the organic colloids
and can be easily transported via pore spaces of the sediment, thereby encouraging
an increase in mobility and bioavailability of PAHs in the sediments (Kafilzadeh
2015). An increase in molecular weight enhances an increase in PAH’s carcinoge-
nicity and at same time acute toxicity reduction (Kim et al. 2013). Additionally, the
proportion of LMW PAHs to HMW determines PAH’s origin (Nwaichi and Ntorgbo
2016).
6.3.2 Biodegradation Pathway of PAHs
Biodegradation has long been applied to address contaminated environment with
PAHs because of its abilities to treat different types of pollutant, low cost of
operation, and no secondary by-product (toxic product) (Vaidya et al. 2018; Ghosal
et al. 2016b). Bacteria are capable of using many toxic hydrocarbons in pollutant
environment as a potential substrate for their removal (Heipieper et al. 2010).
Degradation can be induced when the mechanisms are able to facilitate the proper
use of waste disposal strategies (Wu et al. 2018; Singh and Sharma 2008). Normally,
the nature of microbe and chemical structure of the chemical compounds are the two
basic factors which determine the extent PAHs can be degraded (Haristash and
Kaushik 2009).
122 D. C. Ali and Z. Wang
6.3.2.1 Naphthalene
Naphthalene is a member of PAH which occurs when two aromatic rings can share two
carbon atoms (Tomás-Gallardo et al. 2014) and remain the simplest PAHs which are
generally used as a vibrant instrument to study the enzymatic aromatic degradation
pathways (Selifonov et al. 1996;Jerinaetal.1976; Garrido-Sanz et al. 2019). Albeit
naphthalene possesses a relatively low aqueous solubility (32 mg/L), it is very hazard-
ous. Water has become the main target in the environmental contamination issues
because 15% of contaminants such as naphthalene is discharged into the environments
(water) where most aquatic animal dwell thereby posing health challenges.
The ration of naphthalene concentration in wastewater from the radioisotope
manufacturing facilities is 1.65 mg/L, 6–220 ng/L in municipal wastewater, dyeing
and textile wastewater 0.1–2.1 mg/L, and naphthalene sulfonic acid 285 mg/L in
effluents (Karimi et al. 2015). Naphthalene degradation via metabolic diversities by
Mycobacterium sp., Pseudomonas putida,Rhodococcus opacus,Bacillus pumilus,
and Nocardia otitidiscaviarum has been established by different researchers. Pseu-
domonas aeruginosa is the most studied isolate from soil, plants, and water (Karimi
et al. 2015; Akhtar and Husain 2006). They are gram-negative bacteria which are
capable of causing infections such as cystic fibrosis in human (Soberón-Chávez et al.
2005). The bacteria are known with its ability to utilize hydrocarbons as sources of
carbon and energy. The ability of the Pseudomonas sp. to produce biosurfactant
enhances the effective uptake of hydrophobic compounds (Calfee et al. 2005)to
degrade naphthalene which basically depends on difference in temperature range.
The study revealed that at pH 8, Pseudomonas sp. degraded 96% of naphthalene,
whereas at pH 7, 90% of naphthalene was degraded after 3 days (Karimi et al. 2015).
The degradation of naphthalene by bacteria is effective via catabolic enzymes
encoded by the plasmid Pseudomonas sp. (Seo et al. 2009). The production of
1,2-dihydroxynaphthalenes is produced through the dehalogenation of 1,2-dihydro-
1,2-dihydroxynaphthalenes by Escherichia coli recombinant strain. The Escherichia
coli recombinant strain consists of dihydrodiol naphthalene dehydrogenase gene
cloned from Pseudomonas fluorescens N
3
(Cavallotti et al. 1999). The production of
1,2-dihydroxynaphthalene by pseudomonads is known as an intermediate in the
metabolism of naphthalene which is oxidized by oxygen in a reaction catalyzed by
1,2-dihydroxynaphthalene oxygenase. 1,2-Dihydroxynaphthalene have been
differentiated from catechol-2,3-dioxygenase (C230) by virtue of the greater stability
at 50 C and the differences in control of induction (Patel and Barnsley 1980).
Naphthalene metabolic pathway via 1,2-dihydroxynaphthalene is cleaved by a
dioxygenase to an unstable ring cleavage product by Pseudomonas species and can
degrade naphthalene (Eaton and Chapman 1992).
Normally, naphthalene is degraded by the hydroxylation of phenanthrene, anthra-
cene, and fluorene and monooxygenation of acenaphthene (Resnick 1996; Ferraro
et al. 2017). The degradation pathway of naphthalene remains the base for cell-to-
cell communication via specific regulatory system, enzymes and transporters (elec-
tron) (Díaz et al. 2012). Subsequently, the oxidation of naphthalene and other PAHs
reactions is further classified as central aromatic degradation pathways popularly
6 Biodegradation of Hydrophobic Polycyclic Aromatic Hydrocarbons 123
called catechol and as an intermediate depending on the specific PAHs (Selifonov
et al. 1996), before ring cleavage occurs (Jõesaar et al. 2017).
The microbial (Pseudomonas) degradation of naphthalene (Yen et al. 1988) via
the metabolic steps usually proceeded via hydroxylation of one of the aromatic rings
to produce 1,2-dihydroxynaphthalene. 1,2-digydroxynaphtalene later undergo fur-
ther reaction to metabolized to salicylic acid, and further metabolized through
catechol cleaved either meta- or ortho-rings shown in Fig. 6.2 (Phale et al. 2019).
The resulting enzyme 1,2-dihydroxynaphthalene is regarded as a toxic organic
solvent with aromatic and alicyclic ring that shares two carbon atoms (Tomás-
Gallardo et al. 2009). Naphthalene degradation with bacteria nah gene is arranged
in two operons in plasmid pNAH7, where one operon codes for enzymes and can
covert naphthalene to salicylate (Tomás-Gallardo et al. 2014; Phale et al. 2019),
based on the possibility of phenol and naphthalene to induce salicylate by Pseudo-
monas sp. which further converted to C230. This showed that C230 expression with
phenol- and naphthalene-induced salicylate by Pseudomonas. Therefore, these
enzymes involved in the process can convert salicylate over catechol to pyruvate
and acetyl-coA (Jõesaar et al. 2017; Eaton and Chapman 1992).
6.3.2.2 Pyrene
Pyrene is a member of HMW PAHs and remains one of the compounds with
simplest four-fused benzene ring. They are among the most abundant PAHs in the
environment that occurred as a result of pyrolytic processes (Kafilzadeh 2015).
Pyrene consists of high water solubility of 0.4 mg per ring structure (benzo[a]
pyrene) with 1.7 10
3
mg/L of water solubility (Husain 2008a). Pyrene-
contaminated environment remains a public health concern, and its toxicity to
microinvertebrate Gammarus pulex capable of transmitting into quinone metabolites
termed as the agent of mutagenic and toxicity to organisms in their respective
Fig. 6.2 The degradation pathway of naphthalene
124 D. C. Ali and Z. Wang
habitant (Kim et al. 2007; Ma et al. 2013). The microbial degradation of pyrene with
more than three rings encounter challenges (Ghosh et al. 2014), though bacteria
remain one of the fastest degrading pyrene mechanisms (Shusheng et al. 2014)to
effectively study the biodegradation of HMW PAHs because it has similar structure
with other carcinogenic PAHs (Kim et al. 2007).
The microbial degradation of pyrene using metabolized substrate as sole source
of carbon and energy for the degradation of pyrene has been established (Juhasz
et al. 1997). The isolation of Pseudomonas sp. for the degradation of pyrene has been
successfully done by several researchers, but the effectiveness of its degradation of
pyrene remains a challenge because it is not a very effective degrader of pyrene.
Most work currently relied on using the six consortia such as Pseudomonas and
Burkholderia which can degrade various PAHs (Vaidya et al. 2017). The utilization
of pyrene as sole carbon source by microbes depends on their ability to produce its
metabolite (Xing-Fang et al. 1996). Pseudomonas is the promising bacteria genus for
the catabolism of aromatic compounds (Nogales et al. 2017).
The enzyme degradation pathway of pyrene was determined using an environ-
mental microbial isolate identified as Pseudomonas sp. grown in mineral medium
containing pyrene (14.7 mg/L/day) and was able to degrade 82.38% of the pyrene in
the medium used (Husain 2008b). This is because PAH degradation bacteria via the
action of intracellular dioxygenase enable PAHs to be taken up by the cells and
therefore enhance effective degradation (Obayori et al. 2008).
Pyrene degradation proceeds with oxidization via the monooxygenases and
dioxygenases, thereby encouraging the cleavage of the oxidized ring. The pyrene
can also be oxidized or mineralized by various types of microbes through
oxygenases into the carbons on the PAHs, enhancing the C-C covalent bonds to
cleave, and hydroxyl- and carboxyl-substituted moieties will be produced (Husain
2008a; Priyangshu et al. 2004; Mishra and Singh 2014). The metabolite produced by
Pseudomonas aeruginosa will be converted to dihydroxypyrene, thereby causing the
initial ring oxidation or cleavage at C-4, C-5 (K -region), C-1, and C-2 positions, and
this can form pyrene 4,5-dihydrodiol via ortho-cleavage due to ring fission leading to
the production of cis-3,4-phenanthrene dihydrodiol-4-carboxyclic acid,
4-phenanthroic acid, and phenanthrene (Ghosh et al. 2014; Obayori et al. 2008).
The dioxygenase present is relatively high since the dihydroxylation is a nonspecific
step for metabolic pathways and the dioxygenase involved may be cleaved via ortho or
meta of the aromatic nucleus to produce catechol 1,2-dioxygenase as illustrated in
Fig. 6.3 (Cenci et al. 1999;Sugimotoetal.1999),. The structural gene pcaH of
protocatechuate 3,4-dioxygenase was considered having the ability to dissimilate aro-
matic growth substrate via the β-ketoadipate pathway (Gerischer et al. 1995; Yamanashi
et al. 2015). The decarboxylation of 4-phenanthroic acid can produce monoaromatic and
phthalic acid via cleavage. Then, the phthalic acid was further converted to an interme-
diate called pyruvate which enters the TCA cycle, and once the pyruvate enters the TCA,
it will be converted to carbon dioxide and water (Obayori et al. 2008).
6.3.2.3 Fluoranthene
Fluoranthene is a very toxic PAH mainly found in many factories especially wood
preservation plants (Herwijnen et al. 2003). Fluoranthene is made up of naphthalene
6 Biodegradation of Hydrophobic Polycyclic Aromatic Hydrocarbons 125
Fig. 6.3 The degradation pathway of pyrene
126 D. C. Ali and Z. Wang
benzene rings usually condensed with a five-member ring, widespread in the envi-
ronment, and was referred as genotoxic, mutagenic, and carcinogenic (Samanta et al.
2002) especially benzo[α]pyrene. Report showed that consistent fluoranthene expo-
sure by laboratory animal negatively affects human such as decreased body weight,
decreased blood chemistry tests, and increased liver weight. Fluoranthene can cause
neurobehavioral toxicity, lung airway anion transport defects, and suppression of the
immune system (Saunders 2003). They occur as a result of incomplete combustion
of fossil fuels or as a result of pyrolysis of organic material with high temperature.
Human exposure through inhalation of particulate due to tobacco smoking, air, or
ingestion from water or food contamination remains a health threat. They are
commonly identified in complex mixtures of PAHs in soil surface, water, and
sediments (Yu 2002; Bisht et al. 2015).
The metabolic pathways by isolated strains of bacteria’s ability to metabolize
individual PAHs have been fully established by various researchers (Dean-Ross
et al. 2002). This is because the isolated bacteria strain can utilize fluoranthene as
carbon and energy source as was first described by Weissenfels (Weissenfels et al.
1990), whereas fluoranthene-degrading pathway was discovered in Pseudomonas
sp. by Grifoll (Rehmann et al. 2001; Tersagh et al. 2016). Pseudomonas aeruginosa
on the other hand can cause different types of diseases which may be harmful to
human. Currently, P. aeruginosa has become promising bacteria used in degradation
of PAH because it can easily decompose hydrocarbons and lives in oil field (Patel
et al. 2014; Yan and Wu 2017).
Bacteria produces enzymes for degradation of PAH compound reported to
possess a broad substrate range which is one of the desirable features of bacteria
degrading PAHs (Juhasz et al. 1997). The microbial study of removal of
PAH-contaminated environment through the action of enzymes remains a promising
reward to the current focus on biodegradation of various PAHs. Interestingly, the
enzymes produced by participating bacteria are referred as catabolic enzymes
involving different mechanisms with members of PAHs. This shows that the degra-
dation of PAH is usually enhanced and initiated via hydroxylation, especially
deoxygenation in which oxygenase can be catalyzed (Simarro et al. 2013; Kweon
et al. 2011; Somtrakoon et al. 2008).
Furthermore, fluoranthene metabolism also initiates deoxygenation of the
fluoranthene molecule (Rehmann et al. 2001), which further produces 1,2- and
2, 3-dioxydrofluoranthene. The production of an intermediate 9-fluorenone-1-car-
boxylic acid by 1,2- and 2, 3-dioxydrofluoranthene cleavage through meta- or ortho-
pathway was established (Somtrakoon et al. 2008), in which upon decarboxylation
yields 9-hydroxyfuorene (Reddy et al. 2018). Thus, the production of 9-fluorenone-
1-carboxylic acid further undergoes angular deoxygenation leading to the production
of benzene-1,2,3-tricarboxylic acid in Fig. 6.4 (Dean-Ross et al. 2002; López et al.
2006). The benzene-1,2,3-tricarboxylic acid produced also showed that the degrada-
tion of 9-fluorenone1-carboxylic acid occurred via angular deoxygenation. Further
decarboxylation to phthalate and degradation of phthalate by benzene-1,2,3-tricar-
boxylic enhance central metabolism (López et al. 2006).
6 Biodegradation of Hydrophobic Polycyclic Aromatic Hydrocarbons 127
6.4 Biosurfactants
6.4.1 Biosurfactants
6.4.1.1 Glycolipid
Glycolipids are active compounds with the presence of carbohydrate moiety coupled
to fatty acids. Glycolipid biosurfactant remains the most studied microbial surfactant
and is the best known biosurfactant including mannosylerythritol lipids,
sophorolipids, rhamnolipids, and trehalolipids. Glycolipids are also made up of
mono- or disaccharides which are incorporated with long-chain aliphatic acids and
sometimes hydroxy aliphatic acids as seen in Fig. 6.5 (Sen et al. 2017). Glycolipids
are made up of carbohydrate moiety referred as microbial surface-active compounds
(Mnif and Ghribi 2016; Irorere et al. 2017).
Glycolipid biosurfactants have a strong fungicidal activity (Morita et al. 2013),
and they are used in cosmetic industries because of its amazing moisturizing and
liquid-crystal-forming features (Yamamoto et al. 2012). They are well known by
their ability to lower the surface and interfacial tension (Mnif and Ghribi 2016;
Irorere et al. 2017). It is being classified as low molecular weight biosurfactant
produced by many companies for commercial utilization (Irorere et al. 2017) and has
been regarded as promising biosurfactant with respect to its adaptability such as low
toxicity, biodegradability, and chemical stability (Paulino et al. 2016; Imura et al.
2014). They are very useful in their application in biodegradation, oil recovery, food,
and pharmaceutical industries (Sen et al. 2017). Glycolipids of microbial origin are
ranging from viruses to human cells such as antiparasitic, anticancer, and immuno-
modulatory activities and antimicrobial (Abdel-Mawgoud and Stephanopoulos
2017).
Fig. 6.4 The degradation pathway of fluoranthene
128 D. C. Ali and Z. Wang
Rhamnolipids
Rhamnolipid is one of the most popular glycolipids (Mnif and Ghribi 2016) and is
made up of one or two (L)-rhamnose molecules with a glycosidic linkage. One or
two L-rhamnose molecules are hydrophobic group which contains one or two
β-hydroxyl fatty acids. Microorganisms can produce different types of rhamnolipid
congeners during fermentation. Rhamnolipid congeners vary in chain length and are
different in numbers of rhamnose molecules and unsaturation for the fatty acid chain
potential (Chong and Li 2017).
Rhamnolipid consists of rhamnose units which are composed of β-glycosidic
bond that can assist rhamnose units to link to 3-hydroxyl fatty unit/units. O-Glyco-
sidic bonds help rhamnose units to link to each other. Thus, an ester bond also allows
Fig. 6.5 The molecular structure of glycolipids. (a) rhamnolipids; (b) cellobiose lipids; (c)
sophorolipids; (d) trehalolipids; (e) mannosylerythritol lipids
6 Biodegradation of Hydrophobic Polycyclic Aromatic Hydrocarbons 129
3-hydroxyl fatty acids to link to each other (Reddy et al. 2018; Twigg et al. 2018).
Rhamnolipid is a viscous sticky oily yellowish-brown liquid with fruity odor
produced by Pseudomonas aeruginosa (Irorere et al. 2017). The rhamnolipid pro-
duction by different microbes mainly in area such as soil/water samples or industrial
facilities (Irorere et al. 2017), though P. aeruginosa has been confirmed as the best-
known representative of organisms that produce rhamnolipids (Kaczorek et al.
2018).
Rhamnolipids can be used for degradation of hydrocarbon in a site contaminated
with the compound or in the petroleum industry (Irorere et al. 2017; Müller et al.
2012), and that is why they are good candidate for environmental application (Shao
et al. 2017). Zhong investigated the effect of low concentration mono-rhamnolipid
using Pseudomonas aeruginosa ATCC 9027 which was properly prepared and
grown on glucose or hexadecane to glass beads. It comprises of hydrophobic or
hydrophilic surface and was conducted via batch adsorption experiments. The result
obtained reveals that there is a hydrophobic interaction on the bacteria cells during
adsorption to the surfaces. This results to the reduction of bacterial adsorption with
much implication on cell surface hydrophobicity (Zhong et al. 2015). It confirms that
it enhances an effective degradation of hydrophobic compounds which begins with
solubilization and alters cell affinity to hydrophobic compounds by microorganism
(Zeng et al. 2018). Application of rhamnolipid biosurfactant such as degradation of
hydrophobic compounds by means of solubilization, water treatment, and soil and
waste treatment is referred as promising approach on environmental freedom
(Catherine 2013), although there are challenges with rhamnolipids as they negatively
play a role in the biodegradation of relatively volatile hydrocarbons such as n-
alkanes with short chains (Chen et al. 2013). Rhamnolipid surfactants are capable
of reducing from 72 to 28 mN/m of the surface tension of water and 43 to <1mN/m
of interfacial tension of water-oil (W/O) (Zhou et al. 2019). It was referred as
promising target to the environmental application because it can enhance the
pollutants uptake by increasing the bacterial membrane to targeted compounds/
candidate that aids bioremediation processes (Kaczorek et al. 2018). It is also used
for agricultural control of plant diseases and protecting stored products (Mnif and
Ghribi 2016).
Cellobiose Lipids
Cellobiose lipids (CLs) are biosurfactants and a member of glycolipid which is made
up of cellobiose moiety comprising hydrophilic parts and acetyl groups and fatty
acids referred as hydrophobic part (Morita et al. 2013). CLs are produced extracel-
lularly by yeast and mycelia fungi. The CLs with a cellobiose moiety can be used is
fungicidal application, e.g., yeast which can help to preserve food from fungal attack
(Morital et al. 2011). The cellobiose lipids produced by Cryptococcus humicola have
a high surface activity (López et al. 2006).
They are surface-active compounds owing to its ability to lower the tension of
water solutions. Its specific function depends on pH and temperature stability
making the cellobiose promising compounds in the application of agricultural use
such as fumigation. And that is why the study of cellobiose lipids by biochemists and
130 D. C. Ali and Z. Wang
genetics and its ecological role are relevant (Morital et al. 2011; Trilisenko et al.
2012). The ability of microorganism to degrade cellobiose via cellulolytic
biocatalysts helps in bioprocessing (Guo et al. 2015).
Sophorolipids
Sophorolipids (SLPs) are a family of glycolipid biosurfactant which are made up of
disaccharide sophorose (20-O-β-D-glucopyranosyl-β-glycoside) and have become the
most promising glycolipid biosurfactant. SLPs have various characteristics that
make them superior to synthetic surfactants such as temperature, salinity, and
stability in the wide range of pH (Oliveira et al. 2015). Candida bombicola produces
SLPs which have become the most studied SLPs producing yeast sophorolipids
(Oliveira et al. 2015).
SLPs can also be used as biosurfactants instead of using classical chemistry-
derived surfactants in food, petroleum, cleanings, and cosmetic industries. Reports
showed that SLPs exhibit medical features including anticancer, anti-inflammatory,
anti-HIV, and antiviral activities which are useful in medical application (Ivancic
et al. 2018). Currently, SLPs are the first microbiological biosurfactants on the
market (Müller et al. 2012). SLP biosurfactant can be applied for biofuel, drug
delivery, detergent, and cleaners (Nguyen and Sabatini 2011). Sophorolipid
biosurfactants are biodegradable and less toxic, and they have been approved by
the FDA (Vasudevan and Prabhune 2018).
Trehalolipids
Trehalolipids occur naturally and consist of three isomers (α,α-; α,β-; and β,β-). It is
also called α-D-glucopyranosyl-(1 !1)-α-D-glucopyranoside (Reis et al. 2018). The
structures of trehalose-containing glycolipids are made of hydrophobic moiety with
long complex fatty acid (Kuyukina and Ivshina 2019). Trehalolipids are
characterized with high surfactant activity. The chemical diversity of trehalolipids
is vast which includes monomycolates, trehalose dimycolates, and trimycolates.
Others are nonionic acylated and anionic trehalose tetraesters and succinoyl
trehalolipids (White et al. 2013).
Trehalolipids are produced by various organisms, and the trehalolipids produced
contain different number of atoms, size, structural pattern of mycolic acid, and
degree of unsaturation (Desai and Banat 1997). Rhodococcus sp. is the most notable
bacteria that produce trehalolipid biosurfactant using hydrophobic substrates (sun-
flower oil). They are excellent emulsifying compounds and can be used in the
microbial application for oil recovery and degradation of oil-contaminated environ-
ment (Sen et al. 2017).
Mannosylerythritol Lipid
Mannosylerythritol lipids (MELs) are a glycolipid class of biosurfactants. MELs are
active compound of glycolipid biosurfactant with excellent interfacial biochemical
properties (Souayeh et al. 2014). Thus, MELs possess hydrophilic and hydrophobic
parts (Morita 2013).
6 Biodegradation of Hydrophobic Polycyclic Aromatic Hydrocarbons 131
They are mainly produced by Ustilago and Pseudozyma on n-alkanes. For the fact
that they are active surface compound, it can be applied in pharmaceutical, food, and
cosmetic industries due to their excellent surface activities and other peculiar
bioactivities (Yu et al. 2015). MELs are regarded as promising biosurfactants
because they are environmentally friendly and have structural diversity, mild pro-
duction versatile biochemical functions, and self-assembling properties with high
yield (Souayeh et al. 2014; Niu et al. 2019).
6.4.1.2 Lipopeptides
Lipopeptide is a low molecular weight (LMW) biosurfactant derived from amino
acids. Lipopeptide is obtained via the mixture of cyclic lipopeptides mainly from
hydroxy fatty acid chains and heptapeptides (Gudiña et al. 2013). Lipopeptides can
lower surface tension and are referred as an important bioactive compound (Baltz
2018; Kubicki et al. 2019). It is produced by various clades of microorganisms such
as the bacterial genera Lactobacillus,Bacillus,Pseudomonas,Streptomyces,and
Serratia (Kubicki et al. 2019;Park et al. 2019). Lipopeptide possesses antifungal
activity (Toral et al. 2018). Generally, lipopeptides are promising microbial
surfactants applied in the environment for oil recovery. Bacillus subtilis can produce
cyclic lipopeptide surfactin which can lower 72 to 27.9 mN/m of the surface tension
at 0.005% concentration (Arun et al. 2011; Souayeh et al. 2014).
6.4.1.3 Phospholipids
Phospholipids (PLs) are surfactants which play a vital function during cell growth in
plant and animal (Willem et al. 2015). They are amphiphilic molecules with surface-
active compounds made up of head called polar and lipophilic tail. Thus, amphi-
philic features of PL deemed it fit to be used as emulsifier, solubilizer, and wetting
agent (Jing et al. 2015). It can stablize emulsions due to its good emulsifying
features, though phospholipids enhance the hydrophilic and hydrophobic properties
due to their surface-active wetting features (Rui et al. 2013).
Bacteria and yeasts are organisms that can produce phospholipids mostly on
n-alkanes when isolated, and they are very useful in the application of various
industries such as food, pharmaceutical, and cosmetic industries (Vikbjerg et al.
2005). Meanwhile, they are also useful in the environmental application (Murínová
and Dercová2014).
6.4.2 Polymeric Biosurfactants
Polymeric surfactants are macromolecules with hydrophilic and hydrophobic parts
in their structure. They are also called polymers with surfactant characteristics.
Polymeric microbial surfactant is produced by microorganisms with a combination
of many chemical types, and their chemical structures are exploited for commercial
purposes (Bustamante et al. 2012). The polymeric surfactants are high molecular
weight biosurfactants (Kapadia et al. 2013). They are environmentally friendly and
low toxicity, and its biodegradability can be controlled during extreme condition
132 D. C. Ali and Z. Wang
such as pH, temperature, and salinity (Almeida et al. 2016). Polysaccharide protein
consists of polysaccharide, generally produced from the surface coat of bacteria,
linked to protein carriers. Examples of polysaccharide protein complex are
lipopolysaccharides (Gautam and Tyagi 2006; Floris et al. 2018).
Lipopolysaccharides (LPS) are the primary component of the outer membrane of
Gram-negative bacteria; it is composed of high molecular weight component
associated with phospholipid and protein. LPS are useful in environmental applica-
tion (Zhang et al. 2016a: Saddler 1978).
Liposan, lipomannam, alasan, emulsan and mannoprotein are polysaccharide
protein complexes which are the most studied polymeric biosurfactants. Reports
reveal that emulsan consists of 80%(w/w) lipopolysaccharide and high molecular
weight exopolysaccharide 20% (w/w). Lipopolysaccharide is made up of fatty acid,
charge, and solution behavior, whereas exopolysaccharide is cationic in nature for
the formation of the emulsan complex by electrostatic binding mechanism. The
exopolysaccharide enhances emulsion stabilization (Pandey et al. 2015) and are
produced by Acinetobacter venetianus. Emulsan can be used as emulsifier for
hydrocarbons in water at 0.001% to 0.01% concentration. Emulsan possesses New-
tonian flow features and therefore undergoes conformational changes at W/O inter-
face. Thus, it can be applied in the bioremediation processes for oil removal. It can
also be used in the preparation of cosmetics. Emulsan helps to prevent bacteria from
adhering to buccal epithelial cells and has become a promising candidate for
cosmetic application such as toothpaste production (Saddler 1978; Mercaldi et al.
2008). Liposan can acts as emulsifier in the extracellular water-soluble, and can be
synthesized by Candida lipolytica. They are made up of carbohydrate (83%) and
protein (17%) which can be employed to different industries such as food, cosmetics
(Santos et al. 2016), and oil industries where it has many adverse processing
conditions (de Cássia et al. 2014). Mannoproteins are produced by Saccharomyces
cerevisiae; it is referred as having excellent emulsifier activity toward several oils,
organic solvents, and alkane.
6.5 Enhanced Biodegradation of PAHs by Biosurfactant
The existence of a hydrophobic organic compound in the soil has an impact on the
environmental-related problems (Cheng et al. 2018). PAHs are highly lipophilic and
soluble in organic solvent (Kafilzadeh 2015), and the low aqueous solubility of
PAHs decreases upon molecular weight increase enhancing accumulation in the
bottom sediments because of its ability to settle out of the water (Kafilzadeh 2015).
For the fact that PAHs are nonpolar in water, an increase in their molecular weight
decreases its hydrophobicity. Normally, PAHs bind to particles in the soil and are
absorbed. Therefore, the persistence of hydrophobicity of hydrocarbon in the
contaminated site (soil) affect the degradation potentials of the participating
microbes as it can lower water solubility thereby increasing their sorption to soil
particle thereby limiting their biodegradability (Barkay et al. 1999). There is an
overwhelming setback in biodegradation of PAHs because of their hydrophobicity
6 Biodegradation of Hydrophobic Polycyclic Aromatic Hydrocarbons 133
coupled with low aqueous solubility of different PAHs (Kaczorek et al. 2018; Hou
et al. 2018).
6.5.1 Biodegradation in Micelles
It is very necessary to increase the apparent solubility of hydrophobic hydrocarbon
via addition of bioemulsifier alasan (surfactants) so that biodegradation will be
enhanced which can transfer PAHs to water and PAHs in micelles or emulsion for
microbial action (Kafilzadeh 2015; Barkay et al. 1999; Díaz et al. 2001). The
hydrophobic and hydrophilic nature of surfactant possesses a different degree of
hydrogen bonding and polarity (Olivera et al. 2008). Micelles could be initiated by
biosurfactant and emulsifiers, thereby enhancing biodegradation of solid or liquid
substances because they help to maintain direct cellular contact with several
compounds (Al-Turki et al. 2009). The biosurfactant released from microorganisms
is referred as detergent molecules. Detergent molecules like structure are composed
of hydrophilic head and lipophilic tail and have the ability to form spherical micelles
especially once the micelle concentration and compound are greater than surfactant
concentration (Reis et al. 2013). The hydrophobic and hydrophilic moiety features
can possibly lower surface tension and interfacial tension among molecules at the
surface and interface (Gautam and Tyagi 2006; Karlapudi et al. 2018) and are easily
biodegradable by microorganisms. The nature of their molecular weight determines
its ability to reduce interfacial surface tension such as low molecular weight, whereas
high molecular weights are stabilizing agents (Park et al. 2019; Santos et al. 2016). It
can be applied in many industries including food, pharmaceutical, and cosmetic
industries (Santos et al. 2016). They function with varying temperature, salinity, pH,
greater selectivity range (Zhang et al. 2016b), biodegradability, environmental
compatibility, and its ability to adopt to extreme temperature and low toxicity
(Saddler et al. 1979).
The use of surfactant to increase the bioavailability of poor carbon such as
hydrophobic compounds to allow free mass transfer to a contaminated soil so as to
lower the interfacial tension thereby enhancing an increase in mass transfer of the
contaminants (Norman et al. 2002; Szczepaniak et al. 2016). The surfactant-
mediated biodegradation is a process involved when PAHs in the soil are partitioned
into the hydrophobic core of surfactant micelle solubilization using surfactant at
concentration above critical micelle concentration (CMC) values. Thus, the quantity
and types of the surfactant, time, surfactant-soil interactions, and hydrophobicity of
surfactant determine the efficiency of degradation. Time plays a significant role
because the time at which hydrophobic compounds, e.g., PAHs, get in contact with
the soil is very important during the process of degradation (Szczepaniak et al.
2016).
Enhancing the biodegradation of hydrocarbons using surfactant via solubilization
aids hydrocarbon uptake by microorganisms. Biodegradation can be enhanced in
micellar solution increasing the solubility and bioavailability of substrate to bacteria
under the influence of surfactant which further increases the interfacial area.
134 D. C. Ali and Z. Wang
Importantly, the presence of surfactant enables the bacteria to get in contact at the
hydrocarbon-water interface. Also, the presence of surfactant can influence contact
between cells’nonaqueous liquid phase and reduce diffusion path length between
the site of adsorption and site of microorganism uptake for cell adsorption to
hydrocarbon in soil particles. Thus, the effectiveness of the surfactant in enhancing
biodegradation of PAHs is specific interactions dependent between bacteria and
surfactant (Barkay et al. 1999). The biosurfactant-enhanced solubility of pollutants is
a good strategy for a potential application in biodegradation.
For the fact that low water solubility affects the hydrophobicity degradation of
PAHs, the addition of biosurfactant will enhance biodegradation, (Barkay et al.
1999) which can overcome the problems associated with PAH low aqueous solubil-
ity (Ukiwe et al. 2013). Application of biosurfactant in biodegradation enhances the
bioavailability of hydrophobic compounds. Meanwhile, the microbial growth also
increases the biosurfactant released into environment and also enhances bioavail-
ability of pollutants (Araújo et al. 2019; Borah and Yadav 2017). Finally, the effect
of degradation of hydrocarbon also depends on the growth factor and media
components involved (Ibrahim 2016). The production of biosurfactant enhances
the mechanism needed for biodegradation of water-immiscible pollutants (Schmid
et al. 1998).
6.5.2 Biosurfactant Acting as Bioemulsifier
An aqueous-organic solvent two-phase system usually an organic water-immiscible
solvent and an aqueous solution is used as a model for hydrophobic compound/
product synthesis (Oberbremer 1990). The transportation rate of the lipophilic
substrates from the organic phase to the cells is relatively and may affect the growth
in the systems. There are possible steps to improve this limitation by increasing the
volume fraction of the dispersed organic phase and also increasing the organic-
aqueous interfacial area using surfactants (Sifour et al. 2007). Biosurfactant is a
surface-active compound and can lower the surface and interfacial tension between
different phases such as liquid to liquid with a low CMC and can form stable
emulsions (Satpute 2010).
Bioemulsifiers are surface-active compounds with profound characteristics such
as biodegradability, foaming, biocompatibilities, nontoxicity, low concentrations,
pH, salinity, and temperatures (Alizadeh-Sani et al. 2018; Satpute et al. 2010). It is a
bioactive compound that currently draws much research interest because of its
function and structural diversity in the degradation of hydrocarbons. High emulsifi-
cation helps to facilitate the bioavailability of organic compound for faster hydro-
carbon degradation by the participating bacteria. The ability of the marine organism
to deliver bioemulsifiers can reduce overdependence on synthetic surfactant due to
their influence in the use of environmental biodegradation surface-active molecule
(Amodu et al. 2014).
Biosurfactant produced from Pseudomonas aeruginosa has the ability to emulsify
two liquid phases. The two liquid phases can be hydrocarbon, hydrocarbon mixtures,
and vegetable oil and capable of forming stable emulsions. The emulsification
6 Biodegradation of Hydrophobic Polycyclic Aromatic Hydrocarbons 135
characteristics of rhamnolipid to hydrocarbons and vegetable oils occur simply via
lowering surface tension of the culture media (Satpute 2010). The surface-active
agent functions effectively by lowering the surface tension of the medium, thereby
enhancing the formation of two phases of emulsions and thus enhancing the bio-
availability of hydrophobic compounds (Suryanti et al. 2017). Biosurfactant such as
phospholipid and glycolipid can emulsify the hydrocarbon substrate via
synthetization, enhancing transport into the cell. For example, application of
biosurfactant can lower the surface tension of water and benzene used in the study
of the emulsification of biosurfactant by observing the emulsion stability which
suggests that biosurfactants are good emulsifiers. It also established that
biosurfactant emulsion is water in oil (W/O) emulsion (Uzoigwe et al. 2015). The
stability of emulsion decreases when the temperature is high, and it will affect the
features of oil, interfacial film, water, and solubility of surfactant in the oil and water
phases (Suryanti et al. 2017).
Bioemulsifiers are well known for its ability to emulsify liquids, thereby having
no reduction effect in surface/interfacial tension of their growth medium or between
different phases (Suryanti et al. 2017). Meanwhile, reduction effect on surface
tension reduction is very important because surface tension reduction should be
less than 35 mN/m even though many reports have recorded biosurfactant containing
high emulsification capacity of hydrophobic organic compounds where medium’s
surface tensions are above 35 mN/m. In addition, emulsion stability remains the
basic tool used in the environmental application of biosurfactants. Importantly, pH,
soil variation, salinity, and temperature are environmental factors that can lead to
de-emulsification. De-emulsification occurs as a result of stimulation and ionizations
of acid constituents of interfacial films (Satpute 2010; Suryanti et al. 2017).
6.6 Conclusions
PAHs are common causes of environmental pollution, and they have characteristic
features such as mutagenic, carcinogenic, genotoxic, and toxic. Occupational
exposures to benzo[α]pyrene through inhalation are dangerous to health. Thus, the
persistent exposures to PAHs contributed to an increased rate of cancer and other
related diseases to human. Generally, PAHs are classified into HMW and LMW,
which are based on the different aromatic rings present. HMW PAHs are less
degradable by native microbes, whereas LMW PAHs are less carcinogenic when
compared to HMW PAHs.Utilization of hydrocarbon (fluoranthene, pyrene, and
naphthalene) by microorganisms (Pseudomonas species) as their energy source via
enzymatic pathway is effective. Biosurfactants such as phospholipid and glycolipid
are apparently synthesized by the PAH-degraded microorganism to solubilize via
micelles or on other hand emulsify the hydrocarbon substrate enhancing its transport
into the cells. Meanwhile, the mass transport in emulsion is very essentials for the
movement of oil molecules from emulsion droplets to the surrounding aqueous
medium via surfactant.
Conflict of Interest All authors declare no conflict of interest.
136 D. C. Ali and Z. Wang
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