11.5.1.1 Bioavailability
A key factor that determines the efficiency of utilization of oil pollutants by microbes
is the bioavailability. Mainly due to the hydrophobic nature of the hydrocarbons,
their aqueous solubility is low, thus limiting their bioavailability. In general, as their
molecular weight increases, the bioavailability of hydrocarbons decreases. As the
enzymes that metabolize hydrocarbons are present within the cells and are rarely
secreted, the molecules need to be taken up and transported to the cell interior. Thus,
microbes that have a capacity for biodegradation of petroleum hydrocarbons have
invariably developed a mechanism to obtain access to these oils and for their uptake
into the cell interior.
Essentially three pathways of uptake have been identified, whereby bacteria are
observed to gain access to petroleum hydrocarbons (Hua and Wang 2014): (1) aqueous solubilization of hydrocarbons, (2) pseudo-solubilization through secretion of
biosurfactants, and (3) direct contact with large oil droplets.
Water-soluble aromatics and short-chain hydrocarbons which are more soluble in
the aqueous phase in comparison to the longer length molecules are the most
accessible to microbes and more easily taken up than the less soluble ones.
11.5.2 Biosurfactant Production by Microbes
Many microbes that have the capability of degrading hydrocarbons however have
been shown to secrete biosurfactants. Biosurfactants are able to reduce surface
tension and increase solubility through emulsification and in essence, pseudosolubilization, thereby increasing the chance of direct contact between the bacteria
and oil droplets.
A variety of microbes have been found to secrete biosurfactants; Pseudomonas
aeruginosa is among the best-known biosurfactant producing, hydrocarbondegrading Gram-negative bacteria (Das and Chandran 2011) that produces
rhamnolipids (a glycolipid surfactant) (Abdel-Mawgoud et al. 2009).
P. aeruginosa DS10-129, an indigenous strain isolated from diesel oil and gasoline
contaminated sites, has been reported to produce rhamnolipid biosurfactants
(Varjani and Upasani 2017). Other species of Pseudomonas, namely P. putida and
P. chlororaphis have also been reported to produce glycolipid type biosurfactants
(Das and Chandran 2011).
Bacillus sp. has been reported to produce “surfactins” (Whang et al. 2008), with
B. subtilis being considered to be the most prominent in surfactin production.
Additionally, B. amyloliquifaciens, B. licheniformis, B. pumilus, and B. mojavensis
have also been reported to produce surfactins (Marti et al. 2014; Li et al. 2016;
Uttlová et al. 2016). Acetinetobacter venetianus RAG has been reported to produce a
lipopolysaccharide biosurfactant (Fondi et al. 2016). Mycobacterium sp. and
Rhodococcus erythropolis are known to produce trehalose lipids (White et al.
11 Microbial Bioremediation of Petroleum Hydrocarbons
275
A key factor that determines the efficiency of utilization of oil pollutants by microbes
is the bioavailability. Mainly due to the hydrophobic nature of the hydrocarbons,
their aqueous solubility is low, thus limiting their bioavailability. In general, as their
molecular weight increases, the bioavailability of hydrocarbons decreases. As the
enzymes that metabolize hydrocarbons are present within the cells and are rarely
secreted, the molecules need to be taken up and transported to the cell interior. Thus,
microbes that have a capacity for biodegradation of petroleum hydrocarbons have
invariably developed a mechanism to obtain access to these oils and for their uptake
into the cell interior.
Essentially three pathways of uptake have been identified, whereby bacteria are
observed to gain access to petroleum hydrocarbons (Hua and Wang 2014): (1) aqueous solubilization of hydrocarbons, (2) pseudo-solubilization through secretion of
biosurfactants, and (3) direct contact with large oil droplets.
Water-soluble aromatics and short-chain hydrocarbons which are more soluble in
the aqueous phase in comparison to the longer length molecules are the most
accessible to microbes and more easily taken up than the less soluble ones.
11.5.2 Biosurfactant Production by Microbes
Many microbes that have the capability of degrading hydrocarbons however have
been shown to secrete biosurfactants. Biosurfactants are able to reduce surface
tension and increase solubility through emulsification and in essence, pseudosolubilization, thereby increasing the chance of direct contact between the bacteria
and oil droplets.
A variety of microbes have been found to secrete biosurfactants; Pseudomonas
aeruginosa is among the best-known biosurfactant producing, hydrocarbondegrading Gram-negative bacteria (Das and Chandran 2011) that produces
rhamnolipids (a glycolipid surfactant) (Abdel-Mawgoud et al. 2009).
P. aeruginosa DS10-129, an indigenous strain isolated from diesel oil and gasoline
contaminated sites, has been reported to produce rhamnolipid biosurfactants
(Varjani and Upasani 2017). Other species of Pseudomonas, namely P. putida and
P. chlororaphis have also been reported to produce glycolipid type biosurfactants
(Das and Chandran 2011).
Bacillus sp. has been reported to produce “surfactins” (Whang et al. 2008), with
B. subtilis being considered to be the most prominent in surfactin production.
Additionally, B. amyloliquifaciens, B. licheniformis, B. pumilus, and B. mojavensis
have also been reported to produce surfactins (Marti et al. 2014; Li et al. 2016;
Uttlová et al. 2016). Acetinetobacter venetianus RAG has been reported to produce a
lipopolysaccharide biosurfactant (Fondi et al. 2016). Mycobacterium sp. and
Rhodococcus erythropolis are known to produce trehalose lipids (White et al.
11 Microbial Bioremediation of Petroleum Hydrocarbons
275