14
unique adaptation to the sea environment. Moreover, they are considered to function
as pioneering surface colonizers that contribute to the initiation of biogeochemical
cycling and biodegradation processes and have still many unseen potential to be
discovered (Dang and Lovell 2016). Nutrient scarce conditions in the ocean allow
microbes to adapt easily and utilize whatever nutrients present in their surroundings
might surge their capability to adhere and deteriorate materials. On the other hand,
the addition of starch or other supplements provides contamination opportunities
that would hinder the degradation process (Satpute et al. 2010). There have been
various microorganisms involved in the deterioration of different types of synthetic
polymers that have been reported to deteriorate and utilize different plastics
(Table 1.2). In vitro, marine bacterial strains Rhodococcus ruber reported to degrade
8% of the dry weight of plastic after 30 days of incubation (Orr et al. 2004).
Extracellular enzymes from Streptomyces species were found to cause structural
changes in preheated starch-polyethylene in 3 weeks, and different strains showed
different levels of polyethylene degradation (Pometto et al. 1992). These would be
the footsteps of bioremediation research where the characterization of enzymes
extricating different plastic structures would unveil their mechanism of action. Also,
initial stresses such as heating or photo-oxidation to plastic films induce the brittlement and release of low molecular weight and carbonyl groups in the material which
further help in the microbial degradation process (Eyheraguibel et al. 2017). The gut
microbiome of mealworms was found to be capable of degrading polyethylene and
a mixture of polyethylene and polystyrene, where two bacterial species, Citrobacter
sp. and Kosakonia sp., are associated with the degradation of polyethylene and
polystyrene in the gut of mealworms (Brandon et al. 2018). The low molecular
weight oxidation products from polyethylene can be utilized by microorganisms,
but the exact molecular weight and fragment size of released products are not known
(Koutny et al. 2006).
The process of microbial degradation of plastic can take place either aerobically
or anaerobically based on the nature of microorganisms executing the process. In
general, it is known that the degradation product in the aerobic process is carbon
dioxide and water; however, in anaerobic processes, methane will be produced
(Eubeler et al. 2009). The basis of mechanism behind the structural destabilization
and degradation begins with the action of extracellular oxidoreductive enzymes
(lignin peroxidase and manganese peroxidase); however, detailed characterization
of these enzymes is still lacking (Kumari et al. 2017). The structural and functional
group changes, appearance or disappearance of bonds, reactivity, and reduction in
the hydrophobicity of the plastic surface are the main aspects of the initiation of
biodegradability, where variation in other factors such as molecular weight, porosity, tensile strength, and crystallinity suggests microbial action (O’Brine and
Thompson 2010; Santo et al. 2013; Harshvardhan and Jha 2013; Kanelli et al.
2015). In a few reports, the initial step of microbial hydrolysis of polyolefins is
analogous to n-alkane resulting in hydroxyl group formation followed by further
oxidation to generate aldehydes and ketones further into the corresponding acid;
carboxylated hydrocarbon fragment can enter into beta-oxidation pathway due to
A. Kumari et al.
unique adaptation to the sea environment. Moreover, they are considered to function
as pioneering surface colonizers that contribute to the initiation of biogeochemical
cycling and biodegradation processes and have still many unseen potential to be
discovered (Dang and Lovell 2016). Nutrient scarce conditions in the ocean allow
microbes to adapt easily and utilize whatever nutrients present in their surroundings
might surge their capability to adhere and deteriorate materials. On the other hand,
the addition of starch or other supplements provides contamination opportunities
that would hinder the degradation process (Satpute et al. 2010). There have been
various microorganisms involved in the deterioration of different types of synthetic
polymers that have been reported to deteriorate and utilize different plastics
(Table 1.2). In vitro, marine bacterial strains Rhodococcus ruber reported to degrade
8% of the dry weight of plastic after 30 days of incubation (Orr et al. 2004).
Extracellular enzymes from Streptomyces species were found to cause structural
changes in preheated starch-polyethylene in 3 weeks, and different strains showed
different levels of polyethylene degradation (Pometto et al. 1992). These would be
the footsteps of bioremediation research where the characterization of enzymes
extricating different plastic structures would unveil their mechanism of action. Also,
initial stresses such as heating or photo-oxidation to plastic films induce the brittlement and release of low molecular weight and carbonyl groups in the material which
further help in the microbial degradation process (Eyheraguibel et al. 2017). The gut
microbiome of mealworms was found to be capable of degrading polyethylene and
a mixture of polyethylene and polystyrene, where two bacterial species, Citrobacter
sp. and Kosakonia sp., are associated with the degradation of polyethylene and
polystyrene in the gut of mealworms (Brandon et al. 2018). The low molecular
weight oxidation products from polyethylene can be utilized by microorganisms,
but the exact molecular weight and fragment size of released products are not known
(Koutny et al. 2006).
The process of microbial degradation of plastic can take place either aerobically
or anaerobically based on the nature of microorganisms executing the process. In
general, it is known that the degradation product in the aerobic process is carbon
dioxide and water; however, in anaerobic processes, methane will be produced
(Eubeler et al. 2009). The basis of mechanism behind the structural destabilization
and degradation begins with the action of extracellular oxidoreductive enzymes
(lignin peroxidase and manganese peroxidase); however, detailed characterization
of these enzymes is still lacking (Kumari et al. 2017). The structural and functional
group changes, appearance or disappearance of bonds, reactivity, and reduction in
the hydrophobicity of the plastic surface are the main aspects of the initiation of
biodegradability, where variation in other factors such as molecular weight, porosity, tensile strength, and crystallinity suggests microbial action (O’Brine and
Thompson 2010; Santo et al. 2013; Harshvardhan and Jha 2013; Kanelli et al.
2015). In a few reports, the initial step of microbial hydrolysis of polyolefins is
analogous to n-alkane resulting in hydroxyl group formation followed by further
oxidation to generate aldehydes and ketones further into the corresponding acid;
carboxylated hydrocarbon fragment can enter into beta-oxidation pathway due to
A. Kumari et al.