Degradation

1. Biodegradation
MBH-203: Environmental Microbiology

2. Biodegradation of Plastics
 Conventional plastics (polyethylene, polypropylene,
polystyrene, PVC, etc.)
 Non-biodegradable

3. Biodegradable plastics
 Environment friendly
 Strategies:
 Derive from renewable feedstocks
(e.g. polyhydroxyalkanoates (PHA) and lactic acid (raw materials for PLA) produced by
fermentative biotechnological processes)
 Petroleum-based plastics
(e.g. aliphatic polyesters like PCL, polycaprolactone)
 Recycle non-biodegradable plastics
(e.g. polystyrene)

4. Polymer-degrading microorganisms
 Microbial degradation of plastics:
 Adherence to plastic surface
 Colonization of exposed surface
 Enzymatic degradation of plastics (HYDROLYSIS):
 Enzyme binds to polymer
 Catalyzes a hydrolytic cleavage
 Polymers are degraded into low mol. wt. oligomers, dimers,
monomers
 Mineralization

5. Polymer-degrading microorganisms

6. Polymer-degrading microorganisms
 Enzymes vary with plastics:
 Laccase (actinomycetes): polyethylene
 Proteases: polyethylene
 Lignin degrading fungi and manganese peroxidase (P.
chrysosporium): polyethylene
 Papain and urease: polyurethane
 Lipase and polyurethane esterase: polylactic acid
 Reference
 Microorganisms:
 Polyhydroxybutyrate (PHB): G+, G-, Streptomyces, Fungi
 PHB, PCL, PBS (polybutylene succinate): Firmicutes and
Proteobacteria
 Polymer degradation efficiency: PHB = PCL > PBS > PLA

7. Polymer-degrading microorganisms

8. Factors affecting Plastic Biodegradability
 Chemical and physical properties of plastics
 Surface conditions (surface area, hydrophilic and hydrophobic
properties)
 Structural properties
 Molecular weight
 Melting temperature of polyesters

9. Aliphatic Polyesters from Fossil Resources
 Poly(Ethylene Adipate) (PEA)
 PEA ([-OCH2CH2OOC(CH2)4CO-]n): pre-polymer of polyurethane
 Penicillium
 Enzyme required: Lipase with broad substrate specificity

10. Aliphatic Polyesters from Fossil Resources
 Poly(ε-Caprolactone) (PCL)
 PCL ([-OCH2CH2CH2CH2CH2CO-]n):biodegradable synthetic
partially-crystalline polyester
 Degraded by both aerobic (Aspergillus) and anaerobic (Clostridium)
microorganisms
 Enzymes requires: lipases and esterases
 Copolymerization with aliphatic polyesters: increased
biodegradability (copolymers have lower crystallinity and lower Tm,
thus better degradability)

11. Aliphatic Polyesters from Fossil Resources
 Poly (β-Propiolactone) PPL
 PPL ([-OCH2CH2CO-]n): chemosynthetic biodegradable aliphatic
polyester
 Structural unit is similar to PHB and PCL: degraded both by PHB
depolymerase and lipase
 Bacillus sp., Acidovorax sp., Variovorax paradoxus, Sphingomonas
paucimobilis, Streptomyces

12. Aliphatic Polyesters from Fossil Resources
 Poly(Butylene Succinate) (PBS) and Poly(Ethylene
Succinate) (PES)
 PBS ([-O(CH2)4OOC(CH2)2CO-]n) and PES ([-
O(CH2)2OOC(CH2)2CO-]n): aliphatic synthetic polyesters
 High melting points: 112-114 °C (PBS) and 103-106 °C (PES)
 Synthesized from dicarboxylic acids and glycols
 PBS: Amycolatopsis sp.
 PES: Bacillus sp.

13. Aliphatic Polyesters from Fossil Resources
 Aliphatic-Aromatic Copolyesters (AAC)
 Consists of: PCL and aromatic polyester such as polyethylene
terephthalate (PET), polybutylene terephthalate (PBT) and
polyethylene isophthalate (PEIP)
 Rhizopus delemar lipase
 Thermobifida fusca thermophilic hydrolase
 Susceptibility decreases with increase in aromatic polyester content

14. Aliphatic Polyesters from Renewable
Resources
 Poly(3-Hydroxybutyrate) (PHB)
 PHB ([-O(CH3)CHCH2CO-]n): natural polymer produced by many
bacteria to store carbon and energy
 can be biodegraded in both aerobic and anaerobic environments,
without forming any toxic products
 Bacillus, Pseudomonas and Streptomyces

15. Aliphatic Polyesters from Renewable
Resources
 Poly(Lactic Acid) (PLA)
 PLA ([-O(CH3)CHCO-]n): biodegradable and biocompatible
thermoplastic (produced by fermentation from renewable
resources)
 PLA-degraders are not widely distributed (less susceptible to
microbial attack)
 Amycolatopsis and Saccharotrix
 Proteinase K (Tritirachium album), lipase (Rhizopus delemar),
depolymerase (Amycolatopsis sp) bromelain and pronase

16. Polymer Blends
 Blends of Polyester with other Polymers
 Modify desired properties and degradation rates
 Reduces overall costs
 Combination of PCL with conventional plastics [low density
polyethylene (LDPE), polypropylene (PP), polystyrene (PS), nylon 6
(NY), poly(ethylene terephthalate) (PET) and PHB
 Blends of Polyester and Starch

17. Polycarbonates
 Greater resistance to hydrolysis than polyesters
 Polyethylene carbonate (PEC)
 Polyhexamethylene carbonate (PHC)
 Polybutylene carbonate (PBC)
 Enzymes:
 cholesterol esterase (Candida cylindracea), lipoprotein
 lipase (Pseudomonas sp
 Lipase and lipoprotein lipase (Pseudomonas sp.)

18. Polyurethanes
 Two types: ester and ether
 R. delemar lipase
 No microbe can degrade polyurethane completely

19. Polyamide (Nylon)
 Two types: ester and ether
 R. delemar lipase
 No microbe can degrade polyurethane completely
 Biodegradability of nylon in comparison with aliphatic polyesters is low
(due to strong inter-chain interactions caused by the hydrogen bonds
between molecular chains of nylon)
 Flavobacterium sp. and Pseudomonas sp.

20. Polyethylene
 Stable polymer with long chains of ethylene monomers
 Not easily degraded by microorganisms
 Degradation: synergistic action of photo-and thermo-oxidative
degradation and biological activity
 Blending with additives generally enhances auto-oxidation

21. References (Plastics)

22. Biodegradation of Wood
 Composition of wood: cellulose (~ 45%), hemi cellulose
(~20-30%), lignin (~25-30%)

23. Composition of Wood
 Cellulose: Homopolysaccharide composed of b-D-
glucopyranoside units, linearly linked together by (1→4)-glycosidic
bonds
 Hemicellulose: consist of relatively short, mainly branched
heteropolymers of glucose, xylose, galactose, mannose and
arabinose as well as uronic acids of glucose, galactose and 4-O-
methylglucose linked by (1→3)-, (1→6)- and (1→4)-glycosidic bonds
 Lignin: complex, amorphous, three-dimensional aromatic polymer

24. Causes of Wood Biodegradation
 Due to enzymatic activities of microorganisms
 Primarily by fungi
 Conditions required:
 An adequate supply of oxygen
 A favorable temperature (15°C to 40°C)
 Moisture in excess of Fiber saturation point (25-30%)
 A suitable source of energy and nutrients (i.e. the wood)
 Absence of antagonistic influence of other fungi

25. Fungal Decay of Wood

26. Fungal Decay of Wood
 Brown rot
 Cellulose utilization
 Degradation: oxidation, partially via demethylation of
the aromatic rings(increases the phenolic hydroxyl
content), and partially via introduction of new carbonyl
and carboxyl groups
 By peroxidases
 Brown colored lignin (oxidized) left behind

27. Fungal Decay of Wood
 White rot
 Lignin, cellulose and hemicellulose utilization
 The hyphae of fungi rapidly invade and secrete the
respective enzymes
 Syringyl (S) units of lignin are preferentially degraded,
whereas guaiacyl (G) units are more resistant to
degradation
 Can colonize cell lumina and cause cell wall erosion
 Some white-rot fungi degrade lignin (by lignolytic
peroxidases and laccases) during secondary metabolism

28. Fungal Decay of Wood
 Soft rot
 Ascomycetes usually cause soft-rot decay of wood
 The decayed wood is brown and soft, and the residue
is cracked when dry.
 Daldinia, Hypoxylon and Xylaria

29. Other causes of wood decay
 Beetles
 Longhorn
 Ambrosia
 Powderpost
 Carpenter ants
 Borers
 Termites
 Dampwood
 Subterranean
 Drywood

30. References (Plastics)
http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.370.9642&rep=rep1&type=pdf