Green Microbes and the Production of Biodegradable Polymers: History
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Research efforts have shifted to creating biodegradable polymers to offset the harmful environmental impacts associated with the accumulation of non-degradable synthetic polymers in the environment. A comprehensive examination of the role of green microbes in fostering sustainable bioproduction of these environment-friendly polymers is presented. Green microbes, primarily algae and cyanobacteria, have emerged as promising bio-factories due to their ability to capture carbon dioxide and utilize solar energy efficiently. The metabolic pathways harnessed for the synthesis of biopolymers such as polyhydroxyalkanoates (PHAs) and the potential for genetic engineering to augment their production yields are further discussed. Additionally, the techno-economic feasibility of using green microbes, challenges associated with the up-scaling of biopolymer production, and potential solutions are elaborated upon. With the twin goals of environmental protection and economic viability, green microbes pave the way for a sustainable polymer industry.

  • green microbes
  • biodegradable polymers
  • polyhydroxyalkanoates

1. Bacterial Biopolymers

Bacteria are highly adaptable microorganisms that can survive in a diverse range of ecosystems [1]. Therefore, they can synthesize multiple classes of biopolymers, including polysaccharides, polyamides, and polyesters [1]. The role of bacteria in the production of bacterial polymers and the types of biopolymers that are synthesized by bacteria are well documented in Table 1. Polysaccharides are high-molar-mass polymers that are produced from sugars and sugar acids. The typical metabolic pathway to produce biopolymers involves three enzymatic actions. In the first step, ADP-Glucose pyrophosphorylase (AGPase) catalyzes adenosine diphosphate-glucose (ADP-Glc). In the second step, linear 1-1,4-linked glucose chains are generated with glycogen synthase as the catalyst. The chains give glycogen its helical structure, making it a suitable energy reserve. Finally, a-1,6-linked glucan branches are produced in the polymer, with a branching enzyme as the catalyst [2]. The resulting biopolymers are easy to use and more sustainable compared to their more conventional chemical alternatives.
Table 1. Types of Bacterial Biopolymers.
Type of Microorganisms Species/Strain Biopolymers
Bacteria Acetobacter polysaccharogenes, Acetobacter xylinum, Pseudomonas aeruginosa, Bacillus thuringiensis, Escherichia coli, Pasteurella multocida, Lactobacillus acidophilus, and Geobacillus thermonitrificans Polysaccharides
Pseudomonas putida, Xanthomonas campestris, Corynebacterium glutamicum, and Escherichia coli Polyamides
Lactobacillus bulgaricus, lactobacillus delbrueckii, and lactobacillus leichmanni Biopolyesters
Actinobacteria, Bacteroidetes, Cyanobacteria, Proteobacteria, Clostridia, and Bacilli Polyhydroxylalkanoates
The biopolymers are produced and stored inside cells to serve as carbon and energy reserves [3]. Due to their high molar mass, biopolymers, such as polysaccharides, constitute biofilm matrices with diverse material properties that make them suitable for use in different materials. Depending on their material properties, such as the type of glycosidic linkages, length of the polymers, solubility, ionic strength, extendibility, and molar mass, bacterial polysaccharides have a wide range of applications in the manufacture of natural viscosifiers and thickeners, gel, microcapsules, foams and fibers, nanoparticles and nanotubes, and sponges, with applications in drug delivery, cosmetics, and biomedical and food packaging [1]. The bacterial species that are commonly used in the production of polysaccharides include Acetobacter polysaccharogenes, Acetobacter xylinum, Pseudomonas aeruginosa, Bacillus thuringiensis, Escherichia coli, Pasteurella multocida, Lactobacillus acidophilus, and Geobacillus thermonitrificans [3].
While the most abundantly produced and used polysaccharides are glycogen, cellulose, and starch, which are connected by glycosidic linkages, polyamides are made up of amino acids that are linked through peptide bonds [1]. Most biopolyamides are either co-polymers of diacids and diamines or homopolymers of terminal amino acids [4]. One metabolic pathway for the synthesis of biopolyamides produces amino acids such as 5-AVA as an intermediate in the process of degrading L-lysine through the AMV pathway using Pseudomonas putida or Escherichia coli as the source of carbon and nitrogen [5].
The primary source of the bio-based monomers used in the production of biopolyamides is castor oil, which contains an abundant mixture of saturated and unsaturated fatty acids. Pressed castor oil is converted to octanedicarboxylic acid, decanediamine, and aminoundecanoic through pyrolysis. In the final step, the monomers are polymerized into biopolymyadies [4]. In addition to Pseudomonas putida, Xanthomonas campestris, Corynebacterium glutamicum, and Escherichia coli are used in the production of biopolyamides.
Bacteria are widely used in the production of biopolyesters as well, among them Polyhydroxylalkanoates (PHAs). PHAs are preferred to their synthetic chemical alternatives due to their biodegradability [6]. There are different metabolic pathways for the synthesis of polyester based on the bacteria strain used in the production of the materials. For instance, the process of the manufacture of polyester using lactide begins with the condensation of lactic acid to form a pre-polymer with low metabolic weight. In the second step, the pre-polymer undergoes polymerization to produce lactide, which then undergoes ring-open polymerization to form Polylactic Acid, a type of polyester.
Lactobacillus bulgaricus, lactobacillus delbrueckii, and lactobacillus leichmannii are used in the fermentation process in the production of the monomers that are polymerized in the second step to form the monomers that undergo ROP to produce polyester [7]. The process yields polylactic acid (PLA), a type of biopolyester. Other bacteria used in the production of PHAs include Actinobacteria, Bacteroidetes, Cyanobacteria, Proteobacteria, Clostridia, and Bacilli [7]. Evidently, bacteria provide a wide variety of polymers with extensive technical, industrial, and medical applications. Specific applications include drug delivery, wound dressing, as an additive in reconstructed foods such as cake mixtures and frozen custards, and in the manufacture of regenerative medicines [8].

2. Fungal Biopolymers

The literature on fundal biopolymers recognizes the two primary properties of fungi, on which different industries have capitalized on to produce biopolymers using the microorganisms; they are fast growing and can convert nutrients from low-cost and low-value waste into biopolymers [9]. Consequently, fungal waste from wineries, which includes saccharomyces and non-saccharomyces yeasts, waste from enzyme production, which includes Aspergillus and Trichoderma, penicillium waste from the manufacture of antibiotics, and waste from the mushroom industry are up-scaled for use in the production of fungal exopolysaccharides such as Aureobasidium, Sclerotium, and Botryosphaeria [10].
After cellulose, the second-most abundant biopolymer is chitin, a carbohydrate-based component of the cells of fungi [11]. The metabolic pathway for the synthesis of chitin is a seven-step process that begins with glycogenolysis, which breaks down glucose into glucose-1-phosphate and glucose. After the initial phosphorolysis, a second catalysis occurs in the presence of phosphomutase. The compounds in the subsequent steps are broken down progressively until chitin is formed in the last step [12].
Chitin and its derivative, chitosan, are easily recovered from fungal classes such as Absidia coerulea, Absidia glauca, Absidia blakesleeana, Mucor rouxii, Aspergillus niger, Phycomyces blakesleeanus, Trichoderma reesei, Colletotrichum lindemuthianum, Gongronella butleri, Pleurotus sajocaju, Rhizopus oryzae, and Lentinus edodes (see Table 2). The applications of the chitin and chitosan recovered from these fungi include the production of paper and textiles, biofertilizers, wound-dressing and wound-healing management products, artificial kidney membranes, tendons, cartilage, and skin, drug-delivery systems for the delivery of vaccines, and in burn treatment [12].
Table 2. Types of Fungal Biopolymers.
Type of
Microorganisms
Species/Strain Biopolymers
Fungi Aspergillus Trichoderma, and penicillium Exopolysaccharides
Absidia coerulea, Absidia glauca, Absidia blakesleeana, Mucor rouxii, Aspergillus niger, Phycomyces blakesleeanus, Trichoderma reesei, Colletotrichum lindemuthianum, Gongronella butleri, Pleurotus sajocaju, Rhizopus oryzae, and Lentinus edodes Aminopolysaccharides

3. Algal Biopolymers

Algae are photoautotrophic organisms with high biomass and a high rate of growth. The microorganisms have limited nutritional requirements, the ability to thrive in environments that are not arable and typically do not support the growth of plants, including wastewaters, and the ability to yield biomass regardless of the season, making them perfect for the production of organic compounds [13]. These qualities of algae, combined with the microorganism’s ability to assimilate carbon dioxide into different organic compounds, make them perfect materials for the manufacture of biopolymers. The most common algal biopolymers are Poly Lactic Acid (PLA) and Polyhydroxyalkanoates (PHAs) (see Table 3). The metabolic pathway for the synthesis of biopolymers from algae begins with the transformation of acetyl Co-A to acetoacetyl Co-A using 3-ketothiolase enzyme through condensation. Once condensation is complete, acetoacetyl Co-A reductase reduces acetoacetyl Co-A into hydroxybutyryl. The biopolymer is formed in the third step, in which polymerization occurs in the presence of polymerase [14].
Table 3. Types of Algal Biopolymers.
Type of Organism Biopolymers
Algae Polylactic Acid
Polyhydroxyalkanoates
Polyhydroxybutyrate
This process is commonly used in the production of Polyhydroxybutyrate (PHB), a polymer that is similar to petroleum-based polymers in its quality and properties and is commonly marketed as PHA. In addition to PLA, PHA, and PHB, algae are also used in the production of polyurethane (PU). PU is made from algal oils, whose composition of fatty acids are ideal for the production of similar to polyols. Polyurethane is produced through the epoxidation of algae such as Chlorella with ethylene glycol and lactic acid. The resulting polymers have good physio-mechanical properties and exhibit antibacterial and anticorrosive qualities [15]. Ultimately, algae and cyanobacteria have high nitrogen concentrations, which improves their biofilm formation. Other algal strains, besides Chlorella, that are commonly used in the production of biopolymers include Chlamydomonas, which is the best strain for the production of bioplastic, Scenedesmus, and Spirulina [15].

This entry is adapted from the peer-reviewed paper 10.3390/polym15234617

References

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  3. Khan, R.; Shah, M.D.; Shah, L.; Lee, P.C.; Khan, I. Bacterial polysaccharides—A big source for prebiotics and therapeutics. Front. Nutr. 2022, 9, 1031935.
  4. Radzik, P.; Leszczyńska, A.; Pielichowski, K. Modern biopolyamide-based materials: Synthesis and modification. Polym. Bull. 2020, 77, 501–528.
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  8. Vicente, D.; Proença, D.N.; Morais, P.V. The Role of Bacterial Polyhydroalkanoate (PHA) in a Sustainable Future: A Review on the Biological Diversity. Int. J. Environ. Res. Public Health 2023, 20, 2959.
  9. Delattre, C.; Cabrera-Barjas, G.; Banerjee, A.; Rodriguez-Llamazares, S.; Pierre, G.; Dubessay, P.; Zamani, A. Production of fungal biopolymers and their advanced applications. In Current Developments in Biotechnology and Bioengineering; Elsevier: Amsterdam, The Netherlands, 2023; pp. 497–532.
  10. Tupe, S.G.; Deshmukh, S.K.; Zambare, R.B.; Tripathi, A.A.; Deshpande, M.V. Biopolymers from Fungi and Their Applications. In Fungal Biopolymers and Biocomposites: Prospects and Avenues; Springer: Singapore, 2022; pp. 3–14.
  11. Fernando, L.D.; Dickwella Widanage, M.C.; Penfield, J.; Lipton, A.S.; Washton, N.; Latgé, J.P.; Wang, T. Structural polymorphism of chitin and chitosan in fungal cell walls from solid-state NMR and principal component analysis. Front. Mol. Biosci. 2021, 8, 727053.
  12. Abo Elsoud, M.M.; El Kady, E.M. Current trends in fungal biosynthesis of chitin and chitosan. Bull. Natl. Res. Cent. 2019, 43, 59.
  13. Sreenikethanam, A.; Bajhaiya, A. Algae based bio-plastics: Future of green economy. Biorefineries-Sel. Process. 2021, 9, 77–89.
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  15. Kartik, A.; Akhil, D.; Lakshmi, D.; Gopinath, K.P.; Arun, J.; Sivaramakrishnan, R.; Pugazhendhi, A. A Critical Review on Production of Biopolymers from Algae Biomass and Their Applications. Bioresour. Technol. 2021, 329, 124868.
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