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Abstract
The negative effects of petrochemical-derived plastics on the global environment and depletion of global fossil fuel supplies have paved the way for exploring new technologies for the production of bioplastics. Polyhydroxyalkanoates (PHAs) are considered an alternative for synthetic polymers because of their biodegradability, biocompatibility, and non-toxicity. Many bacteria have been reported to have the ability to synthesize PHAs. Among them, the Aeromonas species seem to be ideal hosts for the industrial production of these biopolyesters due to their robust growth, simple growth requirements, their ability for the synthesis of homopolymers, co-polymers, and terpolymers with unique material properties. Some Aeromonas strains were able to produce PHAs in satisfactory amounts from simple carbon sources. Efforts have been made to use genetically modified Aeromonas strains for enhanced PHAs and to obtain bacteria with modified compositions and improved properties. This review discusses the current state of knowledge of polyhydroxyalkanoates synthesized by Aeromonas species, with a special focus on their potential, challenges, and progress in PHA synthesis.
Highlights
In Europe, 51 million tonnes of plastics are produced every year
This review focuses on the development of fermentation strategies, PHA synthesis by wild and genetically engineered Aeromonas species, the conversion of substrates, and properties of extracted PHAs
The authors proved that the mutant synthesized up to 71% of P(3HB-co-3HV-co-3HHx) terpolyester when fed with 2 g/L valerate, and additional feeding on valerate led to the reduction in PHA concentration in bacterial cells
Summary
In Europe, 51 million tonnes of plastics are produced every year. In the USA, only 8% of plastics were recycled in 2010. Medium-chain-length PHAs (mcl-PHAs) consist of six and more carbon atoms in each unit Belonging to this group are, for example, poly(3-hydroxyhexanoate) (P(3HHx)), poly(3-hydroxyoctanoate) (P(3HO)), and poly(3-hydroxyhexanoate-co-3-hydroxyoctanoate) copolymer (P(3HHx-co-3HO)). These two classes differ mainly due to the substrate specificity of PHA synthases, which play a crucial role in the PHA polymerization process, using (R)-3-hydroxyacyl-CoA as a substrate for PHAs synthesis. Fatty acids are degraded by Aeromonas spp. via the β-oxidation process In this pathway, acyl-CoA derived from fatty acids is degraded, resulting in the formation of enoyl-CoA intermediates encoded by the phaJ gene, which may be converted to (R)-3-hydroxyacyl-CoA by the (R)-specific enoyl-CoA hydratase and may be incorporated into a growing polyester chain by the function of PHA synthase [3]. The two acetyl-CoA molecules are combined into acetoacetyl-CoA by β-ketothiolase (PhaA), and subsequently 3-hydroxybutyryl-CoA is generated by acetoacetyl-CoA dehydrogenase (PhaB) using NADPH as a cofactor, and is polymerized into PHA by PhaC polymerase [5]
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