A Review of Polyhydroxyalkanoates: Characterization, Production, and Application from Waste

Abstract

The search for alternatives to petrochemical plastics has intensified, with increasing attention being directed toward bio-based polymers (bioplastics), which are considered healthier and more environmentally friendly options. In this review, a comprehensive overview of polyhydroxyalkanoates (PHAs) is provided, including their characterization, applications, and the mechanisms underlying their biosynthesis. PHAs are natural polyesters produced by a wide range of prokaryotic and some eukaryotic organisms, positioning them as a significant and widely studied type of bioplastic. Various strategies for the production of PHAs from agroindustrial waste, such as cacao shells, cheese whey, wine, wood, and beet molasses, are reviewed, emphasizing their potential as sustainable feedstocks. Industrial production processes for PHAs, including the complexities associated with extraction and purification, are also examined. Although the use of waste materials offers promise in reducing costs and environmental impact, challenges remain in optimizing these processes to enhance efficiency and cost-effectiveness. The need for continued research and development to improve the sustainability and economic viability of PHA production is emphasized, positioning PHAs as a viable and eco-friendly alternative to conventional petroleum-based plastics.

Keywords: agroindustrial waste; bioplastics; cost-effectiveness; environmental impact; extraction processes; microorganisms; polyhydroxyalkanoates (PHAs); sustainable feedstocks.

Figure 1

The general structure of PHAs is represented as follows: R denotes one of the radicals shown in the table, n represents the number of repeats of the monomer(s) (which can be arranged in tandem or randomly), and x signifies the number of CH₂ groups within the main chain of each monomer. For the classification of PHAs, the total number of carbons present in the monomer is used, calculated as (R + x + 2).

Figure 2

A simplified scheme of PHAs biosynthesis from CO₂ in microalgae (A), cyanobacteria (B), and Cupriavidus necator as an example of a chemolithotroph (C) is presented. CA: carbonic anhydrase; TGA: triacylglycerides; FFA: free fatty acids; GAP: glyceraldehyde-3-phosphate; G6P: glucose-6-phosphate; PHA/PHAs: polyhydroxyalkanoates; CBB cycle: Calvin–Benson–Bassham cycle. Gray arrows indicate CO₂ or bicarbonate capture, dashed lines represent passive diffusion, and solid arrows denote transport that may involve transporter proteins and cofactors. (D) Biosynthesis of polyhydroxyalkanoates from acetyl-CoA is depicted, with PhaA: acetyl-CoA acetyltransferase; PhaB: acetoacetyl-CoA reductase; PhaC: poly-(3-hydroxyalkanoate) polymerase. Gray shading illustrates the input of acyl-CoA other than R-3-hydroxybutyryl-CoA into the synthesis of PHAs.

Figure 3

General Scheme of PHAs Synthesis in Heterotrophic Microorganisms. GAP: glyceraldehyde-3-phosphate; 3HP: 3-hydroxypropionate; 3HB: 3-hydroxybutyrate; 4HB: 4-hydroxybutyrate; P(3HP): poly(3-hydroxypropionate); P(4HB): poly(4-hydroxybutyrate); PHA: polyhydroxyalkanoate; P(3HB-co-4HB): poly(3-hydroxybutyrate-co-4-hydroxybutyrate); TCA cycle: tricarboxylic acid cycle; CH₄: methane; CH₃OH: methanol; pMMO: particulate methane monooxygenase; sMMO: soluble methane monooxygenase; RuMP cycle: ribulose monophosphate cycle.

Figure 4

A schematic diagram illustrating the steps required to obtain PHAs generated by microorganisms is provided. All available options for each step are presented.