Biosynthesis of poly(3-hydroxypropionate-co-3-hydroxybutyrate) with fully controllable structures from glycerol

Copolymers as a turning point for large scale polyhydroxyalkanoates applications

Traditional plastics reshaped the society thanks to their brilliant properties and cut-price manufacturing costs. However, their protracted durability and limited recycling threaten the environment. Worthy alternatives seem to be polyhydroxyalkanoates, compostable biopolymers produced by several microbes. The most common 3-hydroxybutyrate homopolymer has limited applications calling for copolymers biosynthesis to enhance material properties. As a growing number of researches assess the discovery of novel comonomers, great endeavors are dedicated as well to copolymers production scale-up, where the choice of the microbial carbon source significantly affects the overall economic feasibility. Diving into novel metabolic pathways, engineered strains, and cutting-edge bioprocess strategies, this review aims to survey up-to-date publications about copolymers production, focusing primarily on precursors origins. Specifically, in the core of the review, copolymers precursors have been divided into three categories based on their economic value: the costliest structurally related ones, the structurally unrelated ones, and finally various low-cost waste streams. The combination of cheap biomasses, efficient pretreatment strategies, and robust microorganisms paths the way towards the development of versatile and circular polymers. Conceived to researchers and industries interested in tackling polyhydroxyalkanoates production, this review explores an angle often underestimated yet of prime importance: if PHAs copolymers offer advanced properties and sustainable end-of-life, the feedstock choice for their upstream becomes a major factor in the development of plastic substitutes.

Sustainable production and degradation of plastics using microbes

Plastics are indispensable in everyday life and industry, but the environmental impact of plastic waste on ecosystems and human health is a huge concern. Microbial biotechnology offers sustainable routes to plastic production and waste management. Bacteria and fungi can produce plastics, as well as their constituent monomers, from renewable biomass, such as crops, agricultural residues, wood and organic waste. Bacteria and fungi can also degrade plastics. We review state-of-the-art microbial technologies for sustainable production and degradation of bio-based plastics and highlight the potential contributions of microorganisms to a circular economy for plastics.

De Novo Synthesis of Poly(3-hydroxybutyrate-co-3-hydroxypropionate) from Oil by Engineered Cupriavidus necator

Poly(3-hydroxybutyrate-co-3-hydroxypropionate) [P(3HB-co-3HP)] is a biodegradable and biocompatible polyester with improved and expanded material properties compared with poly(3-hydroxybutyrate) (PHB). This study engineered a robust malonyl-CoA pathway in Cupriavidus necator for the efficient supply of a 3HP monomer, and could achieve the production of [P(3HB-co-3HP)] from variable oil substrates. Flask level experiments followed by product purification and characterization found the optimal fermentation condition (soybean oil as carbon source, 0.5 g/L arabinose as induction level) in general consideration of the PHA content, PHA titer and 3HP molar fraction. A 5 L fed-batch fermentation (72 h) further increased the dry cell weight (DCW) to 6.08 g/L, the titer of [P(3HB-co-3HP)] to 3.11 g/L and the 3HP molar fraction to 32.25%. Further improving the 3HP molar fraction by increasing arabinose induction failed as the engineered malonyl-CoA pathway was not properly expressed under the high-level induction condition. With several promising advantages (broader range of economic oil substrates, no need for expensive supplementations such as alanine and VB12), this study indicated a candidate route for the industrial level production of [P(3HB-co-3HP)]. For future prospects, further studies are needed to further improve the strain and the fermentation process and expand the range of relative products.

Production of polyhydroxyalkanoates containing monomers conferring amorphous and elastomeric properties from renewable resources: Current status and future perspectives

Petrochemical-based plastics cause environmental pollution and threaten humans and ecosystems. Polyhydroxyalkanoate (PHA) is considered a promising alternative to nondegradable plastics since it is eco-friendly and biodegradable polymer having similar properties to conventional plastics. PHA's material properties are generally determined by composition and type of monomers in PHA. PHA can be designed in tailor-made manner for their suitable application areas. Among many monomers in PHAs, ω-hydroxalkanoates such as 3-hydroxypropionate (3HP), 4-hydroxybutyrate (4HB), 5-hydroxyvalerate (5HV), and 6-hydroxyhexanoate (6HHx) and medium-chain-length 3-hydroxyalkanoate such as 3-hydroxyhexanoate (3HHx) and 4-hydroxyvalerate (4HV), have been examined as potential monomers able to confer amorphous and elastomer properties when these are incorporated as comonomer in poly(3-hydroxybutyrate) copolymer that has 3HB as main monomer along with comonomers in different monomer fraction. Herein, recent advances in production of PHAs designed to have amorphous and elastomeric properties from renewable sources such as lignocellulose, levulinic acid, crude glycerol, and waste oil are discussed.

Control of D-lactic acid content in P(LA-3HB) copolymer in the yeast Saccharomyces cerevisiae using a synthetic gene expression system

The fully biobased polyhydroxyalkanoate (PHA) polymers provide interesting alternatives for petrochemical derived plastic materials. The mechanical properties of some PHAs, including the common poly(3-hydroxybutyrate) (PHB), are limited, but tunable by addition of other monomers into the polymer chain. In this study we present a precise synthetic biology method to adjust lactate monomer fraction of a polymer by controlling the monomer formation in vivo at gene expression level, independent of cultivation conditions. We used the modified doxycycline-based Tet-On approach to adjust the expression of the stereospecific D-lactate dehydrogenase gene (ldhA) from Leuconostoc mesenteroides to control D-lactic acid formation in yeast Saccharomyces cerevisiae. The synthetic Tet-On transcription factor with a VP16 activation domain was continuously expressed and its binding to a synthetic promoter with eight transcription factor specific binding sites upstream of the ldhA gene was controlled with the doxycycline concentration in the media. The increase in doxycycline concentration correlated positively with ldhA expression, D-lactic acid production, poly(D-lactic acid) (PDLA) accumulation in vivo, and D-lactic acid content in the poly(D-lactate-co-3-hydroxybutyrate) P(LA-3HB) copolymer. We demonstrated that the D-lactic acid content of the P(LA-3HB) copolymer can be adjusted linearly from 6 mol% to 93 mol% in vivo in S. cerevisiae. These results highlight the power of controlling gene expression and monomer formation in the tuning of the polymer composition. In addition, we obtained 5.6% PDLA and 19% P(LA-3HB) of the cell dry weight (CDW), which are over two- and five-fold higher accumulation levels, respectively, than reported in the previous studies with yeast. We also compared two engineered PHA synthases and discovered that in S. cerevisiae the PHA synthase PhaC1437_Ps6-19 produced P(LA-3HB) copolymers with lower D-lactic acid content, but with higher molecular weight, in comparison to the PHA synthase PhaC1Pre.

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P(LA-3HB) monomer structure was adjusted with controlled gene expression.

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Expression of D-lactate dehydrogenase (ldhA) was controlled with Tet-On approach.

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Lactic acid content in copolymer P(LA-3HB) was adjusted from 6 mol% up to 93 mol%.

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5.6% PDLA and 19% P(LA-3HB) of cell dry weight (CDW) were obtained in S. cerevisiae.

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PhaC1437_Ps6-19 P(LA-3HB) had lower D-lactic acid % than PhaC1Pre P(LA-3HB).

Polystyrene nanoplastics change the functional traits of biofilm communities in freshwater environment revealed by GeoChip 5.0

There is an increasing concern regarding the potential effects of nanoplastics (NPs) on freshwater ecosystems. Considering the functional values of biofilms in freshwater, knowledge on whether and to what extent NPs can influence the ecosystem processes of biofilms were still limited. Herein, the freshwater biofilms cultured in lab were exposed to 100 nm polystyrene NPs (PS-NPs) of different dosages (1 and 10 mg/L) for 14 days. Confocal laser scanning microscope observation indicated that biofilms were dominated by filamentous, and spiral algae species and the intensity of extracellular polymeric substances increased under PS-NPs exposure. GeoChip 5.0 analysis revealed that PS-NPs exposure triggered a significant increase in functional genes α diversity (p < 0.05) and altered biofilms' functional structure. Furthermore, the abundance of genes involved in the total carbon and nitrogen cycling were increased under PS-NPs exposure. The abundance of nitrogen fixation genes experienced the most pronounced increase (24.4%) under 1 mg/L PS-NPs treatment, consistent with the increase of ammonium in overlying water. Whereas antibiotic resistance genes and those related to photosynthetic pigments production were suppressed. These results provided direct evidence for PS-NPs' effects on the biofilm functions in terms of biogeochemical cycling, improving our understanding of the potentials of NPs on freshwater ecosystems.

Biosynthesis of Poly(3HB-co-3HP) with Variable Monomer Composition in Recombinant Cupriavidus necator H16

Polyhydroxyalkanoates are attractive alternatives to traditional plastics. However, although polyhydroxybutyrate (PHB) is produced in large quantities by Cupriavidus necator H16, its properties are far from ideal for the manufacture of plastic products. These properties may be improved through its coproduction with 3-hydroxypropionate (3HP), which leads to the formation of the copolymer poly(3-hydroxybutyrate-co-3-hydroxypropionate) (poly(3HB-co-3HP). To achieve this, a pathway was designed to enable C. necator H16 to convert β-alanine to 3HP. The initial low levels of incorporation of 3HP into the copolymer were overcome by the overproduction of the native propionyl-CoA transferase together with PHA synthase from Chromobacterium sp. USM2. Following optimization of 3HP incorporation into the copolymer, the molar fraction of 3HP could be controlled by cultivation in medium containing different concentrations of β-alanine. Between 0 and 80 mol % 3HP could be achieved. Further supplementation with 2 mM cysteine increased the maximum 3HP molar fraction to 89%. Additionally, the effect of deletions of the phaA and phaB1 genes of the phaCAB operon on 3HP molar fraction were investigated. A phaAB1 double knockout resulted in a copolymer containing 91 mol % 3HP without the need for cysteine supplementation.

Strain and process development for poly(3HB-co-3HP) fermentation by engineered Shimwellia blattae from glycerol

Poly(3-hydroxybytyrate-co-3-hydroxypropionate), poly(3HB-co-3HP), is a possible alternative to synthetic polymers such as polypropylene, polystyrene and polyethylene due to its low crystallinity and fragility. We already reported that recombinant strains of Shimwellia blattae expressing 1,3-propanediol dehydrogenase DhaT as well as aldehyde dehydrogenase AldD of Pseudomonas putida KT2442, propionate-CoA transferase Pct of Clostridium propionicum X2 and PHA synthase PhaC1 of Ralstonia eutropha H16 are able to accumulate up to 14.5% (wtPHA/wtCDW) of poly(3-hydroxypropionate), poly(3HP), homopolymer from glycerol as a sole carbon source (Appl Microbiol Biotechnol 98:7409-7422, 2014a). However, the cell density was rather low. In this study, we optimized the medium aiming at a more efficient PHA synthesis, and we engineered a S. blattae strain accumulating poly(3HB-co-3HP) with varying contents of the constituent 3-hydroxypropionate (3HP) depending on the cultivation conditions. Consequently, 7.12, 0.77 and 0.32 gPHA/L of poly(3HB-co-3HP) containing 2.1, 8.3 and 18.1 mol% 3HP under anaerobic/aerobic (the first 24 hours under anaerobic condition, thereafter, aerobic condition), low aeration/agitation (the minimum stirring rate required in medium mixing and small amount of aeration) and anaerobic conditions (the minimum stirring rate required in medium mixing without aeration), respectively, were synthesized from glycerol by the genetically modified S. blattae ATCC33430 strains in optimized culture medium.

Poly(3-hydroxypropionate): a promising alternative to fossil fuel-based materials

Polyhydroxyalkanoates (PHAs) are storage compounds synthesized by numerous microorganisms and have attracted the interest of industry since they are biobased and biodegradable alternatives to fossil fuel-derived plastics. Among PHAs, poly(3-hydroxypropionate) [poly(3HP)] has outstanding material characteristics and exhibits a large variety of applications. As it is not brittle like, e.g., the best-studied PHA, poly(3-hydroxybutyrate) [poly(3HB)], it can be used as a plasticizer in blends to improve their properties. Furthermore, 3-hydroxypropionic acid (3HP) is considered likely to become one of the new industrial building blocks, and it can be obtained from poly(3HP) by simple hydrolysis. Unfortunately, no natural organism is known to accumulate poly(3HP) so far. Thus, several efforts have been made to engineer genetically modified organisms capable of synthesizing the homopolymer or copolymers containing 3HP. In this review, the achievements made so far in efforts to obtain biomass which has accumulated poly(3HP) or 3HP-containing copolymers, as well as the properties of these polyesters and their applications, are compiled and evaluated.

Engineering the diversity of polyesters

Many bacteria have been found to produce various polyhydroxyalkanoates (PHA) biopolyesters. In many cases, it is not easy to control the structures of PHA including homopolymers, random copolymers and block copolymers as well as ratios of monomers in the copolymers. It has become possible to engineer bacteria for controllable synthesis of PHA with the desirable structures by creating new PHA synthesis pathways. Remarkably, the weakening of β-oxidation cycle in Pseudomonas putida and Pseudomonas entomophila led to controllable synthesis of all kinds of PHA structures including monomer ratios in random and/or block copolymers when fatty acids are used as PHA precursors. Introduction of functional groups into PHA polymer chains in predefined proportions has become a reality provided fatty acids containing the functional groups are taken up by the bacteria for PHA synthesis. This allows the formation of functional PHA for further grafting. The PHA diversity is further widened by the endless possibility of controllable homopolymerization, random copolymerization, block copolymerization and grafting on functional PHA site chains.

Production of Block Copolymer Poly(3-hydroxybutyrate)-block-poly(3-hydroxypropionate) with Adjustable Structure from an Inexpensive Carbon Source

The block copolymers poly(3-hydroxybutyrate)-block-poly(3-hydroxypropionate) (P3HB-b-P3HP) with a wide range of 3HP fractions from 7.4 mol % to 75 mol % were biosynthesized from inexpensive carbon sources for the first time, differing from previously reported approaches based on sequential addition of precursors. The engineered Escherichia coli strain carried two parallel synthetic pathways modulated by independent regulatory systems to produce the 3HB and 3HP monomers, respectively. Manipulating the expression levels of 3HB and 3HP pathways resulted in biosynthesis of block copolymers P3HB-b-P3HP with varied compositions. Nuclear magnetic resonance and differential scanning calorimetric studies demonstrated novel microstructure and thermal properties not available in related random copolymers and a blend of P3HB and P3HP homopolymers.