Evaluation of medium-chain-length polyhydroxyalkanoate production by Pseudomonas putida LS46 using biodiesel by-product streams

A Review of Current Achievements and Recent Challenges in Bacterial Medium-Chain-Length Polyhydroxyalkanoates: Production and Potential Applications

Using petroleum-derived plastics has contributed significantly to environmental issues, such as greenhouse gas emissions and the accumulation of plastic waste in ecosystems. Researchers have focused on developing ecofriendly polymers as alternatives to traditional plastics to address these concerns. This review provides a comprehensive overview of medium-chain-length polyhydroxyalkanoates (mcl-PHAs), biodegradable biopolymers produced by microorganisms that show promise in replacing conventional plastics. The review discusses the classification, properties, and potential substrates of less studied mcl-PHAs, highlighting their greater ductility and flexibility compared to poly(3-hydroxybutyrate), a well-known but brittle PHA. The authors summarize existing research to emphasize the potential applications of mcl-PHAs in biomedicine, packaging, biocomposites, water treatment, and energy. Future research should focus on improving production techniques, ensuring economic viability, and addressing challenges associated with industrial implementation. Investigating the biodegradability, stability, mechanical properties, durability, and cost-effectiveness of mcl-PHA-based products compared to petroleum-based counterparts is crucial. The future of mcl-PHAs looks promising, with continued research expected to optimize production techniques, enhance material properties, and expand applications. Interdisciplinary collaborations among microbiologists, engineers, chemists, and materials scientists will drive progress in this field. In conclusion, this review serves as a valuable resource to understand mcl-PHAs as sustainable alternatives to conventional plastics. However, further research is needed to optimize production methods, evaluate long-term ecological impacts, and assess the feasibility and viability in various industries.

Polyhydroxyalkanoate production in Pseudomonas putida from alkanoic acids of varying lengths

Many studies have been conducted to produce microbial polyhydroxyalkanoates (PHA), a biopolymer, from Pseudomonas sp. fed with various alkanoic acids. Because this previous data was collected using methodologies that varied in critical aspects, such as culture media and size range of alkanoic acids, it has been difficult to compare the results for a thorough understanding of the relationship between feedstock and PHA production. Therefore, this study utilized consistent culture media with a wide range of alkanoic acids (C7-C14) to produce medium chain length PHAs. Three strains of Pseudomonas putida (NRRL B-14875, KT2440, and GN112) were used, and growth, cell dry weight, PHA titer, monomer distribution, and molecular weights were all examined. It was determined that although all the strains produced similar PHA titers using C7-C9 alkanoic acids, significant differences were observed with the use of longer chain feedstocks. Specifically, KT2440 and its derivative GN112 produced higher PHA titers compared to B-14875 when fed longer chain alkanoates. We also compared several analytical techniques for determining amounts of PHA and found they produced different results. In addition, the use of an internal standard had a higher risk of calculating inaccurate concentrations compared to an external standard. These observations highlight the importance of considering this aspect of analysis when evaluating different studies.

Advances and trends in microbial production of polyhydroxyalkanoates and their building blocks

With the rapid development of synthetic biology, a variety of biopolymers can be obtained by recombinant microorganisms. Polyhydroxyalkanoates (PHA) is one of the most popular one with promising material properties, such as biodegradability and biocompatibility against the petrol-based plastics. This study reviews the recent studies focusing on the microbial synthesis of PHA, including chassis engineering, pathways engineering for various substrates utilization and PHA monomer synthesis, and PHA synthase modification. In particular, advances in metabolic engineering of dominant workhorses, for example Halomonas, Ralstonia eutropha, Escherichia coli and Pseudomonas, with outstanding PHA accumulation capability, were summarized and discussed, providing a full landscape of diverse PHA biosynthesis. Meanwhile, we also introduced the recent efforts focusing on structural analysis and mutagenesis of PHA synthase, which significantly determines the polymerization activity of varied monomer structures and PHA molecular weight. Besides, perspectives and solutions were thus proposed for achieving scale-up PHA of low cost with customized material property in the coming future.

Enhanced co-production of medium-chain-length polyhydroxyalkanoates and phenazines from crude glycerol by high cell density cultivation of Pseudomonas chlororaphis in membrane bioreactor

Enhanced co-production of medium-chain-length polyhydroxyalkanoates (mcl-PHA) and extracellular phenazines was assessed through a high cell density cultivation of Pseudomonas chlororaphis subsp. aurantiaca (DSM 19603) in a membrane bioreactor using crude glycerol as a fermentative substrate. A maximum dry cell weight (DCW) of 59.25 ± 0.31 g/L was achieved at 90 h of cultivation with a maximum mcl-PHA and extracellular phenazines concentrations of respectively 19.05 ± 0.04 g/L (32.16% of DCW) and 79.42 ± 0.35 mg/L. mcl-PHA concentration achieved through cell retention culture was 28.43-folds higher than that obtained by batch culture. Fourier transform infrared spectroscopy, gas chromatography-mass spectrometry, and ¹H nuclear magnetic resonance analysis identified the produced PHA as a mcl-PHA copolymer of 3-hydroxyhexanoate (0.68%), 3-hydroxyoctanoate (7.76%), 3-hydroxydecanoate (49.18%), 3-hydroxydodecanoate (4.89%), and 3-hydroxytetradecanoate (37.50%). The mcl-PHA exhibited a highly amorphous structure with low crystallinity index (4.19%) and high thermal stability. This is the first report on the enhanced co-production of mcl-PHA and phenazines in a membrane bioreactor.

A four-microorganism three-step fermentation process for producing medium-chain-length polyhydroxyalkanoate from starch

In this study, a four-microorganism three-step fermentation process was established for producing medium-chain-length polyhydroxyalkanoate (mcl-PHA) from starch, which was used as the sole carbon source. The four microorganisms used for this process were Aspergillus niger, Saccharomyces cerevisiae L2612, Acetobacter orientalis, and Pseudomonas putida KT2440-acs. The initial carbon source starch concentration was set to 30 g/L, the maximum glucose concentration reached 17.66 g/L at 48 h after starch hydrolysis, and then, 2.36 g/L of acetic acid was obtained at 96 h. The final output of mcl-PHA was 0.5 g/L at 144 h, overall productivity for mcl-PHA was 3.47 mg/(L·h) and the total starch to mcl-PHA yield for the process was 16.67 mg/g. Although the overall yield and conversion rate of this process were not high, this is the first attempt to produce mcl-PHA using starch as a substrate, and it provides a feasible strategy for producing PHA from kitchen waste. The production process of mcl-PHA with a clear flora structure and short fermentation cycle was realized.

Development of High Cell Density Cultivation Strategies for Improved Medium Chain Length Polyhydroxyalkanoate Productivity Using Pseudomonas putida LS46

High cell density (HCD) fed-batch cultures are widely perceived as a requisite for high-productivity polyhydroxyalkanoate (PHA) cultivation processes. In this work, a reactive pulse feed strategy (based on real-time CO₂ or dissolved oxygen (DO) measurements as feedback variables) was used to control an oxygen-limited fed-batch process for improved productivity of medium chain length (mcl-) PHAs synthesized by Pseudomonas putida LS46. Despite the onset of oxygen limitation half-way through the process (14 h post inoculation), 28.8 ± 3.9 g L⁻¹ total biomass (with PHA content up to 61 ± 8% cell dry mass) was reliably achieved within 27 h using octanoic acid as the carbon source in a bench-scale (7 L) bioreactor operated under atmospheric conditions. This resulted in a final volumetric productivity of 0.66 ± 0.14 g L⁻¹ h⁻¹. Delivering carbon to the bioreactor as a continuous drip feed process (a proactive feeding strategy compared to pulse feeding) made little difference on the final volumetric productivity of 0.60 ± 0.04 g L⁻¹ h⁻¹. However, the drip feed strategy favored production of non-PHA residual biomass during the growth phase, while pulse feeding favored a higher rate of mcl-PHA synthesis and yield during the storage phase. Overall, it was shown that the inherent O₂-limitation brought about by HCD cultures can be used as a simple and effective control strategy for mcl-PHA synthesis from fatty acids. Furthermore, the pulse feed strategy appears to be a relatively easy and reliable method for rapid optimization of fed-batch processes, particularly when using toxic substrates like octanoic acid.

Occurrence of non-toxic bioemulsifiers during polyhydroxyalkanoate production by Pseudomonas strains valorizing crude glycerol by-product

While screening for polyhydroxyalkanoate (PHA) producing strains, using glycerol rich by-product as carbon source, it was observed that extracellular polymers were also secreted into the culture broth. The scope of this study was to characterize both intracellular and extracellular polymers, produced by Pseudomonas putida NRRL B-14875 and Pseudomonas chlororaphis DSM 50083, mostly focusing on those novel extracellular polymers. It was found that they fall into the class of bioemulsifiers (BE), as they showed excellent emulsion stability against different hydrocarbons/oils at various pH conditions, temperature and salinity concentrations. Cytotoxicity tests revealed that BE produced by P. chlororaphis inhibited the growth of highly pigmented human melanoma cells (MNT-1) by 50% at concentrations between 150 and 200 μg/mL, while no effect was observed on normal skin primary keratinocytes and melanocytes. This is the first study reporting mcl-PHA production by P. putida NRRL B-14785 and bioemulsifier production from both P. putida and P. chlororaphis strains.

Biochemistry, genetics and biotechnology of glycerol utilization in Pseudomonas species

The use of renewable waste feedstocks is an environment‐friendly choice contributing to the reduction of waste treatment costs and increasing the economic value of industrial by‐products. Glycerol (1,2,3‐propanetriol), a simple polyol compound widely distributed in biological systems, constitutes a prime example of a relatively cheap and readily available substrate to be used in bioprocesses. Extensively exploited as an ingredient in the food and pharmaceutical industries, glycerol is also the main by‐product of biodiesel production, which has resulted in a progressive drop in substrate price over the years. Consequently, glycerol has become an attractive substrate in biotechnology, and several chemical commodities currently produced from petroleum have been shown to be obtained from this polyol using whole‐cell biocatalysts with both wild‐type and engineered bacterial strains. Pseudomonas species, endowed with a versatile and rich metabolism, have been adopted for the conversion of glycerol into value‐added products (ranging from simple molecules to structurally complex biopolymers, e.g. polyhydroxyalkanoates), and a number of metabolic engineering strategies have been deployed to increase the number of applications of glycerol as a cost‐effective substrate. The unique genetic and metabolic features of glycerol‐grown Pseudomonas are presented in this review, along with relevant examples of bioprocesses based on this substrate – and the synthetic biology and metabolic engineering strategies implemented in bacteria of this genus aimed at glycerol valorization.

Microbial degradation of low-density polyethylene and synthesis of polyhydroxyalkanoate polymers

We have characterized the ability of eight bacterial strains to utilize powdered low-density polyethylene (LDPE) plastic (untreated and without any additives) as a sole carbon source. Cell mass production on LDPE-containing medium after 21 days of incubation varied between 0.083 ± 0.015 g/L cell dry mass (cdm) for Micrococcus luteus IRN20 and 0.39 ± 0.036 g/L for Cupriavidus necator H16. The percent decrease in LDPE mass ranged from 18.9% ± 0.72% for M. luteus IRN20 to 33.7% ± 1.2% for C. necator H16. Linear alkane hydrolysis products from LDPE degradation were detected in the culture media, and the carbon chain lengths of the hydrolysis products detected varied, depending on the species of bacteria. We also determined that C. necator H16 produced short-chain-length polyhydroxyalkanoate biopolymers, while Pseudomonas putida LS46 and Acinetobacter pittii IRN19 produced medium-chain-length biopolymers while growing on polyethylene powder. Cupriavidus necator H16 accumulated poly (3-hydroxybutyrate-co-3-hydroxyvalerate) (PHB-V) polymers to 3.18% ± 0.4% of cdm. The monomer composition of the PHB-V was 94.9% ± 0.61% 3-hydroxybutyrate and 5.03% ± 0.56% 3-hydroxyvalerate. This is the first report that provides direct evidence for simultaneous bioconversion of LDPE plastic to biodegradable polyhydroxyalkanoate polymers.

Bioreactor Operating Strategies for Improved Polyhydroxyalkanoate (PHA) Productivity

Microbial polyhydroxyalkanoates (PHAs) are promising biodegradable polymers that may alleviate some of the environmental burden of petroleum-derived polymers. The requirements for carbon substrates and energy for bioreactor operations are major factors contributing to the high production costs and environmental impact of PHAs. Improving the process productivity is an important aspect of cost reduction, which has been attempted using a variety of fed-batch, continuous, and semi-continuous bioreactor systems, with variable results. The purpose of this review is to summarize the bioreactor operations targeting high PHA productivity using pure cultures. The highest volumetric PHA productivity was reported more than 20 years ago for poly(3-hydroxybutryate) (PHB) production from sucrose (5.1 g L⁻¹ h⁻¹). In the time since, similar results have not been achieved on a scale of more than 100 L. More recently, a number fed-batch and semi-continuous (cyclic) bioreactor operation strategies have reported reasonably high productivities (1 g L⁻¹ h⁻¹ to 2 g L⁻¹ h⁻¹) under more realistic conditions for pilot or industrial-scale production, including the utilization of lower-cost waste carbon substrates and atmospheric air as the aeration medium, as well as cultivation under non-sterile conditions. Little development has occurred in the area of fully continuously fed bioreactor systems over the last eight years.

Carbon flux to growth or polyhydroxyalkanoate synthesis under microaerophilic conditions is affected by fatty acid chain-length in Pseudomonas putida LS46

Economical production of medium-chain length polyhydroxyalkanoates (mcl-PHA) is dependent on efficient cultivation processes. This work describes growth and mcl-PHA synthesis characteristics of Pseudomonas putida LS46 when grown on medium-chain length fatty acids (octanoic acid) and lower-cost long-chain fatty acids (LCFAs, derived from hydrolyzed canola oil) in microaerophilic environments. Growth on octanoic acid ceased when the oxygen uptake rate was limited by the oxygen transfer rate, and mcl-PHA accumulated to 61.9% of the cell dry mass. From LCFAs, production of non-PHA cell mass continued at a rate of 0.36 g L^-1 h^-1 under oxygen-limited conditions, while mcl-PHA accumulated simultaneously to 31% of the cell dry mass. The titer of non-PHA cell mass from LCFAs at 14 h post-inoculation was double that obtained from octanoic acid in bioreactors operated with identical feeding and aeration conditions. While the productivity for octanoic acid was higher by 14 h, prolonged cultivation on LCFAs achieved similar productivity but with twice the PHA titer. Simultaneous co-feeding of each substrate demonstrated the continued cell growth under microaerophilic conditions characteristic of LCFAs, and the resulting polymer was dominant in C8 monomers. Furthermore, co-feeding resulted in improved PHA titer and volumetric productivity compared to either substrate individually. These results suggest that LCFAs improve growth of P. putida in oxygen-limited environments and could reduce production costs since more non-PHA cell mass, the cellular factories required to produce mcl-PHA and the most oxygen-intensive cellular process, can be produced for a given oxygen transfer rate.

Synthesis of polyhydroxyalkanoates (PHAs) from vegetable oils and free fatty acids by wild-type and mutant strains of Pseudomonas chlororaphis

Pseudomonas chlororaphis PA23 was isolated from soybean roots as a plant-growth-promoting rhizobacterium. This strain secretes a wide range of compounds, including the antibiotics phenazine-1-carboxylic acid (PCA), pyrrolnitrin, and 2-hydroxyphenazine. We have determined that P. chlororaphis PA23 can synthesize medium-chain-length polyhydroxyalkanoate (PHA) polymers utilizing free fatty acids, such as octanoic acid and nonanoic acid, as well as vegetable oils as sole carbon sources. Genome analysis identified a pha operon containing 7 genes in P. chlororaphis PA23 that were highly conserved. A nonpigmented strain that does not synthesize PCA, P. chlororaphis PA23-63, was also studied for PHA production. Pseudomonas chlororaphis PA23-63 produced 2.42–5.14 g/L cell biomass and accumulated PHAs from 11.7% to 32.5% cdm when cultured with octanoic acid, nonanoic acid, fresh canola oil, waste canola fryer oil, or biodiesel-derived waste free fatty acids under batch culture conditions. The subunit composition of the PHAs produced from fresh canola oil, waste canola fryer oil, or biodiesel-derived free fatty acids did not differ significantly. Addition of octanoic acid and nonanoic acid to canola oil cultures increased PHA production, but addition of glucose did not. PHA production in the phz mutant, P. chlororaphis PA23-63, was greater than that in the parent strain.

Global changes in the proteome of Cupriavidus necator H16 during poly-(3-hydroxybutyrate) synthesis from various biodiesel by-product substrates

Synthesis of poly-[3-hydroxybutyrate] (PHB) by Cupriavidus necator H16 in batch cultures was evaluated using three biodiesel-derived by-products as the sole carbon sources: waste glycerol (REG-80, refined to 80 % purity with negligible free fatty acids); glycerol bottom (REG-GB, with up to 65 % glycerol and 35 % free fatty acids), and free fatty acids (REG-FFA, with up to 75 % FFA and no glycerol). All the three substrates supported growth and PHB production by C. necator, with polymer accumulation ranging from 9 to 84 % cell dry weight (cdw), depending on the carbon source. To help understand these differences, proteomic analysis indicated that although C. necator H16 was able to accumulate PHB during growth on all three biodiesel by-products, no changes in the levels of PHB synthesis enzymes were observed. However, significant changes in the levels of expression were observed for two Phasin proteins involved with PHB accumulation, and for a number of gene products in the fatty acid β-oxidation pathway, the Glyoxylate Shunt, and the hydrogen (H2) synthesis pathways in C. necator cells cultured with different substrates. The glycerol transport protein (GlpF) was induced in REG-GB and REG-80 glycerol cultures only. Cupriavidus necator cells cultured with REG-GB and REG-FFA showed up-regulation of β-oxidation and Glyoxylate Shunt pathways proteins at 24 h pi, but H2 synthesis pathways enzymes were significantly down-regulated, compared with cells cultured with waste glycerol. Our data confirmed earlier observations of constitutive expression of PHB synthesis proteins, but further suggested that C. necator H16 cells growing on biodiesel-derived glycerol were under oxidative stress.

Efficient production of polyhydroxyalkanoates (PHAs) from Pseudomonas mendocina PSU using a biodiesel liquid waste (BLW) as the sole carbon source

Conditions for the optimal production of polyhydroxyalkanoate (PHA) by Pseudomonas mendocina PSU using a biodiesel liquid waste (BLW) were determined by response surface methodology. These were an initial carbon to nitrogen ratio (C/N) of 40 (mole/mole), an initial pH of 7.0, and a temperature of 35 °C. A biomass and PHA concentration of 3.65 g/L and about 2.6 g/L (77% DCW), respectively, were achieved in a growth associated process using 20 g/L glycerol in the BLW after 36 h of exponential growth. The PHA monomer compositions were 3HB (3-hydroxybutyrate), a short-chain-length-PHA, and the medium-chain-length-PHA e.g. 3-hydroxyoctanoate and 3-hydroxydecanoate. Both the phbC and phaC genes were characterized. The phbC enzyme had not been previously detected in a Pseudomonas mendocina species. A 2.15 g/L of an exopolysaccharide, alginate, was also produced with a similar composition to that of other Pseudomonas species.

Integrated analysis of gene expression and metabolic fluxes in PHA-producing Pseudomonas putida grown on glycerol

Background

Given its high surplus and low cost, glycerol has emerged as interesting carbon substrate for the synthesis of value-added chemicals. The soil bacterium Pseudomonas putida KT2440 can use glycerol to synthesize medium-chain-length poly(3-hydroxyalkanoates) (mcl-PHA), a class of biopolymers of industrial interest. Here, glycerol metabolism in P. putida KT2440 was studied on the level of gene expression (transcriptome) and metabolic fluxes (fluxome), using precisely adjusted chemostat cultures, growth kinetics and stoichiometry, to gain a systematic understanding of the underlying metabolic and regulatory network.

Results

Glycerol-grown P. putida KT2440 has a maintenance energy requirement [0.039 (mmol_glycerol (g_CDW h)⁻¹)] that is about sixteen times lower than that of other bacteria, such as Escherichia coli, which provides a great advantage to use this substrate commercially. The shift from carbon (glycerol) to nitrogen (ammonium) limitation drives the modulation of specific genes involved in glycerol metabolism, transport electron chain, sensors to assess the energy level of the cell, and PHA synthesis, as well as changes in flux distribution to increase the precursor availability for PHA synthesis (Entner–Doudoroff pathway and pyruvate metabolism) and to reduce respiration (glyoxylate shunt). Under PHA-producing conditions (N-limitation), a higher PHA yield was achieved at low dilution rate (29.7 wt% of CDW) as compared to a high rate (12.8 wt% of CDW). By-product formation (succinate, malate) was specifically modulated under these regimes. On top of experimental data, elementary flux mode analysis revealed the metabolic potential of P. putida KT2440 to synthesize PHA and identified metabolic engineering targets towards improved production performance on glycerol.

Conclusion

This study revealed the complex interplay of gene expression levels and metabolic fluxes under PHA- and non-PHA producing conditions using the attractive raw material glycerol as carbon substrate. This knowledge will form the basis for the development of future metabolically engineered hyper-PHA-producing strains derived from the versatile bacterium P. putida KT2440.

Quantitative 'Omics Analyses of Medium Chain Length Polyhydroxyalkanaote Metabolism in Pseudomonas putida LS46 Cultured with Waste Glycerol and Waste Fatty Acids

Transcriptomes and proteomes of Pseudomonas putida LS46 cultured with biodiesel-derived waste glycerol or waste free fatty acids, as sole carbon sources, were compared under conditions that were either permissive or non-permissive for synthesis of medium chain length polyhydroxyalkanoates (mcl-PHA). The objectives of this study were to elucidate mechanisms that influence activation of biopolymer synthesis, intra-cellular accumulation, and monomer composition, and determine if these were physiologically specific to the carbon sources used for growth of P. putida LS46. Active mcl-PHA synthesis by P. putida LS46 was associated with high expression levels of key mcl-PHA biosynthesis genes and/or gene products including monomer-supplying proteins, PHA synthases, and granule-associated proteins. 'Omics data suggested that expression of these genes were regulated by different genetic mechanisms in P. putida LS46 cells in different physiological states, when cultured on the two waste carbon sources. Optimal polymer production by P. putida LS46 was primarily limited by less efficient glycerol metabolism during mcl-PHA synthesis on waste glycerol. Mapping the 'Omics data to the mcl-PHA biosynthetic pathway revealed significant variations in gene expression, primarily involved in: 1) glycerol transportation; 2) enzymatic reactions that recycle reducing equivalents and produce key mcl-PHA biosynthesis pathway intermediates (e.g. NADH/NADPH, acetyl-CoA). Active synthesis of mcl-PHAs was observed during exponential phase in cultures with waste free fatty acids, and was associated with the fatty acid beta-oxidation pathway. A putative Thioesterase in the beta-oxidation pathway that may regulate the level of fatty acid beta-oxidation intermediates, and thus carbon flux to mcl-PHA biosynthesis, was highly up-regulated. Finally, the data suggested that differences in expression of selected fatty acid metabolism and mcl-PHA monomer-supplying enzymes may play a role in determining the monomer composition of mcl-PHA polymers. Understanding the relationships between genome content, gene and gene product expression, and how these factors influence polymer synthesis, will aid in optimization of mcl-PHA production by P. putida LS46 using biodiesel waste streams.

Effects of impurities in biodiesel-derived glycerol on growth and expression of heavy metal ion homeostasis genes and gene products in Pseudomonas putida LS46

Biodiesel production-derived waste glycerol (WG) was previously investigated as potential carbon source for medium chain length polyhydroxyalkanoate (mcl-PHA) production by Pseudomonas putida LS46. In this study, we evaluated the effect of impurities in the WG on P. putida LS46 physiology during exponential growth and corresponding changes in transcription and protein expression profiles compared with cells grown on pure, reagent grade glycerol. High concentration of metal ions, such as Na(+), and numbers of heavy metals ion, such as copper, ion, zinc, were detected in biodiesel-derived WG. Omics analysis from the corresponding cultures suggested altered expression of genes involved in transport and metabolism of ammonia and heavy metal ions. Expression of three groups of heavy metal homeostasis genes was significantly changed (mostly upregulated) in WG cultures and included the following: copper-responded cluster 1 and 2 genes, primarily containing cusABC; two copies of copAB and heavy metal translocating P-type ATPase; Fur-regulated, TonB-dependent siderophore receptor; and several cobalt/zinc/cadmium transporters. Expression of these genes suggests regulation of intracellular concentrations of heavy metals during growth on biodiesel-derived glycerol. Finally, a number of genes involved in adapting to, or metabolizing free fatty acids and other nonheavy metal contaminants, such as Na(+), were also upregulated in P. putida LS46 grown on biodiesel-derived glycerol.

Comparison of mcl-Poly(3-hydroxyalkanoates) synthesis by different Pseudomonas putida strains from crude glycerol: citrate accumulates at high titer under PHA-producing conditions

BACKGROUND

Achieving a sustainable society requires, among other things, the use of renewable feedstocks to replace chemicals obtained from petroleum-derived compounds. Crude glycerol synthesized inexpensively as a byproduct of biodiesel production is currently considered a waste product, which can potentially be converted into value-added compounds by bacterial fermentation. This study aimed at evaluating several characterized P. putida strains to produce medium-chain-length poly(3-hydroxyalkanoates) (mcl-PHA) using raw glycerol as the only carbon/energy source.

RESULTS

Among all tested strains, P. putida KT2440 most efficiently synthesized mcl-PHA under nitrogen-limiting conditions, amassing more than 34% of its cell dry weight as PHA. Disruption of the PHA depolymerase gene (phaZ) in P. putida KT2440 enhanced the biopolymer titer up to 47% PHA (%wt/wt). The low biomass and PHA titer found in the mutant strain and the wild-type strain KT2440 seems to be triggered by the high production of the side-product citrate during the fermentation process which shows a high yield of 0.6 g/g.

CONCLUSIONS

Overall, this work demonstrates the importance of choosing an appropriate microbe for the synthesis of mcl-PHA from waste materials, and a close inspection of the cell metabolism in order to identify undesired compounds that diminish the availability of precursors in the synthesis of biopolymers such as polyhydroxyalkanoates. Future metabolic engineering works should focus on reducing the production of citrate in order to modulate resource allocation in the cell's metabolism of P. putida, and finally increase the biopolymer production.