Polyhydroxybutyrate: an intriguing biopolymer

Taguchi approach for assessing supercritical CO2 (sCO2) fluid extraction of polyhydroxyalkanoate (PHA) from Chlorella Vulgaris sp. microalgae

This research explicitly investigates the utilization of Chlorella Vulgaris sp. microalgae as a renewable source for lipid production, focusing on its application in bioplastic manufacturing. This study employed the supercritical fluid extraction technique employing supercritical CO2 (sCO2) as a green technology to selectively extract and produce PHA's precursor utilizing CO2 solvent as a cleaner solvent compared to conventional extraction method. The study assessed the effects of three extraction parameters, namely temperature (40-60 °C), pressure (15-35 MPa), and solvent flow rate (4-8 ml/min). The pressure, flowrate, and temperature were found to be the most significant parameters affecting the sCO2 extraction. Through Taguchi optimization, the optimal parameters were determined as 60 °C, 35 MPa, and 4 ml/min with the highest lipid yield of 46.74 wt%; above-average findings were reported. Furthermore, the pretreatment process involved significant effects such as crumpled and exhaustive structure, facilitating the efficient extraction of total lipids from the microalgae matrix. This study investigated the microstructure of microalgae biomatrix before and after extraction using scanning electron microscopy (SEM) and thermogravimetric analysis (TGA). Fourier-transform infrared spectroscopy (FTIR) was utilized to assess the potential of the extracted material as a precursor for biodegradable plastic production, with a focus on reduced heavy metal content through inductively coupled plasma-optical emission spectrometry (ICP-OES) analysis. The lipid extracted from Chlorella Vulgaris sp. microalgae was analysed using gas chromatography-mass spectrometry (GC-MS), identifying key constituents, including oleic acid (C18H34O2), n-Hexadecanoic acid (C16H32O2), and octadecanoic acid (C18H36O2), essential for polyhydroxyalkanoate (PHA) formation.

The protective role of PHB and its degradation products against stress situations in bacteria

Many bacteria produce storage biopolymers that are mobilized under conditions of metabolic adaptation, for example, low nutrient availability and cellular stress. Polyhydroxyalkanoates are often found as carbon storage in Bacteria or Archaea, and of these polyhydroxybutyrate (PHB) is the most frequently occurring PHA type. Bacteria usually produce PHB upon availability of a carbon source and limitation of another essential nutrient. Therefore, it is widely believed that the function of PHB is to serve as a mobilizable carbon repository when bacteria face carbon limitation, supporting their survival. However, recent findings indicate that bacteria switch from PHB synthesis to mobilization under stress conditions such as thermal and oxidative shock. The mobilization products, 3-hydroxybutyrate and its oligomers, show a protective effect against protein aggregation and cellular damage caused by reactive oxygen species and heat shock. Thus, bacteria should have an environmental monitoring mechanism directly connected to the regulation of the PHB metabolism. Here, we review the current knowledge on PHB physiology together with a summary of recent findings on novel functions of PHB in stress resistance. Potential applications of these new functions are also presented.

Cyanobacterial PHA Production-Review of Recent Advances and a Summary of Three Years' Working Experience Running a Pilot Plant

Cyanobacteria, as photoautotrophic organisms, provide the opportunity to convert CO2 to biomass with light as the sole energy source. Like many other prokaryotes, especially under nutrient deprivation, most cyanobacteria are able to produce polyhydroxyalkanoates (PHAs) as intracellular energy and carbon storage compounds. In contrast to heterotrophic PHA producers, photoautotrophic cyanobacteria do not consume sugars and, therefore, do not depend on agricultural crops, which makes them a green alternative production system. This review summarizes the recent advances in cyanobacterial PHA production. Furthermore, this study reports the working experience with different strains and cultivating conditions in a 200 L pilot plant. The tubular photobioreactor was built at the coal power plant in Dürnrohr, Austria in 2013 for direct utilization of flue gases. The main challenges were the selection of robust production strains, process optimization, and automation, as well as the CO2 availability.

The methylaspartate cycle in haloarchaea and its possible role in carbon metabolism

Haloarchaea (class Halobacteria) live in extremely halophilic conditions and evolved many unique metabolic features, which help them to adapt to their environment. The methylaspartate cycle, an anaplerotic acetate assimilation pathway recently proposed for Haloarcula marismortui, is one of these special adaptations. In this cycle, acetyl-CoA is oxidized to glyoxylate via methylaspartate as a characteristic intermediate. The following glyoxylate condensation with another molecule of acetyl-CoA yields malate, a starting substrate for anabolism. The proposal of the functioning of the cycle was based mainly on in vitro data, leaving several open questions concerning the enzymology involved and the occurrence of the cycle in halophilic archaea. Using gene deletion mutants of H. hispanica, enzyme assays and metabolite analysis, we now close these gaps by unambiguous identification of the genes encoding all characteristic enzymes of the cycle. Based on these results, we were able to perform a solid study of the distribution of the methylaspartate cycle and the alternative acetate assimilation strategy, the glyoxylate cycle, among haloarchaea. We found that both of these cycles are evenly distributed in haloarchaea. Interestingly, 83% of the species using the methylaspartate cycle possess also the genes for polyhydroxyalkanoate biosynthesis, whereas only 34% of the species with the glyoxylate cycle are capable to synthesize this storage compound. This finding suggests that the methylaspartate cycle is shaped for polyhydroxyalkanoate utilization during carbon starvation, whereas the glyoxylate cycle is probably adapted for growth on substrates metabolized via acetyl-CoA.

Does chlorination of seawater reverse osmosis membranes control biofouling?

Biofouling is the major problem of reverse osmosis (RO) membranes used for desalting seawater (SW). The use of chlorine is a conventional and common practice to control/prevent biofouling. Unlike polyamide RO membranes, cellulose triacetate (CTA) RO membranes display a high chlorine tolerance. Due to this characteristic, CTA membranes are used in most of the RO plants located in the Middle East region where the elevated seawater temperature and water quality promote the risk of membrane biofouling. However, there is no detailed study on the investigation/characterization of CTA-RO membrane fouling. In this investigation, the fouling profile of a full-scale SWRO desalination plant operating with not only continuous chlorination of raw seawater but also intermittent chlorination of CTA-RO membranes was studied. Detailed water quality and membrane fouling analyses were conducted. Profiles of microbiological, inorganic, and organic constituents of analysed fouling layers were extensively discussed. Our results clearly identified biofilm development on these membranes. The incapability of chlorination on preventing biofilm formation on SWRO membranes could be assigned to its failure in effectively reaching throughout the different regions of the permeators. This failure could have occurred due to three main factors: plugging of membrane fibers, chlorine consumption by organics accumulated on the front side fibers, or chlorine adaptation of certain bacterial populations.

Horizontal transfer of carbohydrate metabolism genes into ectomycorrhizal Amanita

The genus Amanita encompasses both symbiotic, ectomycorrhizal fungi and asymbiotic litter decomposers; all species are derived from asymbiotic ancestors. Symbiotic species are no longer able to degrade plant cell walls. The carbohydrate esterases family 1 (CE1s) is a diverse group of enzymes involved in carbon metabolism, including decomposition and carbon storage. CE1 genes of the ectomycorrhizal A. muscaria appear diverged from all other fungal homologues, and more similar to CE1s of bacteria, suggesting a horizontal gene transfer (HGT) event. In order to test whether AmanitaCE1s were acquired horizontally, we built a phylogeny of CE1s collected from across the tree of life, and describe the evolution of CE1 genes among Amanita and relevant lineages of bacteria. CE1s of symbiotic Amanita were very different from CE1s of asymbiotic Amanita, and are more similar to bacterial CE1s. The protein structure of one CE1 gene of A. muscaria matched a depolymerase that degrades the carbon storage molecule poly((R)-3-hydroxybutyrate) (PHB). Asymbiotic Amanita do not carry sequence or structural homologues of these genes. The CE1s acquired through HGT may enable novel metabolisms, or play roles in signaling or defense. This is the first evidence for the horizontal transfer of carbohydrate metabolism genes into ectomycorrhizal fungi.

A synthetic biochemistry molecular purge valve module that maintains redox balance

The greatest potential environmental benefit of metabolic engineering would be the production of high-volume commodity chemicals, such as biofuels. Yet, the high yields required for the economic viability of low-value chemicals is particularly hard to achieve in microbes owing to the myriad competing biochemical pathways. An alternative approach, which we call synthetic biochemistry, is to eliminate the organism by constructing biochemical pathways in vitro. Viable synthetic biochemistry, however, will require simple methods to replace the cellular circuitry that maintains cofactor balance. Here we design a simple purge valve module for maintaining NADP(+)/NADPH balance. We test the purge valve in the production of polyhydroxybutyryl bioplastic and isoprene--pathways where cofactor generation and utilization are unbalanced. We find that the regulatory system is highly robust to variations in cofactor levels and readily transportable. The molecular purge valve provides a step towards developing continuously operating, sustainable synthetic biochemistry systems.

Expression of active recombinant human tissue-type plasminogen activator by using in vivo polyhydroxybutyrate granule display

Recombinant human tissue plasminogen activator (rPA) is a truncated version of tissue plasminogen activator (tPA), which contains nine disulfide bonds and is prone to forming inactive inclusion bodies when expressed in bacteria. To obtain functional rPA expression, we displayed the rPA on the surface of polyhydroxybutyrate (PHB) granules using phasin as the affinity tag. rPA was fused to the N terminus of the phasin protein with a thrombin cleavage site as the linker. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and immunoblot analysis showed that rPA fusion was successfully displayed on the surface of PHB granules. An activity assay indicated that the rPA fusion is active. The in vivo surface display strategy for functional rPA expression in Escherichia coli is distinct for its efficient folding and easier purification and may be expanded to the expression of other eukaryotic proteins with complex conformation.

Application of polyhydroxyalkanoates nanoparticles as intracellular sustained drug-release vectors

Polyhydroxybutyrate (PHB), co-polyesters of 3-hydroxybutyrate and 3-hydroxyhexanoate (PHBHHx), and polylactic acid (PLA) were used to prepare nanoparticles with average sizes of 160, 250 and 150 nm, respectively. A lipid-soluble colorant, rhodamine B isothiocyanate (RBITC), was employed to study drug-release behaviors from these nanoparticles. A high RBITC drug-loading efficiency of over 75% was achieved with all PHA nanoparticles prepared. Macrophage endocytosis led to an intracellular RBITC drug sustained release over a period of at least 20 days for PHB and PHBHHx nanoparticles, while PLA nanoparticles and free drug lasted only 15 days and a week, respectively. Polymer properties and particle sizes showed little effect on drug-release behavior. This study showed for the first time that PHB and PHBHHx can be used effectively to achieve intracellular controlled drug releases.

Flow calorimetry and dielectric spectroscopy to control the bacterial conversion of toxic substrates into polyhydroxyalcanoates

The microbial conversion of toxic substrates into valuable products in continuous culture requires the equivalent of a tight rope walk between formation of the desired product and intoxication of the microbial catalyst. The condition of the latter is reflected immediately by changes in heat flow rate and beta-dispersion in an electrical RF field. Therefore, these were applied to the example of the continuous growth-associated synthesis of polyhydroxyalcanoates (PHA) from phenol by the bacterial strain Variovorax paradoxus DSM 4065. By controlling the supply of phenol to the chemostat, the rates of degradation, biomass formation, and synthesis of target product, respectively, were increasingly elevated until the onset of poisoning the organisms. The boundary between the maximum rates and the initiation of intoxication coincided with a sudden change in the heat flux. Using this occurrence, it was possible to develop a control strategy and test it successfully for a time period of 80 h. After 40 h the process stabilized at mean values, i.e., at rates of 92% phenol degradation, 100% biomass formation, and 70 - 75% of PHA formation compared with the situation shortly before poisoning the organisms. Using a moving-average technique to filter the raw dielectric spectroscope data, changes were followed in biomass concentration of approximately 100 mg/L. However, this technique was not sensitive or rapid enough to control the process.

Synthesis of microbial poly(beta-hydroxybutyrate) modified with oligo(pentaerythritol ethoxylate) by Ralstonia eutropha

Poly(beta-hydroxybutyrate) (PHB) modified with different amounts of pentaerythritol ethoxylate (PEE) has been synthesized using Ralstonia eutropha. The growth kinetics and the synthesis of PHB in the presence of PEE were modeled using appropriate differential equations for the mass balance of the two-stage process. The influence of PEE addition on the morphology of PHB was studied by various microscopic and scattering techniques. Light microscopic and wide-angle X-ray measurements indicated that the addition of PEE had a nucleating effect on the crystallization of PHB. The spherulite growth rate was widely independent of the PEE addition. The lamellae of PHB became more disordered when PEE was added as demonstrated by atomic force microscopy. Furthermore, small-angle X-ray data indicated a decrease in the long period with increasing PEE content of the modified PHB.

Laser cutting: influence on morphological and physicochemical properties of polyhydroxybutyrate

Polyhydroxybutyrate (PHB) is a biocompatible and resorbable implant material. For these reasons, it has been used for the fabrication of temporary stents, bone plates, nails and screws (Peng et al. Biomaterials 1996;17:685). In some cases, the brittle mechanical properties of PHB homopolymer limit its application. A typical plasticizer, triethylcitrate (TEC), was used to overcome such limitations by making the material more pliable. In the past few years, CO2-laser cutting of PHB was used in the manufacturing of small medical devices such as stents. Embrittlement of plasticized PHB tubes has been observed, after laser machining. Consequently, the physicochemical and morphological properties of laser-processed surfaces and cut edges of plasticized polymer samples were examined to determine the extent of changes in polymer properties as a result of laser machining. These studies included determination of the depth of the laser-induced heat affected zone by polariscopy of thin polymer sections. Molecular weight changes and changes in the TEC content as a function of distance from the laser-cut edge were determined. In a preliminary test, the cellular response to the processed material was investigated by cell culture study of L929 mouse fibroblasts on laser-machined surfaces. The heat-affected zone was readily classified into four different regions with a total depth of about 60 to 100 microm (Klamp, Master Thesis, University of Rostock, 1998). These results correspond well with the chemical analysis and molecular weight measurements. Furthermore, it was found that cells grew preferentially on the laser-machined area. These findings have significant implications for the manufacture of medical implants from PHB by laser machining.

Biosynthesis of polyesters from various amino acids by Alcaligenes eutrophus

The possibilities of the biosynthesis of polyesters from 20 kinds of naturally occurring L-amino acids by Alcaligenes eutrophus were studied. It was found that several amino acids were used efficiently to synthesize copolyesters of 3-hydroxybutyrate (3HB) and 3-hydroxyvalerate (3 HV) by A. eutrophus under the 'loose nitrogen-limiting' condition, but the other amino acids were scarcely utilized as a carbon source for the synthesis of polyesters. When L-threonine or L-isoleucine were used as the sole carbon source, copolyesters with higher 3HV content were produced.

Biosynthesis of poly(3-hydroxyalkanoate) from amino acids

It was found that an optically active copolyester, poly(3-hydroxybutyrate-co-3-hydroxyvalerate), denoted as P(3HB-co-3HV), is synthesized by Alcaligenes eutrophus H16 from several amino acids under various fermentation conditions. The optimum condition for the biosynthesis from one amino acid, threonine, was investigated and its biosynthetic pathway was discussed on the basis of the relation between the fermentation condition and the co-monomer composition of the produced polyesters.

Extracellular poly(3-hydroxybutyrate) depolymerase from Penicillium funiculosum: general characteristics and active site studies

An extracellular poly(3-hydroxybutyrate) (PHB) depolymerase has been isolated from Penicillium funiculosum cultural medium by a single hydrophobic column chromatography. The enzyme is a glycoprotein composed of a single polypeptide chain with a molecular mass of about 37,000 Da as analyzed by denatured sodium dodecyl sulfate-polyacrylamide gel electrophoresis and by native gel filtration on Sephadex G-100. Its optimum activity occurs at pH 6.0. It has an isoelectric point of 5.8 and has a Km for PHB (average molecular weight = 45,000 Da) of 0.17 mg/ml. Various nonionic detergents competitively inhibit the enzyme with Ki values of 0.56 and 0.014% for Tween 80 and Triton X-100, respectively. The enzyme is extremely sensitive to diisopropyl fluorophosphate, mercuric ion, and dithiothreitol (DTT). However, sulfhydryl reagents have little or no effect on its activity. The inactivation by mercuric ion and DTT is reversible by mercaptoethanol and hydrogen peroxide, respectively. These data suggest that the enzyme may be a serine esterase and may contain an important disulfide bond. The enzyme is also inactivated by diazoacetyl and epoxide compounds at low pH, which can be prevented by PHB, indicating the presence of a critical carboxyl group at the active site. These characteristics of the enzyme are compared to other extracellular polymerases isolated from bacterial culture media.