Polyhydroxyalkanoates from extremophiles: A review

Green Synthesis of Bioplastics from Microalgae: A State-of-the-Art Review

The synthesis of conventional plastics has increased tremendously in the last decades due to rapid industrialization, population growth, and advancement in the use of modern technologies. However, overuse of these fossil fuel-based plastics has resulted in serious environmental and health hazards by causing pollution, global warming, etc. Therefore, the use of microalgae as a feedstock is a promising, green, and sustainable approach for the production of biobased plastics. Various biopolymers, such as polyhydroxybutyrate, polyurethane, polylactic acid, cellulose-based polymers, starch-based polymers, and protein-based polymers, can be produced from different strains of microalgae under varying culture conditions. Different techniques, including genetic engineering, metabolic engineering, the use of photobioreactors, response surface methodology, and artificial intelligence, are used to alter and improve microalgae stocks for the commercial synthesis of bioplastics at lower costs. In comparison to conventional plastics, these biobased plastics are biodegradable, biocompatible, recyclable, non-toxic, eco-friendly, and sustainable, with robust mechanical and thermoplastic properties. In addition, the bioplastics are suitable for a plethora of applications in the agriculture, construction, healthcare, electrical and electronics, and packaging industries. Thus, this review focuses on techniques for the production of biopolymers and bioplastics from microalgae. In addition, it discusses innovative and efficient strategies for large-scale bioplastic production while also providing insights into the life cycle assessment, end-of-life, and applications of bioplastics. Furthermore, some challenges affecting industrial scale bioplastics production and recommendations for future research are provided.

Development of a genetic system for Haloferax gibbonsii LR2-5, model host for haloarchaeal viruses

Archaeal viruses are among the most enigmatic members of the virosphere, and their diverse morphologies raise many questions about their infection mechanisms. The study of molecular mechanisms underlying virus-host interactions hinges upon robust model organisms with a system for gene expression and deletion. Currently, there are only a limited number of archaea that have associated viruses and have a well-developed genetic system. Here, we report the development of a genetic system for the euryarchaeon Haloferax gibbonsii LR2-5. This strain can be infected by multiple viruses and is a model for the study of virus-host interactions. We created a Hfx. gibbonsii LR2-5 ∆pyrE strain, resulting in uracil auxotrophy, which could be used as a selection marker. An expression plasmid carrying a pyrE gene from the well-established Haloferax volcanii system was tested for functionality. Expression of a GFP-MinD fusion under a tryptophan inducible promoter was fully functional and showed similar cellular localization as in Hfx. volcanii. Thus, the plasmids of the Hfx. volcanii system can be used directly for the Hfx. gibbonsii LR2-5 genetic system, facilitating the transfer of tools between the two. Finally, we tested for the functionality of gene deletions by knocking out two genes of the archaeal motility structure, the archaellum. These deletion mutants were as expected non-motile and the phenotype of one deletion could be rescued by the expression of the deleted archaellum gene from a plasmid. Thus, we developed a functional genetic toolbox for the euryarchaeal virus host Hfx. gibbonsii LR2-5, which will propel future studies on archaeal viruses.

IMPORTANCE

Species from all domains of life are infected by viruses. In some environments, viruses outnumber their microbial hosts by a factor of 10, and viruses are the most important predators of microorganisms. While much has been discovered about the infection mechanisms of bacterial and eukaryotic viruses, archaeal viruses remain understudied. Good model systems are needed to study their virus-host interactions in detail. The salt-loving archaeon Haloferax gibbonsii LR2-5 has been shown to be infected by a variety of different viruses and, thus, is an excellent model to study archaeal viruses. By establishing a genetic system, we have significantly expanded the toolbox for this model organism, which will fuel our understanding of infection strategies of the underexplored archaeal viruses.

Polyhydroxyalkanoates and exopolysaccharides: An alternative for valuation of the co-production of microbial biopolymers

Polyhydroxyalkanoates (PHAs) and exopolysaccharides (EPSs) belong to a class of abundant biopolymers produced by various fermenting microorganisms. These biocompounds have high value-added potential and can be produced concurrently. Co-production of PHAs and EPSs is a strategy employed by researchers to reduce costs associated with large-scale production. EPSs and PHAs are non-toxic, biocompatible, and biodegradable, making them suitable for various industrial sectors, including packaging and the medical and pharmaceutical industries. These biopolymers can be derived from agro-industrial residues, thus contributing to the bioeconomy by producing high-value-added products. This review investigates approaches for simultaneously synthesizing PHAs and EPSs using different carbon sources and microorganisms.

Feasibility of bioplastic production using micro- and macroalgae- A review

Plastic disposal and their degraded products in the environment are global concern due to its adverse effects and persistence in nature. To overcome plastic pollution and its impacts on environment, a sustainable bioplastic production using renewable feedstock's, such as algae, are envisioned. In this review, the production of polymer precursors such as polylactic acid, polyhydroxybutyrates, polyhydroxyalkanoates, agar, carrageenan and alginate from microalgae and macroalgae through direct conversion and fermentation routes are summarized and discussed. The direct conversion of algal biopolymers without any bioprocess (whole algal biomass used emphasizing zero waste discharge concept) favours economic feasibility. Whereas indirect method uses conversion of algal polymers to monomers after pretreatment followed by bioplastic precursor production by fermentation are emphasized. This review paper also outlines the current state of technological developments in the field of algae-based bioplastic, both in industry and in research, and highlights the creation of novel solutions for green bioplastic production employing algal polymers. Finally, the cost economics of the bioplastic production using algal biopolymers are clearly mentioned with future directions of next level bioplastic production. In this review study, the cost estimation was given at laboratory level bioplastic production using casting methods. Further development of bioplastics at pilot scale level may give clear economic feasibility of production at industry. Here, in this review, we emphasized the overview of algal biopolymers for different bioplastic product development and its economic value and also current industries involved in bioplastic production.

Recent Biotechnological Applications of Polyhydroxyalkanoates (PHA) in the Biomedical Sector-A Review

Petroleum-derived plastics are materials of great importance for the contemporary lifestyle, and are widely used commercially because they are low cost, resistant, malleable, and weightless, in addition to their hydrophobic character. However, some factors that confer the qualities of these materials also cause problems, mainly environmental, associated with their use. The COVID-19 pandemic aggravated these impacts due to the high demand for personal protective equipment and the packaging sector. In this scenario, bioplastics are environmentally positive alternatives to these plastics due to their applicability in several areas ranging from packaging, to biomedicine, to agriculture. Polyhydroxyalkanoates (PHAs) are biodegradable biopolymers usually produced by microorganisms as an energy reserve. Their structural variability provides a wide range of applications, making them a viable option to replace polluting materials. PHAs can be applied in various biotechnology sectors, such as producing drug carriers and scaffolds for tissue engineering. This review aimed to survey works published in the last five years on the study and biotechnological application of PHAs in the biomedical sector, exploring the versatility and advantages of their use and helping to understand how to enhance their application.

Microbial glucoamylases: structural and functional properties and biotechnological uses

Glucoamylases (GAs) are one of the principal groups of enzymes involved in starch hydrolysis and belong to the glycosylhydrolase family. They are classified as exo-amylases due to their ability to hydrolyze α-1,4 glycosidic bonds from the non-reducing end of starch, maltooligosaccharides, and related substrates, releasing β-D-glucose. Structurally, GAs possess a characteristic catalytic domain (CD) with an (α/α)6 fold and exhibit five conserved regions within this domain. The CD may or may not be linked to a non-catalytic domain with variable functions depending on its origin. GAs are versatile enzymes with diverse applications in food, biofuel, bioplastic and other chemical industries. Although fungal GAs are commonly employed for these purposes, they have limitations such as their low thermostability and an acidic pH requirement. Alternatively, GAs derived from prokaryotic organisms are a good option to save costs as they exhibit greater thermostability compared to fungal GAs. Moreover, a group of cold-adapted GAs from psychrophilic organisms demonstrates intriguing properties that make them suitable for application in various industries. This review provides a comprehensive overview of the structural and sequential properties as well as biotechnological applications of GAs in different industrial processes.

Deriving high value products from depolymerized lignin oil, aided by (bio)catalytic funneling strategies

Lignin holds tremendous and versatile possibilities to produce value-added chemicals and high performing polymeric materials. Over the years, different cutting-edge lignin depolymerization methodologies have been developed, mainly focusing on achieving excellent yields of mono-phenolic products, some even approaching the theoretical maximum. However, due to lignin's inherent heterogeneity and recalcitrance, its depolymerization leads to relatively complex product streams, also containing dimers, and higher molecular weight fragments in substantial quantities. The subsequent chemo-catalytic valorization of these higher molecular weight streams, containing difficult-to-break, mainly C-C covalent bonds, is tremendously challenging, and has consequently received much less attention. In this minireview, we present an overview of recent advances on the development of sustainable biorefinery strategies aimed at the production of well-defined chemicals and polymeric materials, the prime focus being on depolymerized lignin oils, containing high molecular weight fractions. The key central unit operation to achieve this is (bio)catalytic funneling, which holds great potential to overcome separation and purification challenges.

Biopolymer production from biomass produced by Nordic microalgae grown in wastewater

Biomass from four different Nordic microalgal species, grown in BG-11 medium or synthetic wastewater (SWW), was explored as inexpensive carbohydrate-rich feedstock for polyhydroxybutyrate (PHB) production via microbial fermentation. Thermochemical pre-treatment (acid treatment followed by autoclavation) with 2% hydrochloric acid or 1% sulphuric acid (v/v) was used to maximize sugar yield prior to fermentation. Pre-treatment resulted in ∼5-fold higher sugar yield compared to the control. The sugar-rich hydrolysate was used as carbon source for the PHB-producing extremophilic bacterium Halomonas halophila. Maximal PHB production was achieved with hydrolysate of Chlorococcum sp. (MC-1) grown on BG-11 medium (0.27 ± 0.05 g PHB/ g DW), followed by hydrolysate derived from Desmodesmus sp. (RUC-2) grown on SWW (0.24 ± 0.05 g PHB/ g DW). Nordic microalgal biomass grown on wastewater therefore can be used as cheap feedstock for sustainable bioplastic production. This research highlights the potential of Nordic microalgae to develop a biobased economy.

Medium chain length polyhydroxyalkanoate produced from ethanol by Pseudomonas putida grown in liquid obtained from acidogenic digestion of organic municipal solid waste

Production of medium chain length polyhydroxyalkanoate (mcl-PHA) up to about 6 g.L^-1 was obtained by feeding ethanol to Pseudomonas putida growing in liquid obtained from acidogenic digestion of organic municipal solid waste. Washing the wet, heat-inactivated Pseudomonas cells at the end of the fermentation with ethanol obviated the need of drying the biomass and enabled the removal of contaminating lipids before solvent-mediated extraction of PHA. Using 'green' solvents, 90 to near 100% of the mcl-PHA was extracted and purities of 71-78% mcl-PHA were reached already by centrifugation and decantation without further filtration for biomass removal. The mcl-PHA produced in this way consists of 10-18% C8, 72-78% C10 and 8-12% C12 chains (entirely medium chain length), has a crystallinity and melting temperature of ∼13% and ∼49 °C, respectively, and is a stiff rubberlike, colourless material at room temperature.

Enhanced production of biobased, biodegradable, Poly(3-hydroxybutyrate) using an unexplored marine bacterium Pseudohalocynthiibacter aestuariivivens, isolated from highly polluted coastal environment

The production and disposal of plastics from limited fossil reserves, has prompted research for greener and sustainable alternatives. Polyhydroxyalkanoates (PHAs) are biocompatible, biodegradable, and thermoprocessable polyester produced by microbes. PHAs found several applications but their use is limited due to high production cost and low yields. Herein, for the first time, the isolation and characterization of Pseudohalocynthiibacter aestuariivivens P96, a marine bacterium able to produce surprising amount of PHAs is reported. In the best growth condition P96 was able to reach a maximum production of 4.73 g/L, corresponding to the 87 % of total cell dry-weight. Using scanning and transmission microscopy, lab-scale fermentation, spectroscopic techniques, and genome analysis, the production of thermoprocessable polymer Polyhydroxybutyrate P(3HB), a PHAs class, endowed with mechanical and thermal properties comparable to that of petroleum-based plastics was confirmed. This study represents a milestone toward the use of this unexplored marine bacterium for P(3HB) production.

Cytotoxic indole alkaloids and polyketides produced by a marine-derived fungus Aspergillus flavipes DS720

Marine-derived microorganisms possess the unique metabolic pathways to produce structurally novel secondary metabolites with potent biological activities. In this study, bioactivity-guided isolation of the marine deep-sea-derived fungus Aspergillus flavipes DS720 led to the characterization of four indole alkaloids (compounds 1–4) and four polyketides (compounds 5–8), such as two new indoles, flavonoids A (1) and B (2) with a C-6 reversed prenylation, and a new azaphilone, flaviazaphilone A (5). Their chemical structures were unambiguously established by an extensive interpretation of spectroscopic data, such as 1D/2D NMR and HRESIMS data. The absolute configurations of the new compound 5 were solved by comparing the experimental and calculated Electronic Circular Dichroism (ECD) spectra. Since sufficient amount of flavonoids A (1) was obtained, 1 was subjected to a large-scale cytotoxic activity screening against 20 different human tumor cell lines. The results revealed that 1 showed broad-spectrum cytotoxicities against HeLa, 5637, CAL-62, PATU8988T, A-375, and A-673 cell lines, with the inhibition rates of more than 90%. This study indicated that the newly discovered indole alkaloid 1 may possess certain potential for the development of lead compounds in the future.

Advances in Polyhydroxyalkanoate (PHA) Production, Volume 3

Steadily increasing R&D activities in the field of microbial polyhydroxyalkanoate (PHA) biopolyesters are committed to growing global threats from climate change, aggravating plastic pollution, and the shortage of fossil resources. These prevailing issues paved the way to launch the third Special Issue of Bioengineering dedicated to future-oriented biomaterials, characterized by their versatile plastic-like properties. Fifteen individual contributions to the Special Issue, written by renowned groups of researchers from all over the world, perfectly mirror the current research directions in the PHA sector: inexpensive feedstock like carbon-rich waste from agriculture, mitigation of CO2 for PHA biosynthesis by cyanobacteria or wild type and engineered “knallgas” bacteria, powerful extremophilic PHA production strains, novel tools for rapid in situ determination of PHA in photobioreactors, modelling of the dynamics of PHA production by mixed microbial cultures from inexpensive raw materials, enhanced bioreactor design for high-throughput PHA production by sophisticated cell retention systems, sustainable and efficient PHA recovery from biomass assisted by supercritical water, enhanced processing of PHA by application of novel antioxidant additives, and the development of compatible biopolymer blends. Moreover, elastomeric medium chain length PHA (mcl-PHA) are covered in-depth, inter alia, by introduction of a novel class of bioactive mcl-PHA-based networks, in addition to the first presentation of the new rubber-like polythioester poly(3-mercapto-2-methylpropionate). Finally, the present Special Issue is concluded by a critical essay on past, ongoing, and announced global endeavors for PHA commercialization.

Novel thermophilic polyhydroxyalkanoates producing strain Aneurinibacillus thermoaerophilus CCM 8960

Aneurinibacillus thermoaerophilus CCM 8960 is a thermophilic bacterium isolated from compost in Brno. The bacterium accumulates polyhydroxyalkanoates (PHAs), a biodegradable and renewable alternative to petrochemical polymers. The bacterium reveals several features that make it a very interesting candidate for the industrial production of PHA. At first, due to its thermophilic character, the bacterium can be utilized in agreement with the concept of next-generation industrial biotechnology (NGIB), which relies on extremophiles. Second, the bacterium is capable of producing PHA copolymers containing a very high portion of 4-hydroxybutyrate (4HB). Such materials possess unique properties and can be advantageously used in multiple applications, including but not limited to medicine and healthcare. Therefore, this work focuses on the in-depth characterization of A. thermoaerophilus CCM 8960. In particular, we sequenced and assembled the genome of the bacterium and identified its most important genetic features, such as the presence of plasmids, prophages, CRISPR arrays, antibiotic-resistant genes, and restriction-modification (R-M) systems, which might be crucial for the development of genome editing tools. Furthermore, we focused on genes directly involved in PHA metabolism. We also experimentally studied the kinetics of glycerol and 1,4-butanediol (1,4BD) utilization as well as biomass growth and PHA production during cultivation. Based on these data, we constructed a metabolic model to reveal metabolic fluxes and nodes of glycerol and 1,4BD concerning their incorporation into the poly(3-hydroxybutyrate-co-4-hydroxybutyrate (P(3HB-co-4HB)) structure. KEY POINTS: • Aneurinibacillus sp. H1 was identified as Aneurinibacillus thermoaerophilus. • PHA metabolism pathway with associated genes was presented. • Unique monomer composition of produced PHAs was reported.

The role of polyhydroxyalkanoates in adaptation of Cupriavidus necator to osmotic pressure and high concentration of copper ions

Polyhydroxyalkanoates (PHA) are abundant microbial polyesters accumulated in the form of intracellular granules by numerous prokaryotes primarily as storage of carbon and energy. Apart from their storage function, the presence of PHA also enhances the robustness of the microbial cells against various stressors. In this work, we investigated the role of PHA in Cupriavidus necator, a model organism concerning PHA metabolism, for adaptation to osmotic pressure and copper ions. In long-term laboratory evolution experiments, the bacterial culture was cultivated in presence of elevated doses of sodium chloride or copper ions (incubations lasted 78 passages for Cu²⁺ and 68 passages for NaCl) and the evolved strains were compared with the wild-type strain in terms of growth and PHA production capacity, cell morphology (investigated by various electron microscopy techniques), activities of selected enzymes involved in PHA metabolism and other crucial metabolic pathways, the chemical composition of bacterial biomass (determined by infrared and Raman spectroscopy) and also considering robustness against various stressors. The results confirmed the important role of PHA metabolism for adaptation to both tested stressors.

Current state of the art biotechnological strategies for conversion of watermelon wastes residues to biopolymers production: A review

Poly-3-hydroxyalkanoates (PHA) are biodegradable and compostable polyesters. This review is aimed to provide a unique approach that can help think tanks to frame strategies aiming for clean technology by utilizing cutting edge biotechnological advances to convert fruit and vegetable waste to biopolymer. A PHA manufacturing method based on watermelon waste residue that does not require extensive pretreatment provides a more environmentally friendly and sustainable approach that utilizes an agricultural waste stream. Incorporating fruit processing industry by-products and water, and other resource conservation methods would not only make the manufacturing of microbial bio-plastics like PHA more eco-friendly, but will also help our sector transition to a bioeconomy with circular product streams. The final and most critical element of this review is an in-depth examination of the several hazards inherent in PHA manufacturing.

Established and Emerging Producers of PHA: Redefining the Possibility

The polyhydroxyalkanoate was discovered almost around a century ago. Still, all the efforts to replace the traditional non-biodegradable plastic with much more environmentally friendly alternative are not enough. While the petroleum-based plastic is like a parasite, taking over the planet rapidly and without any feasible cure, its perennial presence has made the ocean a floating island of life-threatening debris and has flooded the landfills with toxic towering mountains. It demands for an immediate solution; most resembling answer would be the polyhydroxyalkanoates. The production cost is yet one of the significant challenges that various corporate is facing to replace the petroleum-based plastic. To deal with the economic constrain better strain, better practices, and a better market can be adopted for superior results. It demands for systems for polyhydroxyalkanoate production namely bacteria, yeast, microalgae, and transgenic plants. Solely strains affect more than 40% of overall production cost, playing a significant role in both upstream and downstream processes. The highly modifiable nature of the biopolymer provides the opportunity to replace the petroleum plastic in almost all sectors from food packaging to medical industry. The review will highlight the recent advancements and techno-economic analysis of current commercial models of polyhydroxyalkanoate production. Bio-compatibility and the biodegradability perks to be utilized highly efficient in the medical applications gives ample reason to tilt the scale in the favor of the polyhydroxyalkanoate as the new conventional and sustainable plastic.

Developing Bioprospecting Strategies for Bioplastics Through the Large-Scale Mining of Microbial Genomes

The accumulation of petroleum-based plastic waste has become a major issue for the environment. A sustainable and biodegradable solution can be found in Polyhydroxyalkanoates (PHAs), a microbially produced biopolymer. An analysis of the global phylogenetic and ecological distribution of potential PHA producing bacteria and archaea was carried out by mining a global genome repository for PHA synthase (PhaC), a key enzyme involved in PHA biosynthesis. Bacteria from the phylum Actinobacteria were found to contain the PhaC Class II genotype which produces medium-chain length PHAs, a physiology until now only found within a few Pseudomonas species. Further, several PhaC genotypes were discovered within Thaumarchaeota, an archaeal phylum with poly-extremophiles and the ability to efficiently use CO2 as a carbon source, a significant ecological group which have thus far been little studied for PHA production. Bacterial and archaeal PhaC genotypes were also observed in high salinity and alkalinity conditions, as well as high-temperature geothermal ecosystems. These genome mining efforts uncovered previously unknown candidate taxa for biopolymer production, as well as microbes from environmental niches with properties that could potentially improve PHA production. This in silico study provides valuable insights into unique PHA producing candidates, supporting future bioprospecting efforts toward better targeted and relevant taxa to further enhance the diversity of exploitable PHA production systems.

Microbial cell factories for the production of polyhydroxyalkanoates

Pollution caused by persistent petro-plastics is the most pressing problem currently, with 8 million tons of plastic waste dumped annually in the oceans. Plastic waste management is not systematized in many countries, because it is laborious and expensive with secondary pollution hazards. Bioplastics, synthesized by microorganisms, are viable alternatives to petrochemical-based thermoplastics due to their biodegradable nature. Polyhydroxyalkanoates (PHAs) are a structurally and functionally diverse group of storage polymers synthesized by many microorganisms, including bacteria and Archaea. Some of the most important PHA accumulating bacteria include Cupriavidus necator, Burkholderia sacchari, Pseudomonas sp., Bacillus sp., recombinant Escherichia coli, and certain halophilic extremophiles. PHAs are synthesized by specialized PHA polymerases with assorted monomers derived from the cellular metabolite pool. In the natural cycle of cellular growth, PHAs are depolymerized by the native host for carbon and energy. The presence of these microbial PHA depolymerases in natural niches is responsible for the degradation of bioplastics. Polyhydroxybutyrate (PHB) is the most common PHA with desirable thermoplastic-like properties. PHAs have widespread applications in various industries including biomedicine, fine chemicals production, drug delivery, packaging, and agriculture. This review provides the updated knowledge on the metabolic pathways for PHAs synthesis in bacteria, and the major microbial hosts for PHAs production. Yeasts are presented as a potential candidate for industrial PHAs production, with their high amenability to genetic engineering and the availability of industrial-scale technology. The major bottlenecks in the commercialization of PHAs as an alternative for plastics and future perspectives are also critically discussed.