Syngas obtained by microwave pyrolysis of household wastes as feedstock for polyhydroxyalkanoate production in Rhodospirillum rubrum

Intensification and biorefinery approaches for the valorization of kitchen wastes - A review

Kitchen wastes (KW) are post-consumption residues from household and food service sector, heterogenous in composition and highly variable depending on the particular origin, which are often treated as municipal. There is a need to improve the management of these continuously produced and worldwidely available resources and their valorization into novel and commercially interesting products will aid in the development of bioeconomy. The successful implementation of such approach requires cooperation between academia, industrial stakeholders, public and private institutions, based on the different dimensions, including social, economic, ecologic and technological involved. This review aims at presenting a survey of technological aspects, regarding current and potential management strategies of KW, following either a single or multiproduct processing according to the biorefineries scheme. Emphasis is given to intensification tools, designed to enhance process efficiency.

The Hyperproduction of Polyhydroxybutyrate Using Bacillus mycoides ICRI89 through Enzymatic Hydrolysis of Affordable Cardboard

Bioplastics are contemplated as remarkable substitutes for conventional plastics to accommodate green technological advancements. However, their industrial production has not been fully implemented owing to the cost of carbon resources. From another perspective, valorizing different paper mill wastes has become a prominent research topic. These materials may serve as an affording sustainable feedstock for bioplastic production. Adjustment of cardboard waste hydrolysate as suitable fermentation media for production of bacterial polyhydroxyalkanoates (PHAs) has been investigated. Cardboard samples were defibered and dried before enzymatic hydrolysis. The enzymatic degradation of commercial cellulase was monitored over 15 days. Interestingly, 18.2 ± 0.2 g/L glucose yield was obtained from 50 g cardboard samples using a 1.5% (v/v) enzyme concentration. The samples exhibited maximum weight loss values of 69-73%. Meanwhile, five soil samples were collected from local sites in Lodz, Poland. A total of 31 bacterial isolates were screened and cultured on Nile blue plates. Analysis of the 16S rRNA gene sequence of the most potent producer revealed 100% similarity to Bacillus mycoides. Cardboard hydrolysates whole medium, modified MSM with cardboard hydrolysate and nitrogen depleted MSM with cardboard hydrolysate were utilized for PHA production, followed by PHA productivity and cell dry weight (CDW) estimation compared to glucose as a standard carbon source. An impressive PHA accumulation of 56% CDW was attained when the waste hydrolysate was used as a carbon source. FTIR and NMR analysis of the isolated PHA indicated that functional groups of the polymer were related to PHB (polyhydroxybutyrate). Thermal analysis demonstrates that PHB and PHB-CB (PHB produced from cardboard hydrolysate) have degradation temperatures of 380 and 369 °C, respectively, which reflect the high thermal stability and heat resistance compared to the same properties for a standard polymer. This is the first demonstration of full saccharification of corrugated cardboard paper waste for high-level production of PHA. In addition, the attained PHB productivity is one of the highest levels achieved from a real lignocellulosic waste.

Microbial autotrophic biorefineries: Perspectives for biopolymer production

The use of autotrophic microorganisms to fabricate biochemical products has attracted much attention in both academia and industry. Unlike heterotrophic microorganisms that require carbohydrates and amino acids for growth, autotrophic microorganisms have evolved to utilize either light (photoautotrophs) or chemical compounds (chemolithotrophs) to fix carbon dioxide (CO2) and drive metabolic processes. Several biotechnological approaches, including synthetic biology and metabolic engineering, have been proposed to harness autotrophic microorganisms as a sustainable/green production platform for commercially essential products such as biofuels, commodity chemicals, and biopolymers. Here, we review the recent advances in natural autotrophic microorganisms (photoautotrophic and chemoautotrophic), focusing on the biopolymer production. We present current state-of-the-art technologies to engineer autotrophic microbial cell factories for efficient biopolymer production.

Strategies for Biosynthesis of C1 Gas-derived Polyhydroxyalkanoates: A review

Biosynthesis of polyhydroxyalkanoates (PHAs) from C1 gases is highly desirable in solving problems such as climate change and microplastic pollution. PHAs are biopolymers synthesized in microbial cells and can be used as alternatives to petroleum-based plastics because of their biodegradability. Because 50% of the cost of PHA production is due to organic carbon sources and salts, the utilization of costless C1 gases as carbon sources is expected to be a promising approach for PHA production. In this review, strategies for PHA production using C1 gases through fermentation and metabolic engineering are discussed. In particular, autotrophs, acetogens, and methanotrophs are strains that can produce PHA from CO₂, CO, and CH₄. In addition, integrated bioprocesses for the efficient utilization of C1 gases are introduced. Biorefinery processes from C1 gas into bioplastics are prospective strategies with promising potential and feasibility to alleviate environmental issues.

Syngas Fermentation for the Production of Bio-Based Polymers: A Review

Increasing environmental awareness among the general public and legislators has driven this modern era to seek alternatives to fossil-derived products such as fuel and plastics. Addressing environmental issues through bio-based products driven from microbial fermentation of synthetic gas (syngas) could be a future endeavor, as this could result in both fuel and plastic in the form of bioethanol and polyhydroxyalkanoates (PHA). Abundant availability in the form of cellulosic, lignocellulosic, and other organic and inorganic wastes presents syngas catalysis as an interesting topic for commercialization. Fascination with syngas fermentation is trending, as it addresses the limitations of conventional technologies like direct biochemical conversion and Fischer–Tropsch’s method for the utilization of lignocellulosic biomass. A plethora of microbial strains is available for syngas fermentation and PHA production, which could be exploited either in an axenic form or in a mixed culture. These microbes constitute diverse biochemical pathways supported by the activity of hydrogenase and carbon monoxide dehydrogenase (CODH), thus resulting in product diversity. There are always possibilities of enzymatic regulation and/or gene tailoring to enhance the process’s effectiveness. PHA productivity drags the techno-economical perspective of syngas fermentation, and this is further influenced by syngas impurities, gas–liquid mass transfer (GLMT), substrate or product inhibition, downstream processing, etc. Product variation and valorization could improve the economical perspective and positively impact commercial sustainability. Moreover, choices of single-stage or multi-stage fermentation processes upon product specification followed by microbial selection could be perceptively optimized.

Biofuel and chemical production from carbon one industry flux gas by acetogenic bacteria

Carbon one industry flux gas generated from fossil fuels, various industrial and domestic waste, as well as lignocellulosic biomass provides an innovative raw material to lead the sustainable development. Through the chemical and biological processing, the gas mixture composed of CO, CO₂, and H₂, also termed as syngas, is converted to biofuels and high-value chemicals. Here, the syngas fermentation process is elaborated to provide an overview. Sources of syngas are summarized and the influences of impurities on biological fermentation are exhibited. Acetogens and carboxydotrophs are the two main clusters of syngas utilizing microorganisms, their essential characters are presented, especially the energy metabolic scheme with CO, CO₂, and H₂. Synthetic biology techniques and microcompartment regulation are further discussed and proposed to create a high-efficiency cell factory. Moreover, the influencing factors in fermentation and products in carboxylic acids, alcohols, and others such like polyhydroxyalkanoate and poly-3-hydroxybutyrate are addressed. Biological fermentation from carbon one industry flux gas is a promising alternative, the latest scientific advances are expatiated hoping to inspire more creative transformation.

Recent progress and challenges in microbial polyhydroxybutyrate (PHB) production from CO2 as a sustainable feedstock: A state-of-the-art review

The recalcitrance of petroleum-based plastics causes severe environmental problems and has accelerated research into production of biodegradable polymers from inexpensive and sustainable feedstocks. Various microorganisms are capable of producing Polyhydroxybutyrate (PHB), a representative biodegradable polymer, under nutrient-limited conditions, among which CO₂-utilizing microorganisms are of primary interest. Herein, we discuss recent progress on bacterial strains including proteobacteria, purple non-sulfur bacteria, and cyanobacteria in terms of CO₂-containing carbon sources, PHB-production capability, and genetic modification. In addition, this review introduces recent technical approaches used to improve PHB production from CO₂ such as two-stage bioprocesses and bioelectrochemical systems. Challenges and future perspectives for the development of economically feasible PHB production are also discussed. Finally, this review might provide insights into the construction of a closed-carbon-loop to cope with climate change.

Biowaste-to-bioplastic (polyhydroxyalkanoates): Conversion technologies, strategies, challenges, and perspective

Biowaste management is a challenging job as it is high in nutrient content and its disposal in open may cause a serious environmental and health risk. Traditional technologies such as landfill, bio-composting, and incineration are used for biowaste management. To gain revenue from biowaste researchers around the world focusing on the integration of biowaste management with other commercial products such as volatile fatty acids (VFA), biohydrogen, and bioplastic (polyhydroxyalkanoates (PHA)), etc. PHA production from various biowastes such as lignocellulosic biomass, municipal waste, waste cooking oils, biodiesel industry waste, and syngas has been reported successfully. Various nutrient factors i.e., carbon and nitrogen source concentration and availability of dissolved oxygen are crucial factors for PHA production. This review is an attempt to summarize the recent advancements in PHA production from various biowaste, its downstream processing, and other challenges that need to overcome making bioplastic an alternate for synthetic plastic.

Molecular Profiling and Optimization Studies for Growth and PHB Production Conditions in Rhodobacter sphaeroides

In the recent climate change regime, industrial demand for renewable materials to replace petroleum-derived polymers continues to rise. Of particular interest is polyhydroxybutyrate (PHB) as a substitute for polypropylene. Accumulating evidence indicates that PHB is highly produced as a carbon storage material in various microorganisms. The effects of growth conditions on PHB production have been widely studied in chemolithotrophs, particularly in Rhodobacter. However, the results on PHB production in Rhodobacter have been somewhat inconsistent due to different strains and experimental conditions, and it is currently unclear how diverse environmental factors are linked with PHB production. Here, we report optimized growth conditions for PHB production and show that the growth conditions are closely related to reactive oxygen species (ROS) regulation. PHB accumulates in cells up to approximately 50% at the highest level under dark-aerobic conditions as opposed to light aerobic/anaerobic conditions. According to the time-course, PHB contents increased at 48 h and then gradually decreased. When observing the effect of temperature and medium composition on PHB production, 30 °C and a carbon/nitrogen ratio of 9:1 or more were found to be most effective. Among PHB biosynthetic genes, PhaA and PhaB are highly correlated with PHB production, whereas PhaC and PhaZ showed little change in overall expression levels. We found that, while the amount of hydrogen peroxide in cells under dark conditions was relatively low compared to the light conditions, peroxidase activities and expression levels of antioxidant-related genes were high. These observations suggest optimal culture conditions for growth and PHB production and the importance of ROS-scavenging signaling with regard to PHB production.

Plastic waste as a global challenge: are biodegradable plastics the answer to the plastic waste problem?

The strength, flexibility and light weight of traditional oil-derived plastics make them ideal materials for a large number of applications, including packaging, medical devices, building, transportation, etc. However, the majority of produced plastics are single-use plastics, which, coupled with a throw-away culture, leads to the accumulation of plastic waste and pollution, as well as the loss of a valuable resource. In this review we discuss the advances and possibilities in the biotransformation and biodegradation of oil-based plastics. We review bio-based and biodegradable polymers and highlight the importance of end-of-life management of biodegradables. Finally, we discuss the role of a circular economy in reducing plastic waste pollution.

Polyhydroxyalkanoate-associated phasins as phylogenetically heterogeneous, multipurpose proteins

Polyhydroxyalkanoates (PHAs) are natural polyesters of increasing biotechnological importance that are synthesized by many prokaryotic organisms as carbon and energy storage compounds in limiting growth conditions. PHAs accumulate intracellularly in form of inclusion bodies that are covered with a proteinaceous surface layer (granule-associated proteins or GAPs) conforming a network-like surface of structural, metabolic and regulatory polypeptides, and configuring the PHA granules as complex and well-organized subcellular structures that have been designated as 'carbonosomes'. GAPs include several enzymes related to PHA metabolism (synthases, depolymerases and hydroxylases) together with the so-called phasins, an heterogeneous group of small-size proteins that cover most of the PHA granule and that are devoid of catalytic functions but nevertheless play an essential role in granule structure and PHA metabolism. Structurally, phasins are amphiphilic proteins that shield the hydrophobic polymer from the cytoplasm. Here, we summarize the characteristics of the different phasins identified so far from PHA producer organisms and highlight the diverse opportunities that they offer in the Biotechnology field.

Studies on the aerobic utilization of synthesis gas (syngas) by wild type and recombinant strains of Ralstonia eutropha H16

The biotechnical platform strain Ralstonia eutropha H16 was genetically engineered to express a cox subcluster of the carboxydotrophic Oligotropha carboxidovoransOM5, including (i) the structural genes coxM, -S and -L, coding for an aerobic carbon monoxide dehydrogenase (CODH) and (ii) the genes coxD, -E, -F and -G, essential for the maturation of CODH. The cox_Oc genes expressed under control of the CO₂ -inducible promoter P_L enabled R. eutropha to oxidize CO to CO₂ for the use as carbon source, as demonstrated by ¹³ CO experiments, but the recombinant strains remained dependent on H₂ as external energy supply. Therefore, a synthetic metabolism, which could be described as 'carboxyhydrogenotrophic', was established in R. eutropha. With this extension of the bacterium's substrate range, growth in CO-, H₂ - and CO₂ -containing artificial synthesis gas atmosphere was enhanced, and poly(3-hydroxybutyrate) synthesis was increased by more than 20%.

Microbial biotechnology addressing the plastic waste disaster

Oceans are a major source of biodiversity, they provide livelihood, and regulate the global ecosystem by absorbing heat and CO2 . However, they are highly polluted with plastic waste. We are discussing here microbial biotechnology advances with the view to improve the start and the end of life of biodegradable polymers, which could contribute to the sustainable use of marine and coastal ecosystems (UN Sustainability development goal 14).

Design of a tailor-made platform for syngas bioconversion into polyhydroxybutyrate

Biodegradable polymers such as polyhydroxybutyrate (PHB) are part of the emerging portfolio of renewable materials, which are addressing the issue of plastic waste. Syngas, as a cheap, renewable and sustainable resource that can be obtained from biomass or waste, is viewed as an excellent feedstock for different bioprocesses, including syngas to PHB bioconversion. However, due to the hazardous nature of syngas, it is of utmost importance to consider safety aspects of the process. This recently developed tailor‐made platform for safe syngas fermentation and PHB production addresses safety aspects and demonstrates the importance of robust online and in‐line analytical tools allowing for monitoring and controlling of this bioprocess.

Biodegradable polymers such as polyhydroxybutyrate (PHB) are part of the emerging portfolio of renewable materials, which are addressing the issue of plastic waste. Syngas, as a cheap, renewable and sustainable resource that can be obtained from biomass or waste, is viewed as an excellent feedstock for different bioprocesses, including syngas to PHB bioconversion. However, due to the hazardous nature of syngas, it is of utmost importance to consider safety aspects of the process. This recently developed tailor‐made platform for safe syngas fermentation and PHB production addresses safety aspects and demonstrates the importance of robust on‐line and in‐line analytical tools allowing for monitoring and controlling of this bioprocess.

Syngas obtained by microwave pyrolysis of household wastes as feedstock for polyhydroxyalkanoate production in Rhodospirillum rubrum

The massive production of urban and agricultural wastes has promoted a clear need for alternative processes of disposal and waste management. The potential use of municipal solid wastes (MSW) as feedstock for the production of polyhydroxyalkanoates (PHA) by a process known as syngas fermentation is considered herein as an attractive bio-economic strategy to reduce these wastes. In this work, we have evaluated the potential of Rhodospirillum rubrum as microbial cell factory for the synthesis of PHA from syngas produced by microwave pyrolysis of the MSW organic fraction from a European city (Seville). Growth rate, uptake rate, biomass yield and PHA production from syngas in R. rubrum have been analysed. The results revealed the strong robustness of this syngas fermentation where the purity of the syngas is not a critical constraint for PHA production. Microwave-induced pyrolysis is a tangible alternative to standard pyrolysis, because it can reduce cost in terms of energy and time as well as increase syngas production, providing a satisfactory PHA yield.