A Review: The Styrene Metabolizing Cascade of Side-Chain Oxygenation as Biotechnological Basis to Gain Various Valuable Compounds

Integrating genomics, molecular docking, and protein expression to explore new perspectives on polystyrene biodegradation

Faced with the escalating challenge of global plastic pollution, this study specifically addresses the research gap in the biodegradation of polystyrene (PS). A PS-degrading bacterial strain was isolated from the gut of Tenebrio molitor, and genomics, molecular docking, and proteomics were employed to thoroughly investigate the biodegradation mechanisms of Pseudomonas putida H-01 against PS. Using scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (ATR-FTIR), and contact angle analysis, significant morphological and structural changes in the PS films under the influence of the H-01 strain were observed. The study revealed several potential degradation genes and ten enzymes that were specifically upregulated in the PS degradation environment. Additionally, a novel protein with laccase-like activity, LacQ1, was purified from this strain for the first time, and its crucial role in the PS degradation process was confirmed. Through molecular docking and molecular dynamics (MD) simulations, the interactions between the enzymes and PS were detailed, elucidating the binding and catalytic mechanisms of the degradative enzymes with the substrate. These findings have deepened our understanding of PS degradation.

Green cleanup of styrene-contaminated soil by carbon-based nanoscale zero-valent iron and phytoremediation: Sunn hemp (Crotalaria juncea), zinnia (Zinnia violacea Cav.), and marigold (Tagetes erecta L.)

Accidental chemical spills can result in styrene-contaminated soil. Styrene negatively affects human health and the environment. The objective of this study was to remediate styrene-contaminated soil using a combination of activated carbon-based nanoscale zero-valent iron (nZVI-AC) and phytoremediation by sunn hemp (Crotalaria juncea), zinnia (Zinnia violacea Cav.) and marigolds (Tagetes erecta L.). The results showed that all three plant types could potentially increase the removal efficiency of styrene-contaminated soil. At 28 days, all three plants showed complete removal of styrene from the soil with 1 g/kg of nZVI-AC, activated carbon-based nZVI synthesized by tea leaves (Camellia sinensis) (T-nZVI-AC), or activated carbon-based nZVI synthesized by red Thai holy basil (Ocimum tenuiflorum L.) (B-nZVI-AC). However, styrene removal efficiencies of sunn hemp, zinnia, and marigold without carbon-based nZVI were 30%, 67%, and 56%, respectively. Statistical analysis (ANOVA) revealed that the removal efficiencies differed significantly from those of phytoremediation alone. With the same removal efficiency (100%), the biomass of sunn hemp in nano-phytoremediation treatments differed by approximately 55%, whereas the biomass of zinnia differed by >67%, compared with that of the control experiment. For marigold, the difference in biomass was only 30%. Styrene was adsorbed on surface of soil and AC and then further oxidized under air-water-nZVI environment, while phytovolatilization played an important role in transporting the remaining styrene from the contaminated soil to the air. Marigold was used as an alternative plant for the nano-phytoremediation of styrene-contaminated soil because of its sturdy nature, high biomass, tolerance to toxic effects, and ease of cultivation. Remediation of one cubic meter of styrene-contaminated soil by a combination of carbon-based nanoscale zero-valent iron and phytoremediation by marigolds emitted 0.0027 kgCO2/m3.

Phylloplane fungus Curvularia dactyloctenicola VJP08 effectively degrades commercially available PS product

Polystyrene (PS), a widely produced plastic with an extended carbon (C-C) backbone that resists microbial attack, is produced in enormous quantities throughout the World. Naturally occurring plasticizers such as plant cuticle and lignocelluloses share similar properties to synthetic plastics such as hydrophobicity, structural complexity, and higher recalcitrance to degradation. In due course of time, phytopathogenic fungi have evolved strategies to overcome these limitations and utilize lignocellulosic waste for their nutrition. The present investigation focuses on the utilization of phylloplane fungus, Curvularia dactyloctenicola VJP08 towards its ability to colonize and degrade commercially available PS lids. The fungus was observed to densely grow onto PS samples over an incubation period of 30 days. The morphological changes showcased extensive fungal growth with mycelial imbrication invading the PS surface for carbon extraction leading to the appearance of cracks and holes in the PS surface. It was further confirmed by EDS analysis which indicated that carbon was extracted from PS for the fungal growth. Further, 3.57% decrease in the weight, 8.8% decrease in the thickness and 2 °C decrease in the glass transition temperature (Tg) confirmed alterations in the structural integrity of PS samples by the fungal action. GC-MS/MS analysis of the treated PS samples also showed significant decrease in the concentration of benzene and associated aromatic derivatives confirming the degradation of PS samples and subsequent utilization of generated by-products by the fungus for growth. Overall, the present study confirmed the degradation and utilization of commercially available PS samples by phylloplane fungus C. dactyloctenicola VJP08. These findings establish a clear cross-assessment of the phylloplane fungi for their prospective use in the development of degradation strategies of synthetic plastics.

Biodegradation of Polystyrene by Galleria mellonella: Identification of Potential Enzymes Involved in the Degradative Pathway

Galleria mellonella is a lepidopteran whose larval stage has shown the ability to degrade polystyrene (PS), one of the most recalcitrant plastics to biodegradation. In the present study, we fed G. mellonella larvae with PS for 54 days and determined candidate enzymes for its degradation. We first confirmed the biodegradation of PS by Fourier transform infrared spectroscopy- Attenuated total reflectance (FTIR-ATR) and then identified candidate enzymes in the larval gut by proteomic analysis using liquid chromatography with tandem mass spectrometry (LC-MS/MS). Two of these proteins have structural similarities to the styrene-degrading enzymes described so far. In addition, potential hydrolases, isomerases, dehydrogenases, and oxidases were identified that show little similarity to the bacterial enzymes that degrade styrene. However, their response to a diet based solely on polystyrene makes them interesting candidates as a potential new group of polystyrene-metabolizing enzymes in eukaryotes.

Identification of key degraders for controlling toxicity risks of disguised toxic pollutants with division of labor mechanisms in activated sludge microbiomes: Using nonylphenol ethoxylate as an example

Efficient management of disguised toxic pollutants (DTPs), which can undergo microbial degradation and convert into more toxic substances, necessitates the collaboration of diverse microbial populations in wastewater treatment plants. However, the identification of key bacterial degraders capable of controlling the toxicity risks of DTPs through division of labor mechanisms in activated sludge microbiomes has received limited attention. In this study, we investigated the key degraders capable of controlling the risk of estrogenicity associated with nonylphenol ethoxylate (NPEO), a representative DTP, in textile activated sludge microbiomes. The results of our batch experiments revealed that the transformation of NPEO into NP and subsequent NP degradation were the rate-limiting processes for controlling the risk of estrogenicity, resulting in an inverted V-shaped curve of estrogenicity in water samples during the biodegradation of NPEO by textile activated sludge. By utilizing enrichment sludge microbiomes treated with NPEO or NP as the sole carbon and energy source, a total of 15 bacterial degraders, including Sphingbium, Pseudomonas, Dokdonella, Comamonas, and Hyphomicrobium, were identified as capable of participating in these processes, Among them, Sphingobium and Pseudomonas were the two key degraders that could cooperatively interact in the degradation of NPEO with division of labor mechanisms. Co-culturing Sphingobium and Pseudomonas isolates exhibited a synergistic effect in degrading NPEO and reducing estrogenicity. Our study underscores the potential of the identified functional bacteria for controlling estrogenicity associated with NPEO and provides a methodological framework for identifying key cooperators engaged in labor division, contributing to the management of risks associated with DTPs by leveraging intrinsic microbial metabolic interactions.

Understanding the in silico aspects of bacterial catabolic cascade for styrene degradation

Styrene is a nonpolar organic compound used in very high volume for the industrial scale production of commercially important polymers such as polystyrene resins as well as copolymers like acrylonitrile butadiene styrene, latex, and rubber. These resins are widely used in the manufacturing of various products including single-use plastics such as disposable cups and containers, protective packaging, heat insulation, and so forth. The large-scale utilization leads to the over-accumulation of styrene waste in the environment causing deleterious health risks including cancer, neurological impairment, dysbiosis of central nervous system, and respiratory problems. To eliminate the accumulating waste. Microbial enzyme-based system represents the most environmental friendly and sustainable approach for elimination of styrene waste. However, comprehensive understanding of the enzyme-substrate interaction and associated pathways would be crucial for developing large-scale disposal systems. This study aims to understand the molecular interaction between the protein-ligand complexes of the styrene catabolic reactions by bacterial enzymes of sty operon. Molecular docking analysis for catalytic enzymes namely, styrene monooxygenase (SMO), styrene oxide isomerase (SOI), and phenylacetaldehyde dehydrogenase (PAD) of the bacterial sty operon was carried out with their individual substrates, that is, styrene, styrene oxide, and phenylacetic acid, respectively. The binding energy, amino acids forming binding cavity, and binding interactions between the protein-ligand binding sites were calculated for each case. The obtained binding energies showed a stable association of these complexes indicating the future scope of their utilization for large-scale bioremediation of styrene, and its commercially used polymers and copolymers.

Styrene Oxide Isomerase-Catalyzed Meinwald Rearrangement in Cascade Biotransformations: Synthesis of Chiral and/or Natural Chemicals

Styrene oxide isomerase (SOI) catalyzes the Meinwald rearrangement of aryl epoxides to carbonyl compounds via a 1,2-trans-shift in a stereospecific manner. A number of cascade biotransformations with SOI-catalyzed epoxide isomerization as a key step have been developed to convert readily available substrates into valuable chiral chemicals. Cascade conversion of terminal or internal alkenes into chiral acids, alcohols or amines was achieved, which involved SOI-catalyzed enantio-retentive isomerization of terminal epoxides via 1,2-H shift, or internal epoxides via 1,2-methyl shift. SOI-involved cascades were also developed to convert racemic epoxides into chiral acids or amines via dynamic kinetic resolution. Additionally, combining SOI-catalyzed isomerization with enantioselective C-C bond forming enzymes enabled the synthesis of chiral amino acids or amino alcohols from racemic epoxides. Finally, integration of SOI-involved cascades with biosynthesis pathways allowed for the direct utilization of renewable substrates for the sustainable synthesis of high-value natural chemicals such as alcohols, acids, and esters.

Co-diet supplementation of low density polyethylene and honeybee wax did not influence the core gut bacteria and associated enzymes of Galleria mellonella larvae (Lepidoptera: Pyralidae)

The current plastic pollution throughout the world is a rising concern that demands the optimization of biodegradation processes. One avenue for this is to identify plastic-degrading bacteria and associated enzymes from the gut bacteria of insect models such as Tenebrio molitor, Plodia interpunctella or Galleria mellonella that have the ability to ingest and rapidly degrade polyethylene. Therefore, this study takes part in understanding the role of the gut bacteria by investigating G. mellonella as a biological model feeding with a diet based on honeybee wax mixed or not with low-density polyethylene. Gut microbiome was analyzed by high throughput 16S rRNA sequencing, and Enterococcaceae and Oxalobacteraceae were found to be the major bacterial families. Compared to the control, the supplementation of low-density polyethylene did not cause significant modification of the bacterial microbiota at community and taxa levels, suggesting bacterial microbiome resilience. The bacterial proteome analysis of gut contents was encouraging for the identification of plastic degrading enzymes such as the phenylacetaldehyde dehydrogenase which participate in styrene degradation. This study allowed a better characterization of the gut bacteria of G. mellonella and provided a basis for the further study of biodegradation of polyethylene based on the bacterial microbiota from insect guts.

Bioconversion of Plastic Waste Based on Mass Full Carbon Backbone Polymeric Materials to Value-Added Polyhydroxyalkanoates (PHAs)

This review article will discuss the ways in which various polymeric materials, such as polyethylene (PE), polypropylene (PP), polystyrene (PS), and poly(ethylene terephthalate) (PET) can potentially be used to produce bioplastics, such as polyhydroxyalkanoates (PHAs) through microbial cultivation. We will present up-to-date information regarding notable microbial strains that are actively used in the biodegradation of polyolefins. We will also review some of the metabolic pathways involved in the process of plastic depolymerization and discuss challenges relevant to the valorization of plastic waste. The aim of this review is also to showcase the importance of methods, including oxidative degradation and microbial-based processes, that are currently being used in the fields of microbiology and biotechnology to limit the environmental burden of waste plastics. It is our hope that this article will contribute to the concept of bio-upcycling plastic waste to value-added products via microbial routes for a more sustainable future.

Oxidative degradation of pre-oxidated polystyrene plastics by dye decolorizing peroxidases from Thermomonospora curvata and Nostocaceae

Biodegradation of PS has attracted lots of public attentions due to its environmental friendliness. However, no specific PS degrading enzyme has been identified yet. Dye decolorizing peroxidases (DyPs) are heme-containing peroxidases named for the ability to degrade a variety of organic dyes. Herein, the abilities of two DyPs from Thermomonospora curvata (TcDyP) and Nostocaceae (AnaPX) to degrade PS were evaluated. Preoxidation methods by ultraviolet (UV) irradiation and chemical oxidants were developed to initially activate C-C bonds in the PS skeleton. DyPs degradation caused obvious etching and enhanced hydrophilicity of UV-PS films, and also generated new CO and C-OH groups. The cleavage of activated C-C bonds by DyPs was experimentally proven by analyzing the degradation products of UV-PS and model substrates. Furthermore, better pre-oxidation was obtained by using chemical oxidants KMnO₄/H₂SO₄ and ^mCPBA to oxidize PS materials in dissolved state. And AnaPX exhibited stronger degradation effects on KMnO₄/H₂SO₄-PS and ^mCPBA-PS by causing greater changes in functional groups CO, C-O, -OH groups and substituted benzenes and higher molecular weight reductions of 19.7% and 31.0%, respectively. To our knowledge, this is the first report on the identification of PS-degrading enzymes that provides experimental evidence.

Polystyrene Degradation by Exiguobacterium sp. RIT 594: Preliminary Evidence for a Pathway Containing an Atypical Oxygenase

The widespread use of plastics has led to their increasing presence in the environment and subsequent pollution. Some microorganisms degrade plastics in natural ecosystems and the associated metabolic pathways can be studied to understand the degradation mechanisms. Polystyrene (PS) is one of the more recalcitrant plastic polymers that is degraded by only a few bacteria. Exiguobacterium is a genus of Gram-positive poly-extremophilic bacteria known to degrade PS, thus being of biotechnological interest, but its biochemical mechanisms of degradation have not yet been elucidated. Based solely on genome annotation, we initially proposed PS degradation by Exiguobacterium sp. RIT 594 via depolymerization and epoxidation catalyzed by a ring epoxidase. However, Fourier transform infrared (FTIR) spectroscopy analysis revealed an increase of carboxyl and hydroxyl groups with biodegradation, as well as of unconjugated C-C double bonds, both consistent with dearomatization of the styrene ring. This excludes any aerobic pathways involving side chain epoxidation and/or hydroxylation. Subsequent experiments confirmed that molecular oxygen is critical to PS degradation by RIT 594 because degradation ceased under oxygen-deprived conditions. Our studies suggest that styrene breakdown by this bacterium occurs via the sequential action of two enzymes encoded in the genome: an orphan aromatic ring-cleaving dioxygenase and a hydrolase.

Enzyme-Catalyzed Meinwald Rearrangement with an Unusual Regioselective and Stereospecific 1,2-Methyl Shift

The Meinwald rearrangement is a synthetically useful reaction but often lacks regioselectivity and stereocontrol. A significant challenge in the Meinwald rearrangement of internal epoxides is the non-regioselective migration of different substituents to give a mixture of products. Herein, an enzyme-catalyzed regioselective and stereospecific 1,2-methyl shift in the Meinwald rearrangement of internal epoxides is reported. Styrene oxide isomerase (SOI) catalyzed the unique isomerization of internal epoxides through 1,2-methyl shift without 1,2-hydride shift to give the corresponding aldehydes and a cyclic ketone as the sole product. SOI-catalyzed isomerization showed high stereospecificity, fully retaining the stereoconfiguration. The synthetic utility of this enzymatic Meinwald rearrangement was demonstrated by its incorporation into three new types of enantioselective cascades, to convert trans-β-methyl styrenes into the corresponding R-configured alcohols, acids, or amines in high ee and yield.

From Organic Wastes and Hydrocarbons Pollutants to Polyhydroxyalkanoates: Bioconversion by Terrestrial and Marine Bacteria

The use of fossil-based plastics has become unsustainable because of the polluting production processes, difficulties for waste management sectors, and high environmental impact. Polyhydroxyalkanoates (PHA) are bio-based biodegradable polymers derived from renewable resources and synthesized by bacteria as intracellular energy and carbon storage materials under nutrients or oxygen limitation and through the optimization of cultivation conditions with both pure and mixed culture systems. The PHA properties are affected by the same principles of oil-derived polyolefins, with a broad range of compositions, due to the incorporation of different monomers into the polymer matrix. As a consequence, the properties of such materials are represented by a broad range depending on tunable PHA composition. Producing waste-derived PHA is technically feasible with mixed microbial cultures (MMC), since no sterilization is required; this technology may represent a solution for waste treatment and valorization, and it has recently been developed at the pilot scale level with different process configurations where aerobic microorganisms are usually subjected to a dynamic feeding regime for their selection and to a high organic load for the intracellular accumulation of PHA. In this review, we report on studies on terrestrial and marine bacteria PHA-producers. The available knowledge on PHA production from the use of different kinds of organic wastes, and otherwise, petroleum-polluted natural matrices coupling bioremediation treatment has been explored. The advancements in these areas have been significant; they generally concern the terrestrial environment, where pilot and industrial processes are already established. Recently, marine bacteria have also offered interesting perspectives due to their advantageous effects on production practices, which they can relieve several constraints. Studies on the use of hydrocarbons as carbon sources offer evidence for the feasibility of the bioconversion of fossil-derived plastics into bioplastics.

Biodegradation of polyethylene and polystyrene: From microbial deterioration to enzyme discovery

The global production of plastics has continuously been soaring over the last decades due to their extensive use in our daily life and in industries. Although synthetic plastics offer great advantages from packaging to construction and electronics, their low biodegradability induce serious plastic pollution that damage the environment, human health and make irreversible changes in the ecological cycle. In particular, plastics containing only carbon-carbon (C-C) backbone are less susceptible to degradation due to the lack of hydrolysable groups. The representative polyethylene (PE) and polystyrene (PS) account for about 40% of the total plastic production. Various chemical and biological processes with great potential have been developed for plastic recycle and reuse, but biodegradation seems to be the most attractive and eco-friendly method to combat this growing environmental problem. In this review, we first summarize the current advances in PE and PS biodegradation, including isolation of microbes and potential degrading enzymes from different sources. Next, the state-of-the-art techniques used for evaluating and monitoring PE and PS degradation, the scientific toolboxes for enzyme discovery as well as the challenges and strategies for plastic biodegradation are intensively discussed. In return, it inspires a further technological exploration in expanding the diversity of species and enzymes, disclosing the essential pathways and developing new approaches to utilize plastic waste as feedstock for recycling and upcycling.

Gut Microbiome and Degradation Product Formation during Biodegradation of Expanded Polystyrene by Mealworm Larvae under Different Feeding Strategies

Polystyrene (PS) is a plastic polymer extensively used for food packaging. PS is difficult to decompose and has low recycling rates, resulting in its accumulation in the environment, in the form of microplastic particles causing pollution and harming oceans and wildlife. Degradation of PS by mealworms (Tenebrio molitor) has been suggested as a possible biological strategy for plastic contamination; however, the biodegradation mechanism of PS by mealworms is poorly understood. It is hypothesized that the gut microbiome plays an important role in the degradation of PS by mealworms. This study carried out a comparative analysis of the gut microbiome of Tenebrio molitor larvae under different feeding strategies, and of the formation of degradation compounds (monomers, oligomers). A diet of bran:PS at 4:1 and 20:1 ratios was tested. The diet with the low ratio of bran:PS led to the presence of higher amounts of these compounds, compared to that with the high ratio. In addition, it was demonstrated that the addition of H₂O significantly improved the biodegradation of PS monomer and oligomer residues, which could be identified only in the frass. The protein and nitrogen contents in insects’ biomass and frass varied amongst treatments. The diets resulted in differences in the gut microbiota, and three potential bacterial strains were identified as candidates involved in the biodegradation of PS.

Microbial Degradation of Plastics and Approaches to Make it More Efficient

Abstract—

The growing worldwide production of synthetic plastics leads to increased amounts of plastic pollution. Even though microbial degradation of plastics is known to be a very slow process, this capacity has been found in many bacteria, including invertebrate symbionts, and microscopic fungi. Research in this field has been mostly focused on microbial degradation of polyethylene, polystyrene, and polyethylene terephthalate (PET). Quite an arsenal of different methods is available today for detecting processes of plastic degradation and measuring their rates. Given the lack of generally accepted protocols, it is difficult to compare results presented by different authors. PET degradation by recombinant hydrolases from thermophilic actinobacteria happens to be the most efficient among the currently known plastic degradation processes. Various approaches to accelerating microbial plastic degradation are also discussed.

Biotechnology of Plastic Waste Degradation, Recycling, and Valorization: Current Advances and Future Perspectives

Although fossil-based plastic products have many attractive characteristics, their production has led to severe environmental burdens that require immediate solutions. Despite these plastics being non-natural chemical compounds, they can be degraded and metabolized by some microorganisms, which suggests the potential application of biotechnologies based on the mechanism of plastic biodegradation. In this context, microbe-based strategies for the degradation, recycling, and valorization of plastic waste offer a feasible approach for alleviating environmental challenges created by the accumulation of plastic waste. This Minireview highlights recent advances in the biotechnology-based biodegradation of both traditional polymers and bio-based plastics, focusing on the mechanisms of biodegradation. From an application perspective, this Minireview also summarizes recent progress in the recycling and valorization of plastic waste, which are feasible solutions for tackling the plastic waste dilemma.

Characterization of the Glutathione S-Transferases Involved in Styrene Degradation in Gordonia rubripertincta CWB2

The glutathione S-transferases carried on the plasmid for the styrene-specific degradation pathway in the Actinobacterium Gordonia rubripertincta CWB2 were heterologously expressed in Escherichia coli. Both enzymes were purified via affinity chromatography and subjected to activity investigations. StyI and StyJ displayed activity toward the commonly used glutathione S-transferase model substrate 1-chloro-2,4-dinitrobenzene (CDNB) with Km values of 0.0682 ± 0.0074 and 2.0281 ± 0.1301 mM and Vmax values of 0.0158 ± 0.0002 and 0.348 ± 0.008 U mg-1 for StyI and StyJ, respectively. The conversion of the natural substrate styrene oxide to the intermediate (1-phenyl-2-hydroxyethyl)glutathione was detected for StyI with 48.3 ± 2.9 U mg-1. This elucidates one more step in the not yet fully resolved styrene-specific degradation pathway of Gordonia rubripertincta CWB2. A characterization of both purified enzymes adds more insight into the scarce research field of actinobacterial glutathione S-transferases. Moreover, a sequence and phylogenetic analysis puts both enzymes into a physiological and evolutionary context. IMPORTANCE Styrene is a toxic compound that is used at a large scale by industry for plastic production. Bacterial degradation of styrene is a possibility for bioremediation and pollution prevention. Intermediates of styrene derivatives degraded in the styrene-specific pathways are precursors for valuable chemical compounds. The pathway in Gordonia rubripertincta CWB2 has proven to accept a broader substrate range than other bacterial styrene degraders. The enzymes characterized in this study, distinguish CWB2s pathway from other known styrene degradation routes and thus might be the main key for its ability to produce ibuprofen from the respective styrene derivative. A biotechnological utilization of this cascade could lead to efficient and sustainable production of drugs, flavors, and fragrances. Moreover, research on glutathione metabolism in Actinobacteria is rare. Here, a characterization of two glutathione S-transferases of actinobacterial origin is presented, and the utilization of glutathione in the metabolism of an Actinobacterium is proven.

Landfill microbiome harbour plastic degrading genes: A metagenomic study of solid waste dumping site of Gujarat, India

Globally, environmental pollution by plastic waste has become a severe ecological and social problem worldwide. The present study aimed to analyse the bacterial community structure and functional potential of the landfill site using high throughput shotgun metagenomic approach to understand plastic degrading capabilities present in the municipal solid waste (MSW) dumping site. In this study, soil, leachate and compost samples were collected from various locations (height and depth) of the Pirana landfill site in Ahmedabad city Gujarat, India. In total 30 phyla, 58 class, 125 order, 278 families, 793 genera, and 2468 species were predicted. The most dominant phyla detected were Proteobacteria, Bacteroidetes, Firmicutes, and Actinobacteria in the soil and compost samples. Whereas, in leachate samples, the predominant phyla belonged to Firmicutes (54.24%) followed by Actinobacteria (43.67%) and Proteobacteria (1.02%). The functional profiling revealed the presence of enzymatic groups and pathways involved in biodegradation of xenobiotics. The results also demonstrated the presence of potential genes that is associated with the biodegradation of different types of plastics such as polyethylene (PE), polyethylene terephthalate (PET), and polystyrene (PS). Present study extablishes the relationship between microbial community structure and rich sources of gene pool, which are actively involved in biodegradation of plastic waste in landfill sites.

Production of Enantiopure Chiral Epoxides with E. coli Expressing Styrene Monooxygenase

Styrene monooxygenases are a group of highly selective enzymes able to catalyse the epoxidation of alkenes to corresponding chiral epoxides in excellent enantiopurity. Chiral compounds containing oxirane ring or products of their hydrolysis represent key building blocks and precursors in organic synthesis in the pharmaceutical industry, and many of them are produced on an industrial scale. Two-component recombinant styrene monooxygenase (SMO) from Marinobacterium litorale was expressed as a fused protein (StyAL2StyB) in Escherichia coli BL21(DE3). By high cell density fermentation, 35 g_DCW/L of biomass with overexpressed SMO was produced. SMO exhibited excellent stability, broad substrate specificity, and enantioselectivity, as it remained active for months and converted a group of alkenes to corresponding chiral epoxides in high enantiomeric excess (˃95–99% ee). Optically pure (S)-4-chlorostyrene oxide, (S)-allylbenzene oxide, (2R,5R)-1,2:5,6-diepoxyhexane, 2-(3-bromopropyl)oxirane, and (S)-4-(oxiran-2-yl)butan-1-ol were prepared by whole-cell SMO.

Biochemical Characterization of Phenylacetaldehyde Dehydrogenases from Styrene-degrading Soil Bacteria

Four phenylacetaldehyde dehydrogenases (designated as FeaB or StyD) originating from styrene-degrading soil bacteria were biochemically investigated. In this study, we focused on the Michaelis-Menten kinetics towards the presumed native substrate phenylacetaldehyde and the obviously preferred co-substrate NAD+. Furthermore, the substrate specificity on four substituted phenylacetaldehydes and the co-substrate preference were studied. Moreover, these enzymes were characterized with respect to their temperature as well as long-term stability. Since aldehyde dehydrogenases are known to show often dehydrogenase as well as esterase activity, we tested this capacity, too. Almost all results showed clearly different characteristics between the FeaB and StyD enzymes. Furthermore, FeaB from Sphingopyxis fribergensis Kp5.2 turned out to be the most active enzyme with an apparent specific activity of 17.8 ± 2.1 U mg-1. Compared with that, both StyDs showed only activities less than 0.2 U mg-1 except the overwhelming esterase activity of StyD-CWB2 (1.4 ± 0.1 U mg-1). The clustering of both FeaB and StyD enzymes with respect to their characteristics could also be mirrored in the phylogenetic analysis of twelve dehydrogenases originating from different soil bacteria.

Potential for and Distribution of Enzymatic Biodegradation of Polystyrene by Environmental Microorganisms

Polystyrene (PS) is one of the main polymer types of plastic wastes and is known to be resistant to biodegradation, resulting in PS waste persistence in the environment. Although previous studies have reported that some microorganisms can degrade PS, enzymes and mechanisms of microorganism PS biodegradation are still unknown. In this study, we summarized microbial species that have been identified to degrade PS. By screening the available genome information of microorganisms that have been reported to degrade PS for enzymes with functional potential to depolymerize PS, we predicted target PS-degrading enzymes. We found that cytochrome P4500s, alkane hydroxylases and monooxygenases ranked as the top potential enzyme classes that can degrade PS since they can break C-C bonds. Ring-hydroxylating dioxygenases may be able to break the side-chain of PS and oxidize the aromatic ring compounds generated from the decomposition of PS. These target enzymes were distributed in Proteobacteria, Actinobacteria, Bacteroidetes, and Firmicutes, suggesting a broad potential for PS biodegradation in various earth environments and microbiomes. Our results provide insight into the enzymatic degradation of PS and suggestions for realizing the biodegradation of this recalcitrant plastic.

The devil is in the details: The chemical basis and mechanistic versatility of flavoprotein monooxygenases

The ubiquitous flavoenzymes commonly catalyze redox chemistry such as the monooxygenation of organic substrates and are both widely utilized in nature (e.g., in primary and secondary metabolism) and of significant industrial interest. In this work, we highlight the structural and mechanistic characteristics of the distinct types of flavoprotein monooxygenases (FPMOs). We thereby illustrate the chemical basis of FPMO catalysis, which enables reactions such as (aromatic) hydroxylation, epoxidation, (de)halogenation, heteroatom oxygenation, Baeyer-Villiger oxidation, α-hydroxylation of ketones, or non-oxidative carbon-hetero bond cleavage. This seemingly unmatched versatility in oxygenation chemistry results from extensive fine-tuning and regiospecific functionalization of the flavin cofactor that is tightly controlled by the surrounding protein matrix. Accordingly, FPMOs steer the formation of covalent flavin-oxygen adducts for oxygen transfer in the form of the classical flavin-C4a-(hydro)peroxide or the recently discovered N5-functionalized flavins (i.e. the flavin-N5-oxide and the flavin-N5-peroxide), while in rare cases covalent oxygen adduct formation may be foregone entirely. Finally, we speculate about hitherto undiscovered flavin-mediated oxygenation reactions and compare FPMOs to cytochrome P450 monooxygenases, before addressing open questions and challenges for the future investigation of FPMOs.

Traditional Human Populations and Nonhuman Primates Show Parallel Gut Microbiome Adaptations to Analogous Ecological Conditions

The results of this study highlight parallel gut microbiome traits in human and nonhuman primates, depending on subsistence strategy. Although these similarities have been reported before, the functional and ecological bases of this convergence are not fully understood.

Compared with urban-industrial populations, small-scale human communities worldwide share a significant number of gut microbiome traits with nonhuman primates. This overlap is thought to be driven by analogous dietary triggers; however, the ecological and functional bases of this similarity are not fully understood. To start addressing this issue, fecal metagenomes of BaAka hunter-gatherers and traditional Bantu agriculturalists from the Central African Republic were profiled and compared with those of a sympatric western lowland gorilla group (Gorillagorilla gorilla) across two seasons of variable dietary intake. Results show that gorilla gut microbiomes shared similar functional traits with each human group, depending on seasonal dietary behavior. Specifically, parallel microbiome traits were observed between hunter-gatherers and gorillas when the latter consumed more structural polysaccharides during dry seasons, while small-scale agriculturalist and gorilla microbiomes showed significant functional overlap when gorillas consumed more seasonal ripe fruit during wet seasons. Notably, dominance of microbial transporters, transduction systems, and gut xenobiotic metabolism was observed in association with traditional agriculture and energy-dense diets in gorillas at the expense of a functional microbiome repertoire capable of metabolizing more complex polysaccharides. Differential abundance of bacterial taxa that typically distinguish traditional from industrialized human populations (e.g., Prevotella spp.) was also recapitulated in the human and gorilla groups studied, possibly reflecting the degree of polysaccharide complexity included in each group’s dietary niche. These results show conserved functional gut microbiome adaptations to analogous diets in small-scale human populations and nonhuman primates, highlighting the role of plant dietary polysaccharides and diverse environmental exposures in this convergence.

IMPORTANCE The results of this study highlight parallel gut microbiome traits in human and nonhuman primates, depending on subsistence strategy. Although these similarities have been reported before, the functional and ecological bases of this convergence are not fully understood. Here, we show that this parallelism is, in part, likely modulated by the complexity of plant carbohydrates consumed and by exposures to diverse xenobiotics of natural and artificial origin. Furthermore, we discuss how divergence from these parallel microbiome traits is typically associated with adverse health outcomes in human populations living under culturally westernized subsistence patterns. This is important information as we trace the specific dietary and environmental triggers associated with the loss and gain of microbial functions as humans adapt to various dietary niches.

Styrene and Bioaerosol Removal from Waste Air with a Combined Biotrickling Filter and DBD–Plasma System

A combined system of a biotrickling filter and a non-thermal plasma (NTP) in a downstream airflow was operated for 1220 days for treatment of emissions of styrene and secondary emissions of germs formed in the biological process. The biotrickling filter was operated at variable inlet concentrations, empty bed residence times (EBRT), type and dosage of fertilizers, irrigation densities, and starvation periods, while dielectric barrier discharge and corona discharge were operated at different specific input energy levels to achieve optimal conditions. Under these conditions, efficiencies in the removal of volatile organic compounds (VOCs), germs and styrene of 96–98%, 1–4 log units and 24.7–50.1 g C m−3 h−1 were achieved, respectively. Fluid simulations of the NTP and a germ emission-based clocking of the discharge reveal further energy saving potentials of more than 90%. The aim of an energy-efficient elimination of VOCs through a biotrickling filter and of secondary germ emissions by a NTP stage in a downstream airflow for potential re-use of purified waste gas as process gas for industrial application was successfully accomplished.

Biodiversity of Microorganisms Colonizing the Surface of Polystyrene Samples Exposed to Different Aqueous Environments

The contamination of marine and freshwater ecosystems with the items from thermoplastics, including polystyrene (PS), necessitates the search for efficient microbial degraders of these polymers. In the present study, the composition of prokaryotes in biofilms formed on PS samples incubated in seawater and the industrial water of a petrochemical plant were investigated. Using a high-throughput sequencing of the V3–V4 region of the 16S rRNA gene, the predominance of Alphaproteobacteria (Blastomonas), Bacteroidetes (Chryseolinea), and Gammaproteobacteria (Arenimonas and Pseudomonas) in the biofilms on PS samples exposed to industrial water was revealed. Alphaproteobacteria (Erythrobacter) predominated on seawater-incubated PS samples. The local degradation of the PS samples was confirmed by scanning microscopy. The PS-colonizing microbial communities in industrial water differed significantly from the PS communities in seawater. Both communities have a high potential ability to carry out the carbohydrates and amino acids metabolism, but the potential for xenobiotic degradation, including styrene degradation, was relatively higher in the biofilms in industrial water. Bacteria of the genera Erythrobacter, Maribacter, and Mycobacterium were potential styrene-degraders in seawater, and Pseudomonas and Arenimonas in industrial water. Our results suggest that marine and industrial waters contain microbial populations potentially capable of degrading PS, and these populations may be used for the isolation of efficient PS degraders.

Microbial Degradation and Valorization of Plastic Wastes

A growing accumulation of plastic wastes has become a severe environmental and social issue. It is urgent to develop innovative approaches for the disposal of plastic wastes. In recent years, reports on biodegradation of synthetic plastics by microorganisms or enzymes have sprung up, and these offer a possibility to develop biological treatment technology for plastic wastes. In this review, we have comprehensively summarized the microorganisms and enzymes that are able to degrade a variety of generally used synthetic plastics, such as polyethylene (PE), polystyrene (PS), polypropylene (PP), polyvinyl chloride (PVC), polyurethane (PUR), and polyethylene terephthalate (PET). In addition, we have highlighted the microbial metabolic pathways for plastic depolymerization products and the current attempts toward utilization of such products as feedstocks for microbial production of chemicals with high value. Taken together, these findings will contribute to building a conception of bio-upcycling plastic wastes by connecting the biodegradation of plastic wastes to the biosynthesis of valuable chemicals in microorganisms. Last, but not least, we have discussed the challenges toward microbial degradation and valorization of plastic wastes.

A Minimized Chemoenzymatic Cascade for Bacterial Luciferase in Bioreporter Applications

Bacterial luciferase (Lux) catalyzes a bioluminescence reaction by using long-chain aldehyde, reduced flavin and molecular oxygen as substrates. The reaction can be applied in reporter gene systems for biomolecular detection in both prokaryotic and eukaryotic organisms. Because reduced flavin is unstable under aerobic conditions, another enzyme, flavin reductase, is needed to supply reduced flavin to the Lux-catalyzed reaction. To create a minimized cascade for Lux that would have greater ease of use, a chemoenzymatic reaction with a biomimetic nicotinamide (BNAH) was used in place of the flavin reductase reaction in the Lux system. The results showed that the minimized cascade reaction can be applied to monitor bioluminescence of the Lux reporter in eukaryotic cells effectively, and that it can achieve higher efficiencies than the system with flavin reductase. This development is useful for future applications as high-throughput detection tools for drug screening applications.

Comparative Genomic Analysis of Capsule-Producing Black Yeasts Exophiala dermatitidis and Exophiala spinifera, Potential Agents of Disseminated Mycoses

The two black yeasts Exophiala dermatitidis and Exophiala spinifera that are clinically considered as the most virulent species potentially causing disseminated infections are both producing extracellular capsule-like material, are compared. In this study, 10 genomes of E. spinifera and E. dermatitidis strains, including both clinical and environmental isolates, were selected based on phylogenetic analysis, physiology tests and virulence tests, sequenced on the Illumina MiSeq sequencer and annotated. Comparison of genome data were performed between intraspecific and interspecific strains. We found capsule-associated genes were however not consistently present in both species by the comparative genomics. The prevalent clinical species, E. dermatitidis, has small genomes containing significantly less virulence-associated genes than E. spinifera, and also than saprobic relatives. Gene OG0012246 and Myb-like DNA-binding domain and SANT/Myb domain, restricted to two strains from human brain, was shared with the neurotropic species Rhinocladiella mackenziei. This study indicated that different virulence profiles existed in the two capsule-producing black yeasts, and the absence of consistent virulence-associated profiles supports the hypothesis that black yeasts are opportunists rather than primary pathogens. The results also provide the key virulence genes and drive the continuing research forward pathogen–host interactions to explore the pathogenesis.

Glutathione: A powerful but rare cofactor among Actinobacteria

Glutathione (γ-l-glutamyl-l-cysteinylglycine, GSH) is a powerful cellular redox agent. In nature only the l,l-form is common among the tree of life. It serves as antioxidant or redox buffer system, protein regeneration and activation by interaction with thiol groups, unspecific reagent for conjugation during detoxification, marker for amino acid or peptide transport even through membranes, activation or solubilization of compounds during degradative pathways or just as redox shuttle. However, the role of GSH production and utilization in bacteria is more complex and especially little is known for the Actinobacteria. Some recent reports on GSH use in degradative pathways came across and this is described herein. GSH is used by transferases to activate and solubilize epoxides. It allows funneling epoxides as isoprene oxide or styrene oxide into central metabolism. Thus, the distribution of GSH synthesis, recycling and application among bacteria and especially Actinobacteria are highlighted including the pathways and contributing enzymes.

Plastics: Environmental and Biotechnological Perspectives on Microbial Degradation

Plastics are widely used in the global economy, and each year, at least 350 to 400 million tons are being produced. Due to poor recycling and low circular use, millions of tons accumulate annually in terrestrial or marine environments. Today it has become clear that plastic causes adverse effects in all ecosystems and that microplastics are of particular concern to our health. Therefore, recent microbial research has addressed the question of if and to what extent microorganisms can degrade plastics in the environment. This review summarizes current knowledge on microbial plastic degradation. Enzymes available act mainly on the high-molecular-weight polymers of polyethylene terephthalate (PET) and ester-based polyurethane (PUR). Unfortunately, the best PUR- and PET-active enzymes and microorganisms known still have moderate turnover rates. While many reports describing microbial communities degrading chemical additives have been published, no enzymes acting on the high-molecular-weight polymers polystyrene, polyamide, polyvinylchloride, polypropylene, ether-based polyurethane, and polyethylene are known. Together, these polymers comprise more than 80% of annual plastic production. Thus, further research is needed to significantly increase the diversity of enzymes and microorganisms acting on these polymers. This can be achieved by tapping into the global metagenomes of noncultivated microorganisms and dark matter proteins. Only then can novel biocatalysts and organisms be delivered that allow rapid degradation, recycling, or value-added use of the vast majority of most human-made polymers.

Functional characterization of an (R)-selective styrene monooxygenase from streptomyces sp. NRRL S-31

Styrene monooxygenases (SMOs) are two-component enzymes known to catalyze the epoxidation of styrene to (S)-styrene oxide. In this work, we identified a new oxygenase component, named StStyA, from the genome of Streptomyces sp. NRRL S-31. StStyA displayed complementary stereoselectivity to all of the known SMOs when coupled with a known reductase component (PsStyB), which made it the first natural SMO that produces (R)-styrene oxide. Accordingly, a plasmid co-expressing StStyA and PsStyB was constructed, which led to an artificial two-component SMO, named StStyA/B. When applied in the bio-epoxidation of nine aromatic alkenes, the enzyme showed activity toward five alkenes, and consistently displayed (R)-selectivity. Excellent stereoselectivity was achieved for all five substrates with enantiomeric excesses ranging from 91% to >99%ee.

The styrene monooxygenase system

Styrene monooxygenases are soluble two-component flavoproteins that catalyze the NADH and FAD-dependent enantioselective epoxidation of styrene to styrene oxide in the aqueous phase. These enzymes present interesting mechanistic features and potential as catalysts in biotechnological applications ranging from green chemical synthesis to bioremediation. This chapter presents approaches for the expression of the reductase (SMOB, StyB) and epoxidase (SMOA, StyA) components of SMO from pET-vectors as native or N-terminally histidine-tagged proteins in commercial strains of E. coli. The two-component structure of SMO and hydrophobic nature of styrene substrate requires some special consideration in evaluating the mechanism of this enzyme. The modular composition of the enzyme allows the flavin-reduction reaction of SMOB and styrene epoxidation reaction of SMOA to be evaluated both independently and as a composite catalytic system. The freedom to independently study the reductase and epoxidase components of SMO significantly simplifies studies of equilibrium-binding and the coupling of the free energy of ligand binding to the electrochemical potential of bound FAD. In this chapter, methods of steady-state and pre-steady-state kinetic assay, experimental approaches to equilibrium-binding reactions of flavin and substrate, and determination of the electrochemical midpoint potential of FAD bound to the reductase and epoxidase components of SMO are presented. This presentation focuses on approaches that have been successfully used in the study of the wild-type styrene monooxygenase system recovered from Pseudomonas putida (S12), but similar approaches may be effective in the characterization of related two-component enzyme systems.

Biocatalytic selective functionalisation of alkenes via single-step and one-pot multi-step reactions

Alkenes are excellent starting materials for organic synthesis due to the versatile reactivity of C[double bond, length as m-dash]C bonds and the easy availability of many unfunctionalised alkenes. Direct regio- and/or enantioselective conversion of alkenes into functionalised (chiral) compounds has enormous potential for industrial applications, and thus has attracted the attention of researchers for extensive development using chemo-catalysis over the past few years. On the other hand, many enzymes have also been employed for conversion of alkenes in a highly selective and much greener manner to offer valuable products. Herein, we review recent advances in seven well-known types of biocatalytic conversion of alkenes. Remarkably, recent mechanism-guided directed evolution and enzyme cascades have enabled the development of seven novel types of single-step and one-pot multi-step functionalisation of alkenes, some of which are even unattainable via chemo-catalysis. These new reactions are particularly highlighted in this feature article. Overall, we present an ever-expanding enzyme toolbox for various alkene functionalisations inspiring further research in this fast-developing theme.

Two-Component FAD-Dependent Monooxygenases: Current Knowledge and Biotechnological Opportunities

Flavoprotein monooxygenases create valuable compounds that are of high interest for the chemical, pharmaceutical, and agrochemical industries, among others. Monooxygenases that use flavin as cofactor are either single- or two-component systems. Here we summarize the current knowledge about two-component flavin adenine dinucleotide (FAD)-dependent monooxygenases and describe their biotechnological relevance. Two-component FAD-dependent monooxygenases catalyze hydroxylation, epoxidation, and halogenation reactions and are physiologically involved in amino acid metabolism, mineralization of aromatic compounds, and biosynthesis of secondary metabolites. The monooxygenase component of these enzymes is strictly dependent on reduced FAD, which is supplied by the reductase component. More and more representatives of two-component FAD-dependent monooxygenases have been discovered and characterized in recent years, which has resulted in the identification of novel physiological roles, functional properties, and a variety of biocatalytic opportunities.

An integrated nanocatalyst combining enzymatic and metal–organic framework catalysts for cascade degradation of organophosphate nerve agents

An integrated nanocatalyst that combined a metal–organic framework-based catalyst and a biocatalyst was developed for cascade degradation of organophosphate nerve agents.