Streptomyces spp. as biocatalyst sources in pulp and paper and textile industries: Biodegradation, bioconversion and valorization of waste

Abstract Complex polymers represent a challenge for remediating environmental pollution and an opportunity for microbial‐catalysed conversion to generate valorized chemicals. Members of the genus Streptomyces are of interest because of their potential use in biotechnological applications. Their versatility makes them excellent sources of biocatalysts for environmentally responsible bioconversion, as they have a broad substrate range and are active over a wide range of pH and temperature. Most Streptomyces studies have focused on the isolation of strains, recombinant work and enzyme characterization for evaluating their potential for biotechnological application. This review discusses reports of Streptomyces‐based technologies for use in the textile and pulp‐milling industry and describes the challenges and recent advances aimed at achieving better biodegradation methods featuring these microbial catalysts. The principal points to be discussed are (1) Streptomyces' enzymes for use in dye decolorization and lignocellulosic biodegradation, (2) biotechnological processes for textile and pulp and paper waste treatment and (3) challenges and advances for textile and pulp and paper effluent treatment.


INTRODUCTION
In recent years, the contribution of wastewater to environmental pollution has been of global concern.The presence of bioactive organic chemicals in water effluents is particularly problematic.Industries such as textile and paper production plants release synthetic dyes and lignin, respectively, as part of their wastewater (Andersson et al., 2021) which could have further negative implications for the environment (Al-Tohamy et al., 2022;Parmar et al., 2022;Yadav & Chandra, 2018).For example, in paper production, the kraft pulping process is used, which consists of digesting wood under alkaline conditions, followed by several steps of washing and screening to separate brownstock and black liquor (Mathews et al., 2015), a process that generates paper sludge wastewater.The effluents become a collection of fibres, dissolved organic solids, salts, chlorinated compounds, heavy metals and a variety of lignocellulose biomass compounds (Brown et al., 2021), ultimately resulting in both liquid and solid wastes that are of environmental concern (Chandra et al., 2017).They could induce toxicity and endocrine damage if released into the environment (Kumar & Chandra, 2020;Yadav & Chandra, 2018), including negatively impacting aquatic life if introduced into waterways (Singh & Chandra, 2019).
Potential pollutants from these effluents are regulated by environmental agencies in different jurisdictions around the world.For example, the United States Environmental Protection Agency provides a list of pollutants and guidelines for discharge and best management practices to manage effluents and waste streams (US EPA, 2016).In the European Union, there is a 'Circular Economy Action Plan for a greener and more competitive Europe' that provides a framework for the prevention of waste, monitoring of toxic substances and the improvement of management practices of secondary raw materials and local waste (European Commission, 2020).However, different countries around the world have varied standards and regulations governing the release of paper and textile industry effluents (Central Pollution Control Board (CPCB), 2000; Meriläinen & Oikari, 2008;Zinabu et al., 2018).
Current treatment approaches for the paper industry effluent have been summarized and are separated into three-step treatments: physicochemical as the primary treatment (sedimentation, flotation and filtration, oxidation and ozonation); biological as secondary treatment, which is divided into aerobic (activated sludge systems and aerated lagoons) and anaerobic and emerging approaches as tertiary treatments (membrane filtration, adsorption and activated carbon, membrane bioreactors; Kumar, Saxena, et al., 2021).Several investigations, reviews and book chapters have described microbial degradation and decolorization of pulp and paper effluents as promising environmentally friendly technologies that need to be developed and further investigated (Singh et al., 2008;Kumar, Saxena, et al., 2021;Kumar, Srivastava, & Gera, 2021;Kumar et al., 2022).
Effluents from industrial textile operations can contain a range of synthetic dyes mixed with other contaminants at various concentrations (Forgacs et al., 2004;Yaseen & Scholz, 2019).Researchers have reported dye contamination levels of textile and printing effluents ranging from 2% to 50% (w/v; Díez-Méndez et al., 2019;Tan et al., 2000) and that from 15% to 30% of the dyestuff used in a dye/printing operations may be released to the environment (Bechtold & Turcanu, 2004), while others present information about the types of common dyes that are found in the wastewater effluent (Petrinić et al., 2015).In addition to the textile industry, synthetic dyes are used in pharmaceutical, food, cosmetics, paper, leather and carpet manufacturing (Castro et al., 2021;Guerra et al., 2018;John Sundar et al., 2011;Pérez-Ibarbia et al., 2016;Saxena et al., 2017).Improper disposal of these dyes represents health risks because of toxic products that biodegradation could produce (Parmar et al., 2022).The increase in fade-resistant fabric production creates a problem because it uses stable dyes that are more resistant to traditional chemical and biological bioremediation methods (Al-Tohamy et al., 2022;Carney Almroth et al., 2021).The possible chemical fates of textile and paper pulping effluents based on interactions with enzymes, which can lead to downstream consequences in the environment, are summarized in Figure 1.Currently, decolorization and degradation methods are expensive and sometimes generate toxic compounds that are difficult to degrade (Parmar et al., 2022).Therefore, developing low-cost, effective bioremediation methods for these colour-fast dyes is needed.
Lignocellulose is the most globally abundant organic renewable source and can be used as a feedstock to produce sustainable bioproducts (i.e.fuels, chemicals and molecules; Riyadi et al., 2020).Lignocellulose is a complex biopolymer composed of cellulose, hemicellulose and lignin.Lignin, the second most abundant component in lignocellulose after cellulose (Xiao et al., 2020), provides rigidity and robustness to the plant cell wall, notably protecting against pathogens and oxidative stress (Achyuthan et al., 2010;Mathews et al., 2015).However, because of its amorphous structure, lignin can be difficult to break down (Kumar & Chandra, 2020).The removal of lignin is necessary to facilitate bioconversion of the hemicellulose and cellulose components of lignocellulose into sugars for fermentation applications (Jönsson & Martín, 2016;Yu et al., 2018;Zhang et al., 2016) as well as for wood pulp processing for paper production (Wang et al., 2013).Once removed, lignin is often burned to use as steam energy (low-cost fuel to power paper mills) instead of being reused (Ko et al., 2016).A recent review describes the process of lignin gasification, including the generation of steam energy from lignin residues in the paper pulping process (Castro Garcia et al., 2022).However, the valorization of lignin for its beneficial properties such as biodegradation, antioxidant activity, thermostability, high carbon content and stiffness has been recently reviewed (Rinaldi et al., 2016) as has the bacterial conversion routes for the valorization of lignin (Liu et al., 2022).Challenges remain in the processing of lignin for complete valorization through its bioconversion to bio-based products (e.g.commodity chemicals like plastics, resins, fibres, bioplastics, resins, fibres and phenols as well as biofuels; Sethupathy et al., 2022).Existing pretreatment methods for lignin degradation may negatively affect downstream stages in energy bioconversion from cellulose biomass and can generate toxic waste products (Klinke et al., 2004;Palmqvist, 2000;Palmqvist & Hahn-Hägerdal, 2000).Generation of harmful pollutants is a potential risk to the environment and human health (Benslama et al., 2021;Yang et al., 2017).Therefore, treatment options are needed to increase the efficiency of lignin degradation and bioconversion.Using microbial enzymes for lignin and dye degradation is promising for enhancing available treatment options.The purpose of this review is to highlight the ability of multiuse enzymes from a specific bacterial genus, Streptomyces spp., to bioremediate textile and pulp-milling waste streams.

TA XONOMY AND LIFE CYCLE OF STR E PTO M YC ES
The genus Streptomyces, phylum Actinobacteriota (Panda et al., 2022), class Actinobacteria are Grampositive, aerobic bacteria with high G + C content (69-78 mol %) that form an extensive mycelium (Barbuto Ferraiuolo et al., 2021;Kämpfer, 2006).Streptomyces are ubiquitous in a variety of ecological niches but are commonly found in soil and are ecologically significant due to their role in decomposing cellulose (Takasuka et al., 2013).The genus is arguably most known for secondary metabolite production (Alam et al., 2022;Dávila Costa et al., 2020;Goel et al., 2021Goel et al., , 2022;;Kinkel et al., 2014;Lee et al., 2014).Streptomyces are well studied as sources for antibiotic production in the pharmaceutical industry (Chevrette et al., 2019).Recent advances in the production of drugs have been reviewed and highlighted as microbial cell factories (Barbuto Ferraiuolo et al., 2021).These microbes contain multiple biosynthetic gene clusters (BGCs) that are sources of bioactive compounds with biomedical and agricultural applications (Nicault et al., 2021;Ward & Allenby, 2018).However, comparably much less discovery has occurred to date focused on developing Streptomycesbased technologies for biodegradation applications.
Physiologically, Streptomyces spp.have a life cycle divided into two phases (vegetative and reproductive) that can vary depending on whether the culture is being cultivated on solid or in liquid media (Lajtai-Szabó et al., 2022).When grown on solid-based media, the spores germinate into the hyphae that produce the vegetative mycelium.The mycelia grow deeply into the solid media, and some proportion of the mycelia could be present during the whole life cycle (Flärdh & Buttner, 2009).Later, programmed cell death starts a degradation process where the hyphae become multinucleated (Manteca et al., 2005).At this stage, the secondary metabolites are generated, and the aerial mycelia appear.This process is followed by a second programmed cell death event where spore formation occurs (Manteca et al., 2008).If suitable conditions exist to promote the germination of the spores, the cycle starts again (Barbuto Ferraiuolo et al., 2021).When Streptomyces are grown in broth cultures, spore germination and mycelium development are similar to growth on solid media; however, differences in morphological characteristics like the propensity for clumping, pellet formation, or having dispersed mycelia can result (Lajtai-Szabó et al., 2022).

STR E PTO M YC ES SPP. ENZ YMES WITH UTILIT Y FOR LIGNIN POLYMER MODIFICATION AND DYE DECOLORIZ ATION AND DEGR ADATION APPLICATIONS
Most lignin degradation studies focus on fungal enzymes, suggesting that fungal lignin metabolism is more efficient for lignin degradation than in bacteria (Liu et al., 2022).Fungal species that have been reported to possess lignin depolymerization properties are white-rot and brown-rot members, where lignin peroxidase (LiP), manganese peroxidase (MnP), versatile peroxidase (VP), dye-decolorizing peroxidases (DyP) and laccases (Lac) have been identified and characterized as the key enzymes for this process (Barrasa et al., 1995;de Eugenio et al., 2021;Salame et al., 2014;Salvachúa et al., 2013;Soden et al., 2002).However, the slow growth of fungi makes the process more challenging.Extracellular activity from fungi with the ability to bioremediate dye and lignocellulosic waste has been demonstrated and extensively studied (Ajaz et al., 2020;del Cerro et al., 2021;Fu & Viraraghavan, 2001;Kita et al., 2022).Challenges in improving the expression and activity of fungal enzymes have opened opportunities to investigate the possible use of oxidase-producing prokaryotic organisms for dye bioremediation (Ajaz et al., 2020).Compared to fungi, bacterial strains are easier to maintain in bioreactor systems.Also, maintaining fungal growth for extended periods (e.g.≥7 days) is unfavourable for high decolorization rates (Banat et al., 1996;Chang et al., 2004).Bacterial-based dye bioremediation applications are preferred because prokaryotes are often faster growers and can generate more biomass that can ultimately support higher functional enzyme titers (Kuhad et al., 2012).In addition, bacterial enzymes can exhibit better stability over a range of temperatures, which is important for in situ bioremediation applications (Wang et al., 2022).
Bacterial lignin degradation has not yet been explored to the degree that fungal processes have been studied, but studies have reported lignin-degrading bacteria in the following classes: Actinobacteria, Alphaproteobacteria, Bacilli and Gammaproteobacteria (Ball et al., 1989;Bugg et al., 2011;Chen et al., 2013;DeAngelis et al., 2011DeAngelis et al., , 2013;;Jiang et al., 2022;Manter et al., 2011;Mathews, Grunden, et al., 2016;Mathews, Smithson, et al., 2016;Tian et al., 2014).Genera belonging to these classes are best known for their degradation capabilities because it has been shown through metagenomic analyses and biochemical characterization that several of them possess highly conserved versions of well-characterized lignin-degrading enzymes (Tian et al., 2014).The study of enzymatic breakdown of lignin and lignocellulose has fortuitously also resulted in the identification of enzymes capable of decolorizing various chromophores such as those found in industrial dye waste streams (Wang, Li, et al., 2018;Wang, Yao et al., 2018).It is been reported that lignocellulose solubilizing microbes were also capable of the decolorization of triphenylmethane and Poly R-478 dyes (Abou-Dobara & Omar, 2015;Ball et al., 1989;Vasdev & Kuhad, 1994).The decolorization process typically occurs through the oxidation of the chromophore portions of dyes or other coloured compounds by enzymes such as laccases, azoreductases and peroxidases (Chen et al., 2003;John et al., 2020;Satheesh Babu et al., 2015).
Several research studies have shown the presence and potential of enzymes from Streptomyces spp.for lignocellulose degradation (Blánquez et al., 2017(Blánquez et al., , 2022;;Cecchini et al., 2018;Feng et al., 2015;Pinheiro et al., 2017;Riyadi et al., 2020;Takasuka et al., 2013;Ventorino et al., 2016;Wadler et al., 2022), and it has been shown that these microbes produce dye-decolorizing, detergent-stable peroxidases that could replace sodium perborate for destaining synthetic textile dyes (Rekik et al., 2015).The presence and/or detected activity of these enzymes in Streptomyces spp.supports the idea that this group should be further investigated as a good source of enzymes for lignocellulolytic-degrading and decolorization applications.

APPLICATIONS OF STR E PTO M YC ES SPP. IN TE X TILE AND PULP AND PAPER INDUSTRIES
For textile and pulp industries, microbial enzymes have proven valuable to treat water effluents or waste by-products (Rajoria & Roy, 2022).Because of these potential capabilities, researchers can use different screening methods to discover the presence of relevant enzymes from a variety of environmental samples (Parmar et al., 2022).For example, a dye decolorization screening method was initially used to identify laccases and other enzymes that can break down aromatic compounds (Glenn & Gold, 1983;Ollikka et al., 1993;Pasti & Crawford, 1991).Dye decolorization screens can employ a solid growth media format wherein the breakdown of the dye results in a distinguishable halo surrounding the microbial colony producing the enzyme(s) of interest, or the microbe can be grown in a liquid medium to which a dye has been added.Colour intensity is then quantified using a spectrophotometer (Shah et al., 2021).The dyes used in the screens (Table 1) are complex in chemical composition and utilized for screening methods for these enzymes because they mimic the structural complexity of lignocellulose compounds and can therefore indicate aromatic-degrading activity (Mathews, Grunden, et al., 2016;Mathews, Smithson, et al., 2016;Tian et al., 2014).These types of plate assays are described in recent reviews as part of the isolation methods for lignin-modifying enzymes secreted by microorganisms (Kameshwar & Qin, 2017;Kaur et al., 2022).
Development of applications using Streptomyces to biotransform recalcitrant materials is an area of significant interest.Better understanding of the metabolism of lignocellulosic materials by Streptomyces spp.has recently opened a door for the valorization of microbialgenerated products from food waste.For example in a were used to produce antifungals and biopigments from potato solid waste.Additionally, in the food waste valorization field, an immobilized polygalacturonase from S. halstedii ATCC 10897 was used in a bioreactor for the degradation of pear and cucumber residues increasing the sugar content up to 15.33 and 9.35 mg/ mL, respectively (Ramírez-Tapias et al., 2018).Another application for lignocellulosic bioconversion was developed for the generation of branched-chain fatty acids for lipid-based biofuel applications where S. lividans bioconverted sunflower stalks and rape straw residues into triacylglycerols with a yield of 19%-44% conversion (Dulermo et al., 2016).Therefore, exploring enzymes from Streptomyces spp.for potential biotechnological applications such as those discussed here represents an opportunity for both industries to treat polymeric waste material before discharge in both textile and pulp and paper industries.Additional reports and details about the application of Streptomyces spp. in these industries are presented in Table 2.

OXIDATI VE LIGNIN-DEGR ADING ENZ YMES FROM STR E PTO M YC ES SPP. FOR USE IN WASTE DETOXIFICATION
Microorganisms can be used for the detoxification (degradation, decolorization) of industrial waste streams.Figure 1 summarizes several interactions between the chemical structure of the polymer and environmental factors which can lead to downstream consequences in the environment.Microbial enzymes have been shown to degrade synthetic dyes into uncoloured compounds or mineralize them in the environment (dos Santos Bazanella et al., 2013;Khan et al., 2013;Saratale et al., 2011).The loss of colour does not necessarily mean or guarantee that complete mineralization has occurred (Albahnasawi et al., 2020;Franca et al., 2015).One possible fate is the production of amines or intermediates that need to be bioconverted by other microorganisms or methods (Figure 1; Chang et al., 2004).To remove these amines, chemical, physical and biological methods have been used.Examples include activated carbon adsorption, membrane separation, steam distillation, bacterial oxidation, chemical oxidation, electrochemical techniques and irradiation (Bhat & Gogate, 2021;Klibanov & Morris, 1981;Reynolds et al., 2016).Biological methods have been receiving attention because of their low cost as an alternative to chemical and physical removal techniques which are expensive and require time-consuming analyses (Rathi et al., 2021).However, research in this area is considered to be in the early stages.The primary goal for implementation is to achieve high degradation rates to avoid the transfer of pollutants from the facility to the environment (Rathi et al., 2021).The ideal scenario is to have microbes with high catalytic versatility to degrade complex mixtures of dyes and that can tolerate harsh conditions such as exposure to detergents, surfactants and metals.Therefore, microbes capable of growing or surviving over a wide range of pH, temperatures and salinity can be helpful for these applications (Anjaneyulu et al., 2005).A summary of enzymes characterized from Streptomyces spp.exhibiting lignocellulosic activity and dye decolorization properties are shown in Table 3.
One of the key enzymes known to be involved in lignin metabolisms is lignin peroxidase (EC 1.11.1.14;LiP) which was first isolated from the fungus Phanerochaete chrysosporium, and it was shown that its heme cofactor is required for enzymatic oxidation of aromatic rings (Tien & Kirk, 1984).Because this type of enzyme needs peroxide for activating catalysis, it is called peroxidase.A peroxidase isoform (P3) from S. viridosporus T7A was characterized and overproduced in the presence of Acid-Precipitable Polymeric Lignin (APPL) and was demonstrated to have the ability to oxidize lignin and phenolic compounds, and because of this, it was classified as a lignin peroxidase called 'Actinomycetes lignin peroxidase' (ALiP-P3; Ramachandra et al., 1988;Spiker et al., 1992).This AliP-P3 could catalyse the oxidation of 2,4-dichlorophenol in the presence of hydrogen peroxide, which suggested its usefulness for degradation of xenobiotics (Yee & Wood, 1997).
Unfortunately, no protein or gene sequence of the lignin peroxidase was deposited which makes it difficult to compare to other studies for further enzyme characterization (de Gonzalo et al., 2016) of this enzyme (Ramachandra et al., 1988;Thomas & Crawford, 1998;Wang et al., 1990).One of the characterization studies demonstrated that the yeast Pichia pastoris can be used as the expression system for recombinant expression of the lignin peroxidase; however, the protein was not purified and biochemically characterized in the study (Thomas & Crawford, 1998).In the same publication, the researchers reported the co-expression of an endoglucanase gene, suggesting that the lignocellulosic system of S. viridosporus was chromosomally clustered (Thomas & Crawford, 1998).
Streptomyces spp.such as S. coelicolor A3 may also have a role in lignin modification since it has been shown to use grass lignocellulose as a growth substrate and for forming APPL (Majumdar et al., 2014).In light of the importance of LiP enzymes for lignin deconstruction, lignin peroxidase-like activity has been screened for in other Streptomyces sp., for example in Streptomyces sp.S6 (Riyadi et al., 2020).However, no homologues or annotated lignin peroxidases have been found in Streptomyces that appear to be similar to fungal lignin peroxidases (de Gonzalo et al., 2016;Riyadi et al., 2020).Moreover, homologues of ligninolytic peroxidases have not been extensively studied in bacteria, and no homologues have been found in lignin-degrading bacteria using gene-sequencing predictions or proteomes (Davis et al., 2013;de Gonzalo et al., 2016).Reviews about these enzymes called to attention the need for bioprospecting for lignin peroxidases in bacteria because little is known about them  (Falade et al., 2017;Lambertz et al., 2016).Recently high LiP activity (1132-2899 U/L) was detected in Vibrio strains which could serve as research models for bacterial LiP (Li et al., 2023).
The genome of Streptomyces sp.S6 encodes peroxidases with high homology to a different peroxidase family: DyP-type peroxidases (DyP); however, low activity was observed (Riyadi et al., 2020).Interestingly, in the genome of S. viridosporous T7A, an annotated gene encoding for a putative TAT-secreted DyP (Davis et al., 2013) was hypothesized to be the enzyme that had been described (de Gonzalo et al., 2016).These dye peroxidases (DyP, EC 1.11.1.19)or dye-decolorizing peroxidases were initially characterized for decolorizing industrial dyes.However, these enzymes act on various substrates, including types of lignin.They differ from typical peroxidases in their substrate preference for anthraquinone dyes and have high peroxidase activity compared to a variety of other organic compounds (Mechouche et al., 2022).Available peroxidases, also utilized for dye decolorization, are heme-containing proteins that require hydrogen peroxide (H 2 O 2 ) or organic hydroperoxides (R-OOH) to oxidize reducing substrates.The ability to oxidize various substrates makes them applicable for multiple biological processes such as dye decolorization and oxidation of small phenolic compounds including molecules that mimic lignin (Chen & Li, 2016).Peroxidase-based technology systems are catalytically active at acidic pH from 4 to 6, a limitation for incorporating peroxidases as biocatalysts in detergent formulations (Rekik et al., 2015).The current status regarding the role of bacterial DyP-type peroxidases in lignin degradation is still under discussion and has recently been reviewed (Sugano & Yoshida, 2021).Interestingly, DyP-type peroxidase expression has been shown to play a role in life cycle control in Streptomyces (Sugano & Yoshida, 2021), and it was demonstrated to participate in switching between vegetative mycelium and aerial hyphae in S. lividans (Chaplin et al., 2015).
Another important oxidative enzyme involved in lignin modification and dye decolorization is laccases (EC 1.10.3.2), which were first discovered in the sap of the Japanese lacquer tree (Hoegger et al., 2006;Yoshida, 1883), and are a type of blue polyphenol oxidase belonging to the family of blue multicopper oxidases.These enzymes are encoded in the genomes of fungi, bacteria and animals.A recent review has highlighted information about the mechanism, structure, enzymatic assays, genomic distribution, biotechnological properties and genetic engineering tools including a mention of their use in the pulp, paper, wood and dye/textile industries (Kaur et al., 2022).Laccases from Trametes versicolor, a white-rot fungi, have been demonstrated to exhibit higher activity (20 times greater) compared to Streptomyces laccases (Margot et al., 2013).However, Streptomyces laccases have shown improved stability over a broader range of temperature, alkaline conditions and salinity, which is helpful for applications in pulp and textile industries for biobleaching and decolorization, respectively (Singh et al., 2011).Several species of Streptomyces are known to express laccases that have been biochemically characterized (Fernandes et al., 2014a(Fernandes et al., , 2014b)).Some examples include a laccase from S. griseus that has an optimal pH and temperature of 6.5 and 40°C, respectively, and was shown to oxidize N,N-dimethylp-phenylenediamine sulfate (Endo et al., 2003).S. cyaneus produces a laccase with an optimal pH and temperature of 4.5 and 70°C, respectively, and reported activity with 2,2′-azino-bis(3-ethylbenzothiazoline-6-su lfonate (ABTS)) and 2,6-dimethoxyphenol (DMP; Arias et al., 2003).The S. ipomoea laccase has an optimal pH of 8 and temperature of 60°C when acting on phenolic substrates (Molina-Guijarro & Pérez, 2009), and S. sviceus laccase has an optimal pH of 9 and temperature of 60°C for reactions with DMP and guaiacol (Gunne & Urlacher, 2012).Reports of laccases associated with lignocellulose and dye deconstruction, degradation or decolorization are provided in Table 3.
Polyphenol oxidases (PPO) are members of the multicopper oxidases that share similar catalytic properties to laccases (Janusz et al., 2020).These enzymes require oxygen to catalyse the oxidation of mono-and di-phenols (Sharma & Kuhad, 2009).Interestingly, these enzymes contribute to melanin pigment formation in bacterial cells (McMahon et al., 2007;Wang et al., 2019).The production of melanin by these enzymes has been observed in Streptomyces and was correlated with the production of laccase (Claus & Decker, 2006).The activity in bacterial cells and spores suggests a protective role against environmental stress factors such as UV radiation, reactive oxygen species (ROS) and toxic heavy metals (Faccio et al., 2012).In addition, extracellular PPOs could have a role in the polymerization and detoxification of plant phenolic compounds in soil environments (Janusz et al., 2020).
Azoreductases are another type of industrially relevant enzyme that has been reported to be expressed in Streptomyces sp. in addition to laccases and peroxidases that have been described earlier.Azoreductases (EC.1.7.1.6)can be defined as oxidoreductases that are mostly known for the degradation of azo dyes by reducing the azo linkage (-N=N-), for example, the reactive dye structures presented in Table 1.These enzymes are categorized into groups according to their cofactor preference (NADH or NADPH) as their electron donor (Dong et al., 2019).A limiting factor in using these enzymes for wastewater treatment is their cofactor requirement as well as their need for redox mediators to facilitate the transfer of electrons from NAD(P) H to the coloured substrate (Mahmood et al., 2016;Verma et al., 2019).However, some research studies have shown that this problem can be overcome by adding coenzymes or integrated enzymatic systems.This was the case for the discovery of a novel azoreductase 'AzoRed2' from Streptomyces sp.S27 which was used in combination with a glucose-1-dehydrogenase from Bacillus subtilis to achieve 99% completion of the degradation of the azo dye within 120 min (Dong et al., 2019).Another study involving S. coelicolor showed the importance of the azoreductase as the main biocatalyst of decolorization, while the presence of an active DyP-type peroxidase and laccase played a role in achieving the mineralization of methylene blue in 72 h at 97.5% decolorization (Preethi & Pathy, 2020).Therefore, azoreductases from Streptomyces spp.can serve as another potential bioremediation catalyst, but they need to be more systematically explored for their use in wastewater treatment technologies.

BIOTE CHN OLO GICAL APPROACHES USING STR E PTO M YC ES SPP. FO R DECONTAMINATION IN TE X TILE AND PULP AND PAPER INDUSTRIES
Bacterial decontamination in general is a promising environment-friendly and cost-effective alternative to physiochemical methods that are commonly used (Parmar et al., 2022).The evaluation of these systems requires the analysis of degradation products or metabolites that are produced during the decontamination process.Suitable methods for these analyses can include analytical techniques such as UV-vis spectroscopy, Fourier transform infrared spectroscopy (FTIR), gas chromatography/mass spectrometry (GC/ MS), high-performance liquid chromatography (HPLC), nuclear magnetic resonance spectroscopy (NMR), among others to gain understanding as to the degree of waste compound degradation and compound fate that can be achieved using the microbial catalysts (Saratale et al., 2011).The current approaches for potential decontamination are discussed in the following sections.

Streptomyces whole-cell technologies
A whole-cell system screening process is helpful in determining if the microbe can grow and/or metabolize a specific substrate under specific conditions required for degradation processes (Joutey et al., 2013).In Streptomyces, whole-cell applications have been developed that involve native planktonic whole-cell biocatalysts, immobilized systems and recombinant whole-cell systems (Salama et al., 2022).Even if the whole-cell approach might not represent an advantage over using enzymes as catalysts in terms of efficiency, it is often more cost-effective because protein isolation and purification steps avoided (de Carvalho, 2016).
Examples of application of this whole-cell approach for Streptomyces bioconversion include the degradation of organophosphorous compounds, which are found in pesticides, using S. phaeochromogenes (Santillan et al., 2020).Another study demonstrated that Streptomyces.sp.MTCC 7546 could convert acrylonitrile into acrylic acid using both immobilized and planktonic cells (Nigam et al., 2009).The biotransformation of nitriles is important because its toxicity has been associated with cancer and respiratory and neuronal disorders (Ramteke et al., 2013).Recombinant whole cells of Streptomyces have been employed to avoid problems that can arise with the use of non-native systems (e.g. the potential for the formation of aggregated recombinant enzyme inclusions when using E.coli expression systems; Salama et al., 2022).For example, S. lividans has been extensively studied since it is readily genetically manipulated and can serve as a robust host for recombinant protein expression (Sevillano et al., 2016).In addition, other Streptomyces spp.have been studied (Hwang et al., 2021) to determine whether different genetic engineering approaches could be used to efficiently produce secondary metabolites, biosynthetic gene clusters (BGC) and recombinant proteins using genetic modification methods as described in Table 4.
To date, several Streptomyces spp.have been identified as sources of enzymes that could be used in dye and lignin degradation either as whole cells or as sources of recombinantly expressed enzymes (Kaur et al., 2022).
Vanillin bioconversion to vanillic acid was performed by whole-cell suspensions of S. viridosporus nearly achieving 96% of purity yield (Pometto & Crawford, 1983).A laccase from S. coelicolor was recombinantly expressed in S. lividans achieving a titre or 350 mg/L with demonstrated activity over a pH range of 4.0-9.0,thermostability up to 70°C, and the ability to decolorize indigo dye in the presence of a mediator system (Dubé, Shareck, Hurtubise, Beauregard, & Daneault, 2008).

Streptomyces applications
The lignin degradation process can be improved by the presence of small aromatic molecules called 'mediators'.In enzymes such as laccases, these mediators can act as electron carriers between the substrate and the enzyme to modulate the redox potential of the reaction system and expand the capacity of the laccase to oxidize structures (Roth & Spiess, 2015).The use of mediators for lignin depolymerization is important since it has been reported that without mediators, laccases can polymerize lignin from small compounds instead of operating in the depolymerization direction (Longe et al., 2018).Furthermore, the polymerization of lignin by a Streptomyces laccase has been observed (Majumdar et al., 2014), and it has been observed that a higher reduction of laccases by mediators favours depolymerization, and a lower reduction of laccases in the absence of mediators favours the repolymerization (Chan et al., 2020).
T A B L E 4 Genomic tools for cloning, assembly and modifying secondary metabolites and recombinant proteins (Updated and modified from Hwang et al., 2021).

Genomic tools
Heterologous host (Streptomyces spp.) An example of a laccase-mediator system in Streptomyces comes from Streptomyces sp.In this case, its small laccase SilA decolorized indigo carmine and diamond black using syringaldehyde as a redox mediator molecule achieving a decolorization of 83.7% and 56.4%, respectively, suggesting that this microbial laccase should be explored for the treatment of textile effluents (Lu et al., 2013).This is a particularly promising application not only because of Streptomyces sp.C1 SilA's decolorization properties but also because of its stability under high pH (up to 10) and temperatures (40-50°C; Lu et al., 2013).Several years later, the first example of the use of SilA in a textile dye decolorization application was published, demonstrating that SilA from S. ipomoneae CECT 3341 can improve the degradation of textile dyes by up to 60-fold and 20-fold, respectively.To achieve these results, a laccase-mediator system along with acetosyringone and methyl syringate was used (Blánquez et al., 2019).In addition, the same small laccase SilA was recombinantly expressed in E. coli and shown to achieve high decolorization levels (more than 80% indigo carmine and malachite green at different pH (6.8-8.0)within 2 h (Coria-Oriundo et al., 2021)).This study also revealed that β-(10-phenothyazyl)-propionic acid (PhCOOG) could be used as an efficient and affordable mediator in combination with recombinant SilA and could support the decolorization of remanzol brilliant blue R (an anthraquinone dye) by >40% and 80% decolorization of xylidine ponceau (azo dye; Coria-Oriundo et al., 2021).
For biobleaching applications, S. cyaneus 3335 was reported as a source of purified laccase able to catalyse the biobleaching of eucalyptus kraft pulps in combination with 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfo nate; ABTS) as a redox mediator (Arias et al., 2003).It was shown to achieve a decrease of 2.3 U in the kappa number, which is a measurement of potassium permanganate solution that is consumed by pulp (Li & Gellerstedt, 1997) and serves as an indication of lignin content or pulp biobleachability.A brightness increase of 2.2% was observed for this study (Arias et al., 2003).
An additional successful laccase-mediator system was developed using S. cyaneus 3335, where the decolorization and detoxification of azo dyes were achieved (Moya et al., 2010).In this case, Methyl Orange and Orange II were decolorized by 90% in the presence of the mediator acetosyringone; however, the viability analyses using Vibrio fischeri revealed that toxicity increased by 500% with Methyl Orange and 200% with Orange II after the treatment, while New Coccine and Chromotrope 2R were decolorized and decreased in toxicity by (300 and 185%, respectively, Moya et al., 2010).Based on these findings, it is important to include appropriate toxicological analyses of the treatments and chromatographic analyses for a better understanding of the process.It is also essential to highlight the importance of the cost of laccase-mediator systems.Mediators can be expensive and incompatible with industrial fermentation processes (Debnath & Saha, 2020).This is a disadvantage that needs to be addressed in future process optimization.

Streptomyces immobilized biocatalyst systems
Enzyme immobilization is also a promising method to consider for textile wastewater treatment.This has been demonstrated in pharmaceutical production (Barbuto Ferraiuolo et al., 2021).Enzyme immobilization can be achieved by any of the following: entrapment, adsorption, binding covalently, self-immobilization and encapsulation (Fernández-Fernández et al., 2013).The immobilization technique is favourable to implement since it allows high cell density in the continuous reactors; therefore, the process could be scaled up.Immobilization allows the increment of nutrient availability and consequently, the improvement of catalytic activity and substrate uptake (Rajoria & Roy, 2022).

Application of Streptomyces consortia
Most of the dye degradation studies featuring Streptomyces as a biocatalyst source describe the utility of recombinant enzymes or whole-cell catalysts in pure culture.To maximize decolorization or degradation, consortia studies have also been performed.A Streptomyces consortium was used to biodegrade Reactive Blue 222, a reactive sulfonated di azo dye (Pillai, 2018).This process enhanced the biodegradation of the dye by 89% using two different Streptomyces spp.This rationale for microbial consortium degradation of waste products has been seen using Streptomyces spp.along with yeast species to bioconvert lignocellulosic components into biofuels (Wadler et al., 2022), where it was observed that some individual strains were able to use about 40% of the soluble lignocellulosic components, while the mixed consortia increased the degradation of the soluble lignocellulosic components by up to 70%.

Bioreactors
Optimization studies using bioreactors have been employed to evaluate the proposed biodegradation of waste at an industrial scale (Barbuto Ferraiuolo et al., 2021;Buntić et al., 2016;Olmos et al., 2013).The optimization of laccase production from Actinobacteria was performed with S. psammoticus with different scale-up strategies, resulting in 215.6 U/g as the best yield in the presence of mediators to achieve decolorization of azo dyes (Niladevi et al., 2008;Niladevi & Prema, 2005).The peroxidase-like activity production of Streptomyces.sp.strain BSII#1 was scaled up to 3 L culture volumes with an airlift bioreactor achieving 4.76 U/mL in the presence of veratryl alcohol as an inducer.Bioreactor-based degradation of xylene, a toxic aromatic compound, by S. sp.AB1 was investigated, and it was shown that the biocatalyst was able to achieve high elimination levels (90%) of xylene from contaminated water (Chikhi et al., 2016).
Solid-state fermentation has been reported as an ecofriendly tool in the biopulping process as a method for biological treatment of the wheat straw using S. cyaneus (Berrocal et al., 2004).It was also shown that this microorganism was able to increase acid-soluble lignin from wood chips by reducing the energy required for the process by 24% (Hernández et al., 2005).Furthermore, Streptomyces sp.MDG147 has been employed for the valorization of soda lignin from wheat straw to produce oleogels which are used in lubricant applications (Borrero-López et al., 2018).The same strain, along with MDG301 was also used for the valorization of agricultural residues (barley and wheat straw) to generate APPL and alkali lignin, which after soda pulping exhibited adhesive properties (Blánquez et al., 2022).

CONCLUSIONS AND FUTURE DIRECTIONS
The textile and paper production can generate hazardous waste containing dyes from the industrial processing of synthetic and natural polymers.Therefore, safe degradation or decolorization applicable to industrial residues is needed (Saxena & Bharagava, 2020).Streptomyces spp.offer a valuable source for generating effective enzyme formulations and the development of large-scale methods for microbial-based waste treatment.Identifying methods as novel alternatives requires expanding the available enzymatic repertoire and understanding the mechanisms for degradation and bioconversion.Improvements in existing and forthcoming technologies can be realized through rigorous biochemical characterization of enzymes of interest, expansion of the enzymatic repertoire for biodegradation, and through evaluation of application scalability and reproducibility.
Immobilized Streptomyces enzymes are promising for large-scale waste treatment.However, research is needed to apply those technologies to other biotechnological processes.To do so, it is necessary to understand the microbial mechanisms employed by the potential biocatalysts for waste compound degradation.Metabolomic and biochemical insights generated from studies of degradation, decolorization and bioconversion need to be coupled to better understanding of the microbial metabolism that controls the biochemical processes.A way to advance a systematic understanding of the underlying microbial metabolic processes is through the implementation of functional -omics (e.g.transcriptomics, metatranscriptomics, genomics, proteomics and metagenomics) in conjunction with the waste compound degradation studies.These analysis approaches could be applied to the examination of pure cultures as well as when evaluating consortia.In addition, the design of mutagenesis experiments including genome editing and directed evolution could add to the understanding of the physiology and mechanisms of biodegradation, which is key to improving the application development for scalable use in both industries.
A strong case has been made that Streptomyces are suitable sources of enzymes that could be used for waste treatment.The main challenge remains in the development of large-scale production methods which can be achieved through a combination of strategic genetic engineering of Streptomyces biocatalysts to improve enzyme expression, activity, and robustness and by using our growing understanding of Streptomyces physiology to enhance growth and biocatalyst performance.

T A B L E 1
Dyes of environmental concern in the textile industry and examples of Streptomyces spp. that have been involved in the treatment of these dyes.by Schalchli et al. (2021) Streptomyces spp.
Mara F. Cuebas-Irizarry: Conceptualization (equal); writing -original draft (equal).Amy M. Grunden: Conceptualization (equal); project administration (equal); resources (lead); supervision (lead); writingreview and editing (lead).A C K N O W L E D G E M E N T SThis work was supported by Hanes Brands Inc. and North Carolina State University's Department of Plant and Microbial Biology (MRA-20170213) and U.S. Department of Agriculture.C O N F L I C T O F I N T E R E S T S TAT E M E N TMara F. Cuebas-Irizarry declares that she has no conflict of interest.Amy M. Grunden declares that she has no conflict of interest.
review article does not contain studies with human participants or animals conducted by any of the authors.O R C I D Mara F. Cuebas-Irizarry https://orcid.org/0000-0003-0751-7750 Amy M. Grunden https://orcid.org/0000-0002-8025-753X R E F E R E N C E S

Dye type Commercial name Material dye is applied to Structure Strains involved in decontamination References
T A B L E 1 (Continued)T A B L E 2 Examples of the applied technologies using Streptomyces spp. in pulp, paper and textile industries.