Biodegradation of Microplastics by Filamentous Fungi: A Novel Approach for Polymer Remediation

Abstract

Microplastic pollution has become a significant environmental concern due to its persistence and widespread impact across ecosystems. These plastic particles (1 μm to 5 mm), originating from larger plastic debris or industrial sources, accumulate in diverse habitats, affecting biodiversity and human health. Microplastics resist natural degradation, posing challenges to both ecological sustainability and waste management strategies. Although numerous studies have explored microbial degradation, most existing research focuses primarily on bacteria, leaving the role of filamentous fungi comparatively underexplored. This represents a significant research gap, because fungi secrete a variety of extracellular enzymes, including laccases, peroxidases, and esterases, which play crucial roles in the breakdown of synthetic polymers. These enzymes facilitate the depolymerization of microplastics by targeting polymer chains and increasing their susceptibility to further microbial degradation. However, the underlying enzymatic mechanisms and their effectiveness in microplastic remediation remain insufficiently characterized. Here, we critically review the potential of filamentous fungi for microplastic biodegradation, emphasizing their oxidative and hydrolytic enzyme systems, biosurfactant production, and mechanisms of adsorption and mineralization. The novelty of this review lies in consolidating the most recent mechanistic insights into fungal-driven depolymerization pathways, integrating them with advances in genetic engineering, bioprocess scale-up, and regulatory perspectives, areas rarely combined in previous reviews. We identify current limitations related to environmental applicability, enzyme accessibility, and the lack of standardized protocols, and propose strategies to overcome these challenges through enzyme immobilization, microbial consortia design, and synthetic biology approaches. By addressing these gaps, filamentous fungi may contribute to the development of sustainable strategies for plastic pollution mitigation and support circular economy approaches toward polymer biodegradation.

1. Introduction

Microplastic pollution has emerged as one of our most pressing environmental concerns due to its widespread distribution and persistence in ecosystems [1]. These small particles, in the range of 1 μm to 5 mm, originate from the degradation of larger plastic debris or primary sources such as microbeads used in cosmetics and personal care products [2,3]. Once released into the environment, microplastics resist natural degradation processes, accumulating in soils, rivers, oceans, and even living organisms [4]. Microplastics originate from the fragmentation of larger plastic materials, which means that their environmental persistence is directly linked to the inherent resistance of synthetic polymers to biological degradation. Therefore, understanding the ability of microorganisms to degrade artificial polymers is fundamental not only for the treatment of macroplastics waste, but also for preventing the continuous formation and accumulation of microplastics in natural and engineered environments [5]. In this context, microplastic removal can be considered a downstream application of microbial polymer degradation, as the same enzymatic mechanisms that act on bulk plastics are responsible for transforming microplastic particles [6]. Studies suggest that their presence poses significant threats to biodiversity, human health, and ecological balance, exacerbated by their ability to adsorb chemical pollutants and be ingested by aquatic organisms [7].

On a global scale, the magnitude of microplastic pollution has reached alarming levels. It is estimated that more than 11 million metric tons of plastic waste enter aquatic environments every year, and this figure could triple by 2040 if current production and waste management practices remain unchanged [8,9]. Microplastics have now been detected in all major ecosystems, from the deep ocean and polar regions to mountain soil and atmospheric dust [10,11,12,13]. Recent meta-analyses report that an alarming percentage of marine species are exposed to plastic debris at some stage of their life cycle [14,15,16], and microplastics have been found in drinking water, table salt, and even human blood and placenta, underscoring the global dimension of exposure [17,18,19]. Economically, plastic pollution imposes losses exceeding USD 13 billion per year on marine industries, particularly fisheries and tourism [9]. These findings highlight the transboundary nature of microplastic contamination and the urgent need for biotechnological solutions capable of mitigating its persistence at a planetary scale.

In this context, the search for sustainable strategies to address plastic pollution, including both macroplastics and microplastics, has gained significant attention. Among these, biodegradation has been explored as a complementary approach due to its potential for the complete mineralization of polymers into harmless end products, low energy requirements, and applicability under environmentally relevant conditions [20]. This process involves microorganisms capable of metabolizing plastic polymers and breaking them down into simpler molecules such as carbon dioxide, water, and biomass [21]. However, the application of microbial biodegradation for the direct removal of microplastics in natural environments remains challenging, as degradation rates are often slow and strongly dependent on environmental conditions, enzyme accessibility, and polymer characteristics. Therefore, biodegradation is currently considered more feasible in controlled systems or as a preventive strategy to reduce the accumulation and persistence of plastic materials before their fragmentation into microplastics. Microbial degradation of artificial polymers represents a key strategy for reducing plastic accumulation at its source, limiting the formation of secondary microplastics and potentially contributing to the transformation of microplastic particles under controlled or favorable conditions [22,23]. However, its effectiveness depends on the identification and development of specialized microorganisms and a deeper understanding of the underlying degradation mechanisms [23].

Among the microorganisms, filamentous fungi have shown unique advantages over bacteria in plastic degradation, with remarkable potential due to their metabolic versatility and widespread presence in natural and anthropogenic environments. Their ability to degrade complex artificial polymers is particularly relevant for both macroplastic treatment and microplastic removal, as fungal enzymatic systems can act on polymer surfaces at different scales, from bulk plastic materials to fragmented microplastic particles [24]. Fungi are renowned for secreting diverse extracellular enzymes such as laccases, peroxidases, cutinases, and esterases, capable of degrading recalcitrant compounds, such as lignin, cellulose, and, more recently, synthetic polymers. These enzymes act synergistically and can penetrate solid substrates through extensive mycelial networks [25]. In contrast, bacterial systems often rely on single-enzyme mechanisms and limited surface colonization capacity [26]. For example, representatives of genera such as Aspergillus, Penicillium, and Trichoderma have demonstrated significant potential to degrade microplastics through the production of specific enzymes or direct interactions with plastic particles [27,28,29], while bacterial strains typically require prolonged adaptation or pretreatment to achieve similar results [30].

Fungal mycelia also confer mechanical stability in biofilm formation, facilitating persistent contact with hydrophobic polymer surfaces [31]. These physiological and enzymatic attributes make fungi particularly promising biocatalysts for complex, recalcitrant polymers where bacterial systems show limited performance. Furthermore, their ease of cultivation and adaptability to industrial conditions enhance their biotechnological relevance [32,33].

This review aims to explore the potential of filamentous fungi in the degradation of artificial polymers, with a particular focus on microplastics as an environmentally relevant application, focusing on the mechanisms involved, the conditions that enhance their activity, and the challenges and limitations for large-scale applications. Additionally, the analytical tools employed in these studies and the prospects for integrating these organisms into environmental bioremediation strategies will be discussed.

Unlike previous reviews that broadly address microbial degradation or focus predominantly on bacterial systems, the novelty of this study lies in providing a targeted and critical synthesis of fungal-based approaches, with special attention to enzymatic pathways and environmental compatibility. The review is limited in scope to filamentous fungi and does not cover bacterial degradation, photodegradation, or physicochemical treatments to deepen the analysis on fungal-specific mechanisms and technological readiness. Its novel contribution lies in consolidating recent literature on the fungal biodegradation of artificial polymers, with a specific focus on microplastics as a critical environmental challenge, highlighting the gaps between laboratory results and environmental implementation, and critically evaluating the realistic scalability of fungal strategies within current environmental and regulatory constraints.

2. Methods

This narrative review was performed following three steps: conducting the search, reviewing abstracts and full texts, and discussing the results. For this, the PubMed, Scopus, Science Direct, Web of Science, and Google Scholar databases were searched to identify relevant studies, according to the development of the review. The final search was conducted in February 2026 and included international English-language articles, online reports, and electronic books. The keyword “microplastics” and “filamentous fungi” were used in combination with other terms such as biodegradation, remediation, environmental impacts, biomass, adsorption, hydrolytic and oxidative enzymes, biosurfactants, mineralization, microbial consortia, challenges, or regulation. After the complete search, the abstracts were read to ensure that they addressed the topic of interest. All duplicates were removed, and the abstracts of the remaining articles were reviewed to ensure that they addressed the inclusion criteria of the review. The eligible criteria were studies that analyzed microplastics and filamentous fungi in combination with the other terms mentioned above. Therefore, the studies of interest were summarized and synthesized to integrate the narrative review. Since it is a narrative review, it was not necessary to document the literature search on specific platforms [34].

3. Microplastics: Definition and Environmental Impacts

Microplastics are plastic particles in the range of 1 μm to 5 mm that are recognized as persistent and ubiquitous pollutants in terrestrial and aquatic environments [2,35]. They originate from both primary and secondary sources [36]. Primary sources of microplastics include microbeads used in cosmetic and personal care products, such as facial scrubs and toothpaste, as well as plastic pellets used in industrial manufacturing, and synthetic fibers released during laundry. These particles are directly released into the environment through sewage systems or improper disposal [1,3]. Secondary sources, on the other hand, result from the fragmentation of larger plastics due to exposure to environmental factors such as ultraviolet light, mechanical abrasion, and biological activity. Common examples include plastic bags, fishing nets, and vehicle tires. These fragmented microplastics exhibit more significant heterogeneity in terms of shape and composition [37,38].

Microplastics are composed of a wide range of synthetic polymers, with common examples including polyethylene (PE), polypropylene (PP), polystyrene (PS), polyurethane (PU), polyamides (nylon), polyethylene terephthalate (PET), and polyvinyl chloride (PVC), among many others used in packaging, textiles, construction materials, and consumer products [39]. Each polymer type presents specific physicochemical characteristics, such as density, thermal resistance, and chemical properties, which determine its degradation in the environment (

Table 1. Characteristics of synthetic polymers found in microplastics.

Synthetic Polymer	Chemical Structure	Common Use	Density (g/cm3)	Thermal Resistance (°C)	Chemical Properties	References
Polyethylene (PE)		Plastic bags, films, containers, pipes	0.91–0.96 (Low)	80–120	Hydrophobic, chemically inert, resistant to most acids and bases, degrades under UV. Classified into four types: High-Density Polyethylene (HDPE), Low-Density Polyethylene (LDPE), Linear Low-Density Polyethylene (LLDPE), and Ultra High Molecular Weight Polyethylene (UHMWPE)	[40,41]
Polypropylene (PP)		Food containers, caps, textiles, packaging	0.90–0.92 (Low)	130–170	Low density, hydrophobic, resistant to chemicals and fatigue, susceptible to UV damage	[42,43]
Polystyrene (PS)		Disposable cups, insulation, packaging materials	1.04–1.07 (Intermediate)	100–120	Brittle, hydrophobic, can leach styrene monomers, degraded by strong acids and bases	[44,45]
Polyurethane (PU)		Foams, coatings, adhesives, elastomers	1.20–1.30 (Intermediate)	150–200	Highly versatile, resistant to abrasion and oil, chemically complex, partly biodegradable	[46,47]
Polyamides (Nylon)		Textiles, fishing nets, carpets	1.13–1.15 (Intermediate)	160–260	High strength, hydrophilic, chemically resistant to oils and solvents, absorbs water	[48,49]
Polyethylene Terephthalate (PET)		Bottles, food packaging, films, synthetic textiles (polyester fabrics), clothing fibers	1.34–1.39 (High)	120–150	Transparent, resistant to moisture, chemical degradation, can adsorb organic pollutants	[50]
Polyvinyl Chloride (PVC)		Pipes, toys, construction materials, coatings	1.30–1.45 (High)	70–100	High density, resistant to fire, chemically stable, releases toxins when incinerated	[51]

).

For instance, PE and PP, commonly used in packaging and disposable products, are less dense than water and tend to float in aquatic environments [52], whereas PET, widely used in bottles and synthetic textiles (polyester fabrics), tends to sink due to its higher density [53,54]. Textile-derived microplastics are particularly relevant, as a large proportion of synthetic fibers released during washing processes originate from polyester-based materials, which are chemically classified as PET [54]. In addition to polymers, microplastics may contain chemical additives like plasticizers, stabilizers, and dyes. These compounds, added during manufacturing, can leach into the environment during plastic degradation, exacerbating ecological and human health risks [55].

The growing accumulation of microplastics raises severe concerns about their ecological and toxicological impacts. Due to their resistance to degradation, they can persist in the environment for hundreds or even thousands of years [56]. Microplastics are distributed across all ecosystems, from agricultural soils and marine sediments to atmospheric air [57]. Their presence in aquatic environments is particularly concerning, as rivers and oceans act as convergence points for particles transported by surface runoff, drainage systems, and industrial emissions. Microplastics can affect marine fauna when mistaken for food by organisms such as fish, crustaceans, and birds, leading to bioaccumulation and biomagnification of toxic substances within the food chain [35]. The ingestion of microplastics can cause blockages in the digestive system, reduced food intake, and, in some cases, death due to intestinal obstruction [58]. Furthermore, microplastics serve as vectors for chemical contaminants and pathogens. They can adsorb toxic compounds present in water, such as pesticides, heavy metals, and hydrocarbons. When ingested by organisms, these contaminants can be transferred, amplifying ecological and human health impacts [59].

One of the most serious toxicological concerns related to microplastics is the potential for endocrine disruption [60]. Many microplastics contain additives such as phthalates and bisphenol A (BPA), chemicals known for their ability to interfere with hormonal systems [61]. These additives can disrupt normal hormonal regulation, affecting reproductive, behavioral, and metabolic functions [62,63]. In marine organisms, these disruptions can lead to reproductive issues, such as reduced fertility, changes in growth rates, and even failure in larval development. In some cases, the effects can be so severe that they result in a decrease in a species’ population [64]. For humans, the ingestion of microplastic-contaminated foods poses a significant risk of exposure to these endocrine disruptors, with potential long-term health implications [65].

Direct ingestion of microplastics can cause inflammation in organisms. Non-degradable microplastics can accumulate in the gastrointestinal tract and tissues, triggering an inflammatory response [66]. Additionally, the abrasiveness of microplastics can cause cellular and tissue damage, resulting in necrosis (unprogrammed cell death) or apoptosis (programmed cell death) [67]. This inflammation can affect the immune system, making organisms more susceptible to infections and other diseases [68].

The persistence, widespread distribution, and environmental interactions make microplastics an urgent issue for research and intervention. A detailed understanding of their physical and chemical characteristics and sources is essential for developing effective mitigation and remediation strategies [69].

The environmental fate and degradability of microplastics are strongly influenced by the intrinsic physicochemical properties of their constituent polymers. Factors such as molecular weight, crystallinity, functional groups, cross-linking degree, and hydrophobicity determine how these materials interact with environmental agents and microorganisms [70,71]. For instance, highly crystalline polymers like PET resist enzymatic attack due to their compact molecular packing [72,73], whereas amorphous polymers are more accessible to microbial enzymes [74]. Similarly, the absence of hydrolyzable bonds in polymers with carbon–carbon backbones, such as PE and PP, makes them far more resistant to biodegradation compared to polyesters or polyurethanes [74]. The hydrophobic nature of most synthetic plastics also hinders microbial colonization and enzymatic activity in aqueous environments, prolonging their persistence [75]. These structural attributes not only control the degradation kinetics, but also influence the transport, fragmentation, and bioaccumulation of microplastics across ecosystems. A more detailed discussion of how polymer chemistry affects biodegradability is presented in Section 4.

4. Filamentous Fungi as Biodegrading Agents of Plastics and Microplastics

Filamentous fungi are eukaryotic organisms widely distributed in the environment, characterized by their ability to form multicellular structures called hyphae, which organize into a network known as mycelium [76]. They belong to several phyla within the Fungi kingdom, including Ascomycota and Basidiomycota, the most studied [77]. The structural and functional characteristics make filamentous fungi highly adaptable to a wide variety of ecological niches, playing key roles in biogeochemical cycles, organic matter decomposition, and interactions with other organisms [78].

The hyphae of filamentous fungi are tubular structures that can be septate (with transverse divisions called septa) or coenocytic (without septa, forming a continuous multinucleated cell) [79]. The hyphal tip, known as the apex, is the site of active growth, where new cells are added, and extracellular enzymes are secreted [80]. These enzymes are crucial for the extracellular digestion of substrates, enabling filamentous fungi to derive nutrients from complex sources such as lignin, cellulose, and, more recently recognized, microplastics [28,81,82].

The mycelium, formed by the branching and interconnection of hyphae, provides a large surface area for contact with the environment, allowing for the efficient absorption of nutrients and water. Additionally, the organization of the mycelium can vary depending on the substrate and environmental conditions, contributing to filamentous fungi’s adaptability [83].

A hallmark of filamentous fungi is their metabolic versatility. They can produce a wide range of enzymes, including cellulases, ligninases, lipases, and proteases, which facilitate the degradation of a vast array of organic and inorganic compounds [84,85,86]. This capability is particularly relevant in the context of bioremediation [86,87,88]. Moreover, filamentous fungi can produce secondary metabolites, such as organic acids, that contribute to the chemical modification of the environment, making it more conducive to the degradation of recalcitrant materials [89].

Filamentous fungi are found in all habitats, from soils and aquatic environments to extreme conditions such as deserts, polar regions, and zones with high pollution levels [90]. Their ability to colonize surfaces and survive under adverse conditions is due to several adaptations such as: (i) production of resistant spores, such as conidia and sporangiospores, which facilitate dispersion and survival in unfavorable conditions; (ii) tolerance to variations in pH, temperature, and nutrient availability, enabling their activity in both natural and industrial environments; (iii) ability to form biofilms, which protect against physical and chemical stresses [91,92,93].

Filamentous fungi are ecologically essential as decomposers, nutrient recyclers, and symbionts in mycorrhizal or endophytic associations [94]. In biotechnological terms, their ability to produce enzymes, bioactive metabolites, and biomass makes them promising in fields such as: bioremediation, including the degradation of organic and inorganic pollutants; production of food, medicines, and biofuels; and development of bioplastics and other sustainable materials [86,87,88].

For instance, Trametes spp. and Pleurotus spp. are well-known for their ability to degrade synthetic dyes through the secretion of laccases and peroxidases, achieving over 90% color removal in textile effluents [32,87,88]. Aspergillus spp. and Penicillium spp. have shown potential in the biodegradation of hydrocarbons and petroleum derivatives, metabolizing compounds such as n-alkanes, toluene, and phenanthrene [95,96]. Certain Geotrichum and Trichoderma strains can also bioaccumulate or transform heavy metals (e.g., cadmium, chromium, and lead) via chelation or biosorption mechanisms [97,98]. Additionally, Ganoderma lucidum and Trametes versicolor have been reported to degrade pesticides and endocrine-disrupting compounds, reducing their toxicity and persistence [62,63].

These examples highlight the broad bioremediation capacity of filamentous fungi, encompassing organic pollutants, heavy metals, and xenobiotic compounds. Their success arises from their extracellular enzymatic machinery, production of biosurfactants and organic acids, and ability to colonize solid substrates through extensive mycelial growth. This combination of metabolic and structural features positions filamentous fungi as powerful biodegrading agents for a wide spectrum of contaminants.

Among the different contaminants that filamentous fungi can degrade, synthetic polymers and plastic-derived materials have gained increasing attention in recent years. The same enzymatic systems involved in lignin degradation, dye removal, and hydrocarbon metabolism are also capable of attacking complex plastic polymers, promoting the oxidative and hydrolytic breakdown of polymer chains [82,99,100]. This functional similarity explains why filamentous fungi are now considered promising biological agents for plastic and microplastic degradation. In this context, understanding how filamentous fungi interact with plastic materials and the mechanisms involved in polymer degradation is essential to evaluate their real potential in microplastic mitigation.

In plastics and microplastics degradation, the combination of their structural, metabolic, and adaptive properties suggests that filamentous fungi could play an innovative role in mitigating environmental pollution caused by plastics, as reported in

Table 2. Taxonomic diversity and mechanisms of filamentous fungi in microplastic biodegradation.

Filamentous Fungus	Microplastic Degraded	Degradation Mechanism	% Mass Loss or CO2 Evolution *	Additional Information	Reference
Agrocybe aegerita and Ganoderma lucidum	LDPE	Evidence of mycelium penetrating the polymer and evidence of oxidation	-	Pre-treatment by blending the plastic with fatty acids	[101]
Alternaria alternata	PE	Colonization/erosion, depolymerization, assimilation and mineralization	-	PE polymer was degraded into small fragments and converted to energy and carbon source	[102]
Alternaria sp. and Trametes sp.	LDPE	Formation of alcohols, ethers, acids, and esters during the degradation process	-	The combined fungal treatment is more effective at degrading LDPE than the single strain treatment	[103]
Aspergillus flavus	HDPE	Two laccase-like multicopper oxidase (LMCOs) genes displayed up-regulated expression during the degradation process, which may be the candidate PE-degrading enzymes	3.9025 ± 1.18%	HDPE was degraded after 28 days incubation	[104]
Aspergillus niger	PE, PET, PS	Surface disintegration, biocatalytic degradation, and physical and chemical modification of treated polymers by lipase	-	Application of wheat bran made the procedure economical	[105]
Aspergillus oryzae	PET	The hydrophobin RolA enhanced PET hydrolysis in the presence of the recombinant PETase	-	The hydrolysis of PET bottle by RolA-PETase achieved the highest weight loss of 26% in 4 days	[106]
Bjerkandera adusta	HDPE	Secretes laccases	-	Performs better under lignocellulose substrate treatment	[107]
Chaetomium globosum	PVC	Changes in color and significant mass loss	9%	Biodegradation of the samples subjected to biotreatments was dependent on the contact and adhesion of the fungus to the surface of the polymer	[108]
Cladosporium cladosporioides	LDPE	Small cavities and depressed areas of circular shape were visible in the treated samples	-	Significant decrease in the relative intensity of the methylene group bands	[109]
Cochliobolus sp.	PVC	Degrades low molecular weight polyvinyl chloride (PVC) by the enzyme laccase	-	Fungal treatment technology has the potential to be used in the full-scale biodegradation of plastic contaminated sites	[110]
Fusarium solani	PBAT	Produces cutinases and biofilms, enhancing adhesion and enzymatic degradation	-	Effectively degraded the amorphous and crystalline regions of PBAT, and the crystalline regions were transformed into amorphous regions with the increase in degradation time	[111]
Humicola insolens	PET	Produces cutinases specific to polyesters	-	Cutinases not to be inhibited by any of the main PET hydrolysis products such as terephthalic acid (TPA), mono-(2-hydroxyethyl) terephthalate (MHET), and bis-(2-hydroxyethyl) terephthalate (BHET)	[112]
Lasiodiplodia theobromae	PP	Produces laccases	-	Higher doses of Gamma rays can increase the sensitivity of plastics toward microorganisms	[113]
Mixed culture of Aspergillus carbonarius and Aspergillus fumigatus	LDPE	Produces oxidase enzymes targeting polymer backbones	~5%	Performs better with thermal pretreatment	[114]
Phanerochaete chrysosporium	PVC	Higher activity of hydrolases (esterases, lipases, and proteases) in the presence of PVC	-	Blending of recycled polylactide/poly(butylene terephthalate-co-butylene sebacate) (rPLA/PBTSe) with PVC resulted in a change in the profile of enzymes secreted by fungi	[115]
Penicillium citrinum	LDPE	Produces laccase, lipase, esterase, and manganese peroxidase	38.82 ± 1.08% before nitric acid pretreatment 47.22 ± 2.04% after nitric acid pretreatment	Nitric acid pretreatment was performed to improve the degradation capacity	[116]
Pleurotus ostreatus	LDPE	Produces laccases and peroxidases; catalyzes oxidative and enzymatic degradation of complex polymers	CO2 production of 2323 mg Kg−1	Commonly studied for plastic and lignin breakdown	[117]
Pleurotus ostreatus	LDPE	LDPE degradation is initiated by laccase (Lac) followed by lignin peroxidase (LiP), whereas manganese peroxidase (MnP) and unspecific peroxygenase (UP) are involved in the final degradation process	-	The biodegradation of LDPE proceeded faster in recycled than in unused samples, which can be enhanced by exposing LDPE to UV radiation	[118]
Rhizopus arrhizus	LDPE	Highlight alterations of LDPE films through cracks, veins, and holes	23.77%	Utilize LDPE as a sole carbon source in batch (α-LDPE) and continuous (γ-LDPE) cultures	[119]
Schizophyllum commune	LDPE	Produces laccases; accelerates bond oxidation in polymer backbones	9.65 ± 1.52%	Promising results in degrading LDPE sheets under in vitro conditions	[120]

LDPE: low-density polyethylene; PE: polyethylene; HDPE: high-density polyethylene; PET: polyethylene terephthalate; PS: polystyrene; PVC: polyvinyl chloride; PBAT: poly(butylene adipate-co-terephthalate); PP: polypropylene. * In some works, this information was not available.

.

The process primarily involves oxidative and hydrolytic mechanisms, which break down polymer chains, facilitating the depolymerization of plastics. Notably, basidiomycetes like Ganoderma lucidum and Pleurotus ostreatus demonstrate strong potential due to their production of peroxidases and laccases, which are particularly effective in oxidizing complex polymer structures [101,118]. This diversity in fungal species and degradation strategies suggests that filamentous fungi could be key players in the biodegradation of microplastic pollution, offering a more sustainable alternative to chemical degradation methods [104,105,108].

The degradation of microplastics by filamentous fungi is influenced by several environmental conditions, which can either promote or hinder the biodegradation process. These conditions are important in determining the efficiency of fungal degradation and involve factors such as temperature, moisture, pH, nutrient availability, oxygen levels, and UV radiation and light (Figure 1) [121,122].

Fungal interactions with other microorganisms, such as bacteria, can also impact plastic degradation. Symbiotic relationships or microbial consortia can sometimes enhance the degradation rate, as some bacteria may produce enzymes that complement fungal degradation processes. The presence of bacteria capable of breaking down plastic components could facilitate fungal access to these polymers, thus speeding up the degradation process [5]. For instance, a study by Salinas et al. [123] enhanced consortia’s linear LDPE plastic degradation capabilities between Fusarium oxysporum and Bacillus subtilis compared to individual strains.

Overall, the combination of enzymatic diversity, structural adaptability, and ecological resilience makes filamentous fungi key players in the biological degradation of plastics, offering a sustainable alternative to physicochemical degradation methods and aligning with circular economy principles.

5. Mechanisms of Biodegradation

5.1. Microplastic Adsorption by Fungal Biomass

The adsorption of microplastics onto fungal biomass is an essential initial step in the fungal biodegradation process, as it facilitates the interaction between the plastic particles and the fungal enzymes. Filamentous fungi are well-suited for this process due to their robust cell wall structures, high surface area, and ability to interact with both hydrophobic and hydrophilic materials [25,28].

The fungal cell wall is composed primarily of chitin, glucans, and proteins, which provides a versatile surface capable of interacting with microplastic particles [24]. The β-glucans contain hydrophilic and hydrophobic domains, allowing them to bind to several types of plastics [124]. In addition, specific proteins embedded in the fungal cell wall, such as hydrophobins, enhance the hydrophobic interaction between fungal biomass and microplastics, particularly those made of hydrophobic polymers like PE and PP [106,125,126].

Microplastic adsorption occurs through a combination of physicochemical and structural mechanisms [127]. Surface charges generated by additives or environmental weathering allow electrostatic interactions with fungal cell wall components, strengthening particle adhesion [128]. At the same time, the filamentous mycelial network acts as a physical trapping system, where hyphae entangle microplastic particles and maintain close contact with enzymatic secretions [129,130]. This combined mechanism enhances microplastic retention and supports subsequent biodegradation processes.

5.2. Role of Hydrolytic and Oxidative Enzymes

Hydrolytic and oxidative enzymes play a central role in the fungal degradation of microplastics, acting after the initial adsorption of plastic particles onto fungal biomass. Once microplastics are immobilized on the fungal surface, extracellular enzymes can interact with the polymer structure, promoting chemical modifications, and in some cases, polymer breakdown [131].

Enzymatic activity generally involves two distinct but complementary stages: surface modification and polymer degradation [131]. In the first stage, oxidative enzymes introduce functional groups into the polymer structure, increasing surface reactivity and facilitating subsequent enzymatic action. This modification does not necessarily lead to immediate degradation but enhances the accessibility of polymer chains to catalytic processes [132]. In the second stage, hydrolytic enzymes act on susceptible chemical bonds, promoting chain scission and the formation of smaller molecules that can be assimilated or further mineralized [129].

Oxidative enzymes, such as laccases and peroxidases, are particularly important in the initial transformation of microplastics. These enzymes generate reactive radicals capable of oxidizing polymer surfaces and introducing oxygen-containing functional groups, which increase polymer susceptibility to further degradation [133], while hydrolytic enzymes, including cutinases, esterases, and lipases, mainly act on polymers containing hydrolyzable bonds. These enzymes lead to polymer chain fragmentation and the formation of oligomers and monomers that can be further metabolized by fungi [129].

In

Table 3. Fungal enzymes and their role in microplastic biodegradation.

Enzyme	Description	Mechanism of Action	Application in Microplastic Biodegradation	References
Laccases	Multicopper oxidases produced by fungi, particularly white-rot fungi.	Oxidize phenolic and non-phenolic compounds using oxygen as an electron acceptor, generating reactive radicals. Mediators such as ABTS or HBT extend the activity to non-phenolic substrates	Modify and destabilize polymer surfaces, especially aromatic-based plastics like PS and PET. Effective in breaking down additives and dyes in plastics, enhancing susceptibility to further enzymatic or environmental degradation	[134,135]
Peroxidases	Oxidative enzymes such as manganese peroxidase (MnP) and lignin peroxidase (LiP).	Catalyze the oxidation of polymers through H2O2-dependent reactions, cleaving C-C and C-H bonds	Target aromatic polymers and modify surface properties of plastics like PET and PS, introducing hydrophilic groups that promote fragmentation and biodegradation	[27,135]
Esterases	Hydrolytic enzymes that cleave ester bonds in synthetic and natural polymers.	Hydrolyze ester linkages, breaking down polyesters into oligomers and monomers (e.g., terephthalic acid and ethylene glycol in PET)	Highly effective for polyesters such as PET and PU, contributing to depolymerization and enabling further microbial or chemical degradation	[27,136]
Cutinases	A subclass of esterases with broader substrate specificity produced by thermophilic fungi.	Hydrolyze ester bonds in polyesters, including both crystalline and amorphous regions	Break down PET and PU microplastics into smaller, more biodegradable molecules	[137]
Proteases	Enzymes that hydrolyze peptide bonds in proteins.	Hydrolyze protein-based additives in plastics, reducing polymer integrity and promoting depolymerization	Useful for addressing microplastics with protein-based components or additives, such as certain composites and coatings, enhancing their overall biodegradability	[21,138]
Lipases	Enzymes that hydrolyze ester bonds in lipids but show activity against specific polymers.	Target ester bonds in polyesters and lipid-based coatings on plastics, breaking down hydrophobic barriers	Contribute to the degradation of PU and lipid-coated microplastics, facilitating microbial colonization and further enzymatic breakdown	[27,139]
Cellulases	Enzymes that degrade cellulose but can aid in biofilm formation on microplastics.	Hydrolyze glycosidic bonds in cellulose, aiding fungi in adhering to microplastic surfaces and creating pathways for other enzymes	Indirectly enhance biodegradation by improving fungal attachment and colonization, enabling a coordinated attack on the plastic material	[131,140]
Ureases	Enzymes that hydrolyze urea, impacting PU-based plastics.	Catalyze the breakdown of urea linkages in PU polymers, destabilizing their structure	Specific for PU degradation, breaking down key structural elements of the polymer and enhancing its fragmentation for microbial consumption	[27,141]

, each enzyme class is explored, as well as its mechanisms and relevance to microplastic degradation. Figure 2 illustrates the enzymatic processes by which hydrolytic and oxidative enzymes contribute to microplastic modification and fragmentation.

However, challenges remain in optimizing the enzymatic degradation of microplastics, particularly regarding enzyme accessibility, environmental conditions, and industrial scalability. The crystalline structure of many synthetic polymers often limits enzymatic access to polymer chains. Thus, strategies such as surface oxidation or pretreatment with physical methods like UV radiation can enhance enzyme–polymer interactions [142]. Additionally, the optimal conditions for enzymatic activity, such as temperature and pH, often differ significantly from natural environmental settings, necessitating the development of robust enzymes capable of functioning efficiently in variable and often suboptimal conditions [90]. To transition from laboratory research to commercial applications, it is essential to scale enzymatic processes for industrial use [143]. Promising strategies include the use of immobilized enzymes, which enhance stability and reusability, and the implementation of bioreactor systems designed for large-scale plastic degradation, offering a sustainable and economically viable approach to tackling microplastic pollution [32,81,143].

5.3. Role of Biosurfactants

Fungi also produce biosurfactants, which reduce surface tension and facilitate closer contact between microplastics and fungal cells. These biosurfactants can also emulsify microplastic particles, enhancing their dispersal in the fungal growth medium. They are environmentally friendly and fully biodegradable, ensuring that their use does not contribute to additional pollution. Biosurfactants are effective even in low concentrations, making them suitable for large-scale applications. Their ability to work under a variety of environmental conditions (e.g., salinity, pH, temperature) further enhances their applicability [144]. Additionally, biosurfactants often work synergistically with fungal enzymes. Emulsifying and dispersing microplastics improves the accessibility of the enzymatic sites, accelerating the degradation process. Moreover, biosurfactants facilitate fungal colonization by modifying the microplastic surface to be more hydrophilic. This improved adhesion allows fungi to establish biofilms on the plastic surface, anchoring them for sustained enzymatic activity [145]. These synergistic mechanisms are summarized in Figure 3.

Fungi produce several classes of biosurfactants, each with unique properties contributing to microplastic interaction [146]. The most prominent types are shown in

Table 4. Fungal biosurfactants and their roles in microplastic adsorption.

Biosurfactant	Producing Fungi	Mechanisms of Action in Microplastic Adsorption	References
Sophorolipids	Candida bombicola, Candida apicola	Reduces surface tension, enhances emulsification of hydrophobic microplastics, improves polymer wettability, and facilitates fungal adhesion to plastic surfaces	[147,148]
Mannosylerythritol Lipids (MELs)	Ustilago maydis, Ustilago scitaminea	Creates stable emulsions, disperses plastic particles in aqueous media, and improves fungal colonization by modifying hydrophobic plastic surfaces	[21,149]
Rhamnolipids	Aspergillus niger Rhizopus oryzae	Reduces surface tension, increases polymer wettability, emulsifies microplastics, and desorbs surface contaminants/additives from plastic particles	[148,150]
Lipopeptides	Trichoderma harzianum, Penicillium italicum	Improves the hydrophobic interaction between fungal biomass and plastics, enhances fungal biofilm formation on the microplastic surface, and facilitates enzymatic degradation	[151,152]

.

5.4. Mineralization and Metabolic Pathways

The degradation of microplastics by filamentous fungi can be divided into two main phases: primary degradation and complete mineralization. These processes are important for understanding the overall fate of microplastics in the environment and the role of microorganisms in reducing their environmental impact. While primary degradation involves breaking down large plastic particles into smaller fragments or simpler compounds, complete mineralization involves converting these materials into inorganic end-products, such as carbon dioxide, water, and biomass [131].

Primary degradation refers to the initial breakdown of microplastic polymers into smaller fragments or oligomers. This stage typically involves partially altering the chemical structure of plastics through physical, chemical, or biological mechanisms. Physical fragmentation occurs due to environmental factors such as UV radiation, mechanical abrasion, and thermal stress, which create cracks and fractures in the polymer matrix, increasing the surface area for microbial colonization and enzymatic attack. Chemical alteration, often initiated by oxidative or hydrolytic processes, introduces functional groups such as hydroxyl, carbonyl, or carboxyl onto the polymer surface, enhancing the susceptibility of plastics to enzymatic degradation. Biological degradation involves the secretion of extracellular enzymes by microorganisms, which cleave polymer chains and generate smaller molecules like oligomers or monomers [122,131].

Complete mineralization, or ultimate degradation, involves the transformation of microplastics into their simplest inorganic forms, such as carbon dioxide (CO₂), water (H₂O), methane (CH₄, under anaerobic conditions), and microbial biomass. This stage signifies the complete removal of plastic materials from the environment. However, it is a slower process than primary degradation, requiring extensive enzymatic and metabolic effort. Biochemical pathways play a fundamental role in this process, as fungi and other microorganisms metabolize simpler compounds through enzymatic activity, with pathways varying according to polymer type and environmental conditions. The mineralization process contributes to the carbon cycle by incorporating degraded polymer carbon into microbial biomass or releasing it as CO₂ through cellular respiration. The synergistic action of fungi and bacteria is also significant, as fungi excel at breaking down complex polymers, while bacteria can metabolize smaller intermediates and complete the mineralization process [153,154].

6. Factors Affecting Plastic Degradation by Filamentous Fungi

The biodegradation of plastics by filamentous fungi is a multifactorial process influenced by both polymer-related structural properties and environmental or biological conditions that determine fungal activity. Understanding these parameters is essential to optimize degradation rates and predict the environmental fate of microplastics under natural or laboratory settings.

One of the main factors that significantly impacts the degradation behavior is the chemical structure of synthetic polymers, particularly the rate and mechanisms by which they break down under environmental or biological conditions. Synthetic polymers, the primary constituents of microplastics, are designed to be durable and resistant to natural decomposition processes, posing significant challenges for degradation. Their molecular architecture, functional groups, crystallinity, and degree of polymerization all play critical roles in determining how these materials interact with physical, chemical, and biological agents [155].

In general, polymers with high molecular weights and long-chain structures tend to be more resistant to degradation due to reduced chain mobility and limited accessibility to degrading agents, although this behavior can vary depending on polymer crystallinity, functional groups, and environmental conditions. Higher degrees of polymerization mean that the polymer chains are more tightly packed and less likely to be attacked by hydrolytic or oxidative enzymes. For instance, PE and PP, which consist of long hydrocarbon chains, exhibit significant resistance to microbial degradation due to their inert chemical structure and lack of hydrolyzable bonds [156,157].

The degree of crystallinity in a polymer refers to the arrangement of polymer chains in an ordered, tightly packed structure. Polymers with high crystallinity, such as PET and nylon, have regions highly resistant to enzymatic or microbial attack. These crystalline domains act as physical barriers, protecting the polymer chains from interacting with degrading enzymes. In contrast, amorphous regions, which are disordered and loosely packed, are more accessible to enzymatic and microbial activity. Thus, the proportion of crystalline versus amorphous regions directly influences the degradation rate [158,159].

The functional groups present in a polymer’s backbone or side chains also affect its susceptibility to degradation. Polymers with hydrolyzable functional groups, such as ester, amide, or carbonate bonds, are more prone to enzymatic degradation. In contrast, polymers composed of inert carbon–carbon backbones, such as PE and PP, lack easily cleavable bonds and are highly resistant to both chemical and enzymatic degradation. These polymers rely heavily on oxidative processes, such as those mediated by laccases or peroxidases, to introduce reactive groups before further breakdown [153,160].

The hydrophobic nature of many polymers, such as PE, PP, and PS, makes them less compatible with aqueous environments, where microbial degradation primarily occurs. Hydrophobic polymers repel water, limiting the ability of enzymes or microorganisms to access and degrade them [161]. However, oxidative processes that introduce polar functional groups, such as hydroxyl or carbonyl groups, can enhance hydrophilicity and make the polymer more susceptible to enzymatic attack [162].

Polymers with cross-linked structures, such as thermosetting plastics, are particularly resistant to degradation. Cross-linking forms covalent bonds between polymer chains, creating a highly stable three-dimensional network that is less accessible to degrading agents [153]. For example, due to their cross-linked networks, vulcanized rubber and certain polyurethanes exhibit extreme resistance to enzymatic and microbial attack [163].

Beyond polymer structure, enzyme accessibility represents a major limitation in fungal plastic degradation. Extracellular enzymes are large macromolecules that cannot easily penetrate dense or non-porous polymer matrices, making degradation predominantly a surface-driven process. As a result, the effectiveness of enzymatic activity is strongly dependent on surface area, porosity, and prior environmental or physicochemical modifications of microplastics. Pretreatment strategies such as UV radiation, mechanical fragmentation, thermal oxidation, or chemical oxidation can increase surface roughness and introduce reactive groups, facilitating enzyme attachment and catalytic activity. Without such modifications, enzymatic degradation tends to be slow and restricted to superficial layers of the polymer [164,165].

Environmental conditions strongly influence the performance of filamentous fungi in plastic degradation. Temperature affects both fungal growth and enzymatic activity: most mesophilic fungi exhibit optimal degradation between 25–35 °C [116], while thermophilic strains such as Thielavia terrestris can degrade polyesters at higher temperatures [166]. pH is another critical factor, as many fungal oxidative and hydrolytic enzymes exhibit optimal activity under acidic conditions (pH 4–6), which are not always representative of natural environments. Variations in pH can alter enzyme stability, catalytic efficiency, and protein structure, directly affecting degradation rates [167]. In aquatic systems, salinity and ionic strength may further influence enzyme stability and fungal physiology by modifying osmotic balance and protein conformation, potentially reducing enzymatic performance in marine environments compared to freshwater or soil systems [168,169]. Additionally, the presence of organic matter and competing substrates can either stimulate co-metabolic degradation or inhibit enzyme production, depending on nutrient availability and environmental stress conditions [170,171].

Fungal colonization and biofilm formation also play an important role in microplastic degradation. The attachment of fungal hyphae to plastic surfaces enables localized enzyme secretion and creates microenvironments that enhance moisture retention, nutrient accumulation, and catalytic efficiency. Biofilm formation increases contact between fungal biomass and microplastics, facilitating the concentration of oxidative and hydrolytic enzymes at the polymer interface. This localized interaction can significantly improve the degradation efficiency compared to free-floating fungal cells, particularly in complex environmental systems where stable colonization enhances long-term enzymatic activity [172,173].

Synthetic polymers often contain additives such as plasticizers, stabilizers, flame retardants, and pigments, which can influence their degradation. These additives may act as barriers to microbial or enzymatic activity by altering the polymer’s chemical properties or creating toxic by-products that inhibit microbial growth [174]. Conversely, certain additives, such as pro-oxidant catalysts, can accelerate degradation by promoting oxidative fragmentation of the polymer chains [175]. An example of a pro-oxidant catalyst is dicobalt octacarbonyl (Co₂(CO)₈). This compound has been used to promote the degradation of plastics, such as PE, through oxidation processes. Pro-oxidant catalysts generally help accelerate the oxidation reaction of polymers by generating free radicals or other reactive species that can break the bonds in the polymer chains. In the case of dicobalt octacarbonyl, it facilitates the introduction of oxygen into the polymer chains, making it more susceptible to microbial or enzymatic degradation [176]. Another typical example of a pro-oxidant catalyst is iron (Fe) in its ionic form, especially Fe²⁺, which can promote the formation of free radicals and enhance the oxidation of plastics [177].

Polymers exposed to environmental conditions, such as UV radiation, oxidation, and mechanical abrasion, undergo surface modifications that enhance or inhibit degradation. UV radiation can break polymer chains and introduce oxygen-containing groups, such as ketones and aldehydes, which facilitate microbial colonization and enzymatic action. However, prolonged environmental aging may also lead to the formation of crystallized, oxidized layers that act as barriers to further degradation [178].

Nutrient availability influences degradation via co-metabolic mechanisms. When supplementary carbon sources (e.g., glucose or cellulose) are present, fungi such as Fusarium oxysporum and Aspergillus niger exhibit enhanced degradation rates, as energy from these substrates supports enzyme production and oxidative stress responses [179,180]. Conversely, nutrient scarcity may induce secondary metabolism and sporulation, reducing degradation efficiency [181].

Additionally, biotic interactions can modulate degradation performance. Mixed microbial consortia often outperform individual strains, as bacterial partners may produce surfactants, organic acids, or complementary enzymes that assist fungal action. For instance, Sarocladium strictum (formerly Acremonium strictum) in co-culture with Bacillus velezensis exhibited enhanced LDPE degradation efficiency twice as fast [182].

Understanding the relationship between polymer chemical structure and degradation mechanisms is crucial for developing targeted strategies to enhance the biodegradability of microplastics. Advances in enzyme engineering, pretreatment methods, and the design of biodegradable plastics can help overcome the inherent resistance of specific polymers to microbial degradation, contributing to more effective solutions for microplastic pollution [23].

From a practical perspective, economic and scalability challenges remain significant barriers to the widespread application of fungal enzymatic degradation. The production, purification, and stabilization of enzymes are costly, and degradation rates are typically slow under natural conditions. Large-scale applications require optimized bioreactor systems, immobilized enzymes, engineered fungal strains, and controlled environmental parameters to achieve efficient and economically viable processes. Without technological optimization, the enzymatic degradation of microplastics may remain limited to laboratory-scale or niche applications [183,184].

In summary, both intrinsic polymer characteristics and extrinsic environmental conditions collectively determine the rate and extent of fungal biodegradation. Optimal performance is achieved when physical and chemical modifications increase the polymer hydrophilicity and surface area, while environmental parameters such as temperature, pH, oxygen, and nutrient availability sustain fungal enzymatic activity. A schematic representation of some of the most relevant oxidative and enzymatic mechanisms involved in the degradation of synthetic polymers is shown in Figure 4. This includes (A) the enzymatic hydrolysis of ester bonds in PET by cutinases or esterases; (B) the oxidation of PE by laccase and molecular oxygen; and (C) the radical-mediated fragmentation of PE via the Fenton reaction involving Fe²⁺ and hydrogen peroxide (H₂O₂). These visualizations support the understanding of how specific chemical structures influence polymer degradation pathways.

7. Methods for Studying Biodegradation by Filamentous Fungi

The study of microplastic degradation by filamentous fungi involves a combination of controlled laboratory experiments and strategies for scaling up these findings for industrial applications. These methods aim to evaluate the degradation efficiency, understand the underlying mechanisms, and assess the feasibility of deploying fungal systems in environmental settings [25].

Laboratory studies are the cornerstone of investigating the fungal degradation of microplastics, providing a controlled environment to isolate variables and study processes at a molecular level. Before testing, microplastics are typically prepared by shredding larger plastic materials into fine particles or beads. The physical and chemical properties of these plastics (e.g., molecular weight, crystallinity, and surface morphology) are analyzed using techniques such as: (i) Fourier transform infrared spectroscopy (FTIR), which identifies functional groups on the polymer surface; (ii) scanning electron microscopy (SEM), which observes changes in surface morphology before and after fungal exposure, revealing cracks, pits, or erosion caused by fungal activity; and (iii) thermogravimetric analysis (TGA), which monitors the thermal degradation behavior, indicating polymer integrity [154].

While these methods are widely employed in biodegradation studies, they present specific advantages and limitations in the context of the fungal degradation of microplastics. FTIR is surface sensitive and may not detect changes in the bulk polymer [185]. SEM provides visual evidence of physical deterioration, but it is qualitative and may overlook chemical alterations [186]. TGA offers insight into changes in thermal stability, which can indicate polymer degradation, but it does not identify specific degradation products and may conflate fungal degradation with thermal artifacts [187]. Therefore, these tools must be interpreted with caution and used in combination to strengthen conclusions.

Beyond FTIR, SEM, and TGA, several complementary analytical tools are essential for characterizing microplastic degradation by filamentous fungi. X-ray diffraction is widely used to determine changes in polymer crystallinity, which reflects structural alterations during enzymatic attack [188]. Differential scanning calorimetry (DSC) provides insights into modifications in melting temperature and glass transition temperature, indicating chain scission or oxidation [189]. Contact angle analysis can evaluate changes in surface hydrophobicity, directly correlating with microbial adhesion and biofilm formation efficiency [190]. Zeta potential measurements help determine alterations in surface charge, which influence fungal adsorption and electrostatic interactions with polymer particles [191]. Additionally, atomic force microscopy (AFM) allows for nanoscale visualization of topographical and mechanical changes on plastic surfaces after fungal colonization, offering quantitative assessment of roughness and elasticity [192]. Combining these complementary techniques provides a more comprehensive understanding of how physicochemical parameters evolve during fungal degradation, beyond the chemical and morphological observations captured by FTIR and SEM.

However, one limitation in many current studies is the reliance on short-term incubations and the absence of kinetic modeling, which impairs the ability to distinguish between superficial oxidation and effective biodegradation (mineralization). Few studies incorporate real-time spectroscopy or isotope tracing to elucidate metabolic assimilation pathways, which would strengthen the mechanistic interpretation of fungal action [193].

Fungi are typically grown in liquid or solid media containing microplastics as the primary or sole carbon source. Typical experimental setups include (i) liquid cultures, where microplastics are suspended in fungal culture media, allowing enzyme diffusion; (ii) solid-state cultures, where fungi grow on agar plates with embedded microplastics; and (iii) biofilm assays to study the fungal biofilm formation on plastic surfaces, which enhances adhesion and degradation. These assays often use quantitative and qualitative methods to assess fungal growth (e.g., dry weight measurements) and enzyme production (e.g., laccase or peroxidase assays) [132].

Nonetheless, there is a growing recognition that conventional cultivation methods may not accurately reflect the complexity of fungal–plastic interactions in real environments. For instance, studies have shown that fungal strains exhibit different degradation capabilities depending on the surface hydrophobicity and charge of the plastic substrate [25,129,194]. Incorporating surface energy and contact angle measurements could offer better correlation with biodegradation efficiency [195]. Moreover, standardized protocols are lacking, making cross-study comparisons difficult and undermining the development of predictive models [196].

Key metrics for assessing microplastic degradation by filamentous fungi include weight loss, CO₂ evolution, and molecular weight reduction: (i) weight loss is a direct indicator of material breakdown, reflecting the overall decrease in plastic mass after fungal exposure; (ii) CO₂ evolution tests measure the release of CO₂, signaling polymer mineralization as fungi metabolize the plastic carbon; and (iii) molecular weight reduction, analyzed using gel permeation chromatography (GPC), reveals depolymerization, where long polymer chains break down into smaller, simpler molecules. Together, these methods provide a detailed view of both the physical and chemical transformations occurring during the degradation process [121].

Each of these approaches, however, has limitations that can affect interpretation. Weight loss cannot distinguish between physical fragmentation, leaching of additives, and true biodegradation [129]. CO₂ evolution is considered the most robust indicator of mineralization, but it requires tightly controlled conditions and can be confounded by other carbon sources present in the medium [197]. GPC provides quantitative insight into polymer chain scission, but its accuracy depends on sample solubility and proper calibration, which can be difficult with aged or oxidized microplastics [198]. Hence, multi-parametric assessments are recommended to corroborate degradation outcomes.

However, a critical gap in most biodegradation studies is the insufficient differentiation between abiotic and biotic degradation. Abiotic pretreatment (e.g., UV, heat) can falsely inflate fungal degradation performance. Future studies should include abiotic controls under identical conditions and report biodegradation using cumulative CO₂ evolution per unit biomass or enzyme activity [199].

Purified fungal enzymes are tested in vitro to determine their direct effects on microplastics. Assays can quantify enzyme activity and identify degradation by-products, including smaller polymer fragments, monomers, and secondary metabolites [25]. They are typically analyzed using high-performance liquid chromatography (HPLC), which separates and quantifies non-volatile compounds, such as organic acids and alcohols; or gas chromatography-mass spectrometry (GC-MS), which analyzes volatile and semi-volatile compounds, such as monomers or oligomers [200].

These analytical tools provide valuable chemical insights, yet they also have constraints. HPLC is highly sensitive to polar, water-soluble metabolites but may miss volatile degradation products unless coupled with mass spectrometry [201]. GC-MS is ideal for small, volatile fragments but requires derivatization for polar compounds and may not detect high-molecular-weight oligomers [202]. Both techniques require careful validation with authentic standards to avoid misidentifying leached additives as true degradation products.

Despite the utility of these approaches, few studies validate that the detected compounds arise specifically from enzymatic cleavage. For robust mechanistic elucidation, isotope-labeled substrates should be employed to trace carbon flux and discriminate between fungal transformation and the simple adsorption or leaching of additives. Additionally, metagenomic or transcriptomic profiling of fungi exposed to microplastics could uncover novel degradation pathways and regulatory mechanisms.

The toxicity of degradation products is a significant concern, as smaller plastic fragments or secondary by-products can interact with living organisms and the environment in unintended ways. Smaller plastic fragments or soluble by-products may enter biological systems more readily than larger microplastics. This increases the potential for bioaccumulation and interaction with cellular processes [203].

The use of cytotoxicity and ecotoxicity assays is important to evaluate the safety of fungal degradation products and their potential biological impacts. In in vitro assays, several standardized tests are commonly applied. The MTT assay measures mitochondrial metabolic activity and is used to assess cell viability after exposure to microplastic degradation by-products [204]. The ROS (reactive oxygen species) assay quantifies oxidative stress within cells, which reflects the pro-oxidant or antioxidant nature of degradation intermediates [205]. The lactate dehydrogenase (LDH) leakage assay evaluates plasma membrane integrity, indicating cytotoxicity associated with polymer fragments or chemical additives [206]. Other assays, such as Comet and Neutral Red Uptake, provide complementary insights into genotoxicity and lysosomal function, respectively [207,208].

In in vivo models, the exposure of aquatic organisms, such as Danio rerio (zebrafish), Daphnia magna, or nematodes like Caenorhabditis elegans, helps identify developmental, behavioral, and reproductive effects caused by microplastic degradation products [209,210]. Ecotoxicological analyses should also incorporate biochemical biomarkers, such as acetylcholinesterase inhibition and lipid peroxidation levels, to reveal potential neurotoxic or oxidative stress responses in exposed organisms [211,212]. Implementing a combination of these assays provides a multidimensional view of toxicity, ensuring that fungal-based remediation strategies are not only effective but also environmentally safe.

While laboratory studies provide insights, translating these findings into large-scale applications requires overcoming numerous challenges, such as ensuring fungal growth in natural environments and maintaining degradation efficiency in non-sterile conditions. Scaling up often involves bioreactors tailored for fungal growth and microplastic degradation. Optimizing conditions such as aeration, agitation, and nutrient supply is essential to maintaining fungal activity at scale [143].

However, fungal systems remain underutilized in applied bioremediation technologies. Compared to bacteria, fungi are less studied in pilot-scale or field studies, despite their broader enzymatic arsenal and capacity to colonize hydrophobic surfaces [213,214]. Therefore, the development of fungal-based bioreactors or immobilized enzyme systems deserves more attention and funding [143]. Moreover, life cycle assessment (LCA) and techno-economic analysis (TEA) should be incorporated early in the research pipeline to guide the feasibility and environmental impact of fungal bioremediation at scale [215,216].

8. Challenges and Future Perspectives

While the use of filamentous fungi offers promising avenues for addressing microplastic pollution, scaling these processes for industrial or environmental applications faces numerous challenges. These limitations encompass biological, environmental, technological, and economic factors, which collectively hinder the widespread adoption of fungal-based degradation methods [217].

Among the biological challenges, fungal degradation is often polymer-specific, meaning that a given species or enzyme may only effectively degrade certain types of plastics. For instance, fungi that excel at degrading PET may show limited efficacy against PE or PS [194]. Additionally, fungal degradation is generally slow compared to physical or chemical methods, and many fungi exhibit optimal degradation activity under controlled laboratory conditions, such as specific pH, temperature, and humidity. Replicating these conditions in diverse natural environments or industrial settings can still be a challenge [129]. Recent studies highlight that the enzymatic arsenal of fungi is versatile, but their activity is highly dependent on the accessibility of the polymer surface and the presence of hydrophilic functional groups [218,219,220]. Consequently, oxidative pre-treatment or enzyme cocktails may be essential to initiate biodegradation [221]. Furthermore, fungal biofilm formation, while beneficial for adherence to hydrophobic surfaces, can be inhibited in dynamic or harsh environmental conditions, reducing long-term efficacy [222].

Moreover, in natural settings, fungi must compete with other microorganisms, such as bacteria, which may inhibit their colonization or enzymatic efficiency. This competition can reduce the overall effectiveness of fungal degradation [223]. Although some reports suggest that co-cultivation strategies with bacteria may improve degradation efficiency via synergistic enzyme secretion [224,225], the dynamics of fungal–bacterial interactions in plastic-rich niches remain poorly understood and underexplored. Such knowledge could lead to better engineering of microbial consortia.

An often-overlooked but critical barrier to implementing the fungal biodegradation of microplastics is the initial step of microplastic collection, separation, and concentration from environmental matrices. In natural environments, especially marine and freshwater systems, microplastics are dispersed, heterogeneously distributed, and often embedded in sediments or biofilms [226]. These characteristics make their recovery technically challenging and resource intensive. Current recovery strategies include density separation, membrane filtration, electrostatic collection, and magnetic extraction using functionalized particles, but none are yet viable at large scale with high efficiency [227]. Therefore, even if fungal degradation systems were fully optimized, their practical deployment depends on the integration of upstream technologies capable of concentrating microplastics into manageable, processable fractions. This reinforces the need for coupled solutions that address both the physical recovery and biological transformation of microplastics.

Like economic considerations, it is important to stand out from the competition with established methods, where existing plastic recycling and disposal methods, such as mechanical recycling and incineration, are often more cost-effective and faster, even though they may be less environmentally friendly. In addition, current waste management policies and market structures often do not incentivize the adoption of innovative biodegradation technologies. To support fungal-based solutions, subsidies, regulatory frameworks, or market-driven incentives are needed [228].

Furthermore, economic assessments of fungal biodegradation systems are rare in the literature. LCA and TEA are essential to determine whether these systems can compete with existing industrial technologies in terms of energy efficiency, CO₂ footprint, and processing time [215,216]. Studies show that integrating fungal systems with waste valorization, for example, converting plastic-derived intermediates into biofuels or specialty chemicals, can improve economic feasibility within a circular bioeconomy [229,230].

As regulatory and safety concerns, some filamentous fungi, such as Aspergillus species, are known as opportunistic pathogens. Scaling their use in open environments or industrial applications raises concerns about unintended exposure and health risks to humans or animals [231]. To mitigate these risks, biosafety assessments must accompany any scale-up strategy, including testing for allergenic spores or potential mycotoxin production [232]. Advances in synthetic biology allow for the design of chassis strains with minimal virulence and controlled proliferation [233].

To overcome these hurdles, the potential for fungal organisms to address the environmental crisis posed by synthetic polymers has driven research into advanced strategies, such as the development of genetically modified fungi and optimized microbial consortia [234]. These approaches aim to enhance the efficiency, specificity, and versatility of biodegradation processes by leveraging the natural abilities of fungi and augmenting them with biotechnological tools.

Genetically modified fungi (GMF) are designed to express or overexpress specific enzymes or metabolic pathways that enhance their ability to degrade microplastics. This approach involves genetic engineering techniques such as clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9, homologous recombination, or heterologous gene expression [235].

GMF can be engineered to overexpress key enzymes involved in plastic degradation. Overexpression ensures higher enzymatic activity, reducing the time required for degradation. For instance, Moniliophthora roreri has been engineered to produce higher levels of cutinases, enhancing its ability to degrade PET [236], and Trichoderma reesei, a model organism for cellulase production, has been modified to secrete esterases that break down polymer ester bonds [237].

Another alternative is that genes from other microorganisms, such as bacteria capable of plastic degradation, can be inserted into fungi to create hybrid systems. For example, the PETase from Ideonella sakaiensis has been successfully expressed in fungal hosts, expanding the range of plastics that fungi can target [238]. However, horizontal gene transfer risks must be considered in environmental applications, especially if these traits confer ecological advantages to non-target species.

Genetic engineering can optimize pathways for energy utilization, ensuring that the degradation of polymers is efficient and supports fungal growth. By rewiring metabolic networks, fungi can convert plastic-derived molecules into biomass or valuable biochemicals, creating a circular economy model.

However, the release of genetically modified organisms (GMOs) into the environment is subject to strict regulations. Public acceptance and biosafety assessments are essential for widespread application, and the introduction of genetically modified fungi or consortia could disrupt local ecosystems. Therefore, risk assessments and containment strategies must be developed.

However, the environmental release of genetically modified fungi (GMF) remains one of the most critical and controversial aspects of bioremediation technology. Although international and national biosafety frameworks such as the U.S. EPA’s Toxic Substances Control Act (TSCA) and European Union Directive 2001/18/EC provide regulatory guidance, their stringent requirements often limit open-field deployment of GMF. The regulatory pathway for fungal biodegraders is further complicated by the dual classification of many species as biotechnological tools and potential opportunistic pathogens, demanding exhaustive safety evaluations before approval.

Critical challenges include the need for validated containment strategies, assessment of horizontal gene transfer risks, and monitoring of potential ecological perturbations. While physical containment (e.g., bioreactors, immobilized matrices) is feasible for industrial applications, environmental-scale deployments face significant biosafety and monitoring barriers. Few jurisdictions currently have clear regulatory categories for “environmental biotechnology” applied to plastics, resulting in case-by-case evaluations that delay innovation.

To move forward, tiered regulatory models could be adopted, similar to those used in agricultural biotechnology, allowing for progressive field trials under controlled containment before full release. The establishment of standardized biosafety protocols for GMF, including genomic stability tests and toxin/allergenicity assessments, would enhance transparency and public acceptance. Moreover, adopting synthetic biology safeguards (e.g., auxotrophy systems, kill-switch circuits) can minimize ecological risks and ensure containment even under environmental stress. In parallel, international initiatives such as the Basel Convention, UNEP’s Global Partnership on Marine Litter, and emerging OECD guidelines for microbial bioremediation can serve as policy frameworks to harmonize global governance and accelerate the safe application of GMF for microplastic degradation [8].

While consortia and GMF have shown promise in laboratory settings, scaling these solutions for industrial applications remains a challenge. Advances in bioreactor technology and immobilization techniques are essential. Recent approaches include the use of 3D-printed scaffolds or encapsulation systems for enzyme immobilization, which improve operational stability and recyclability [32,239]. Moreover, fed-batch bioreactors and co-culture fermenters designed for fungi and bacteria are being tested for continuous plastic feed degradation, albeit at the experimental scale [240,241].

Despite promising laboratory advances, the scale-up potential of fungal biodegradation systems remains constrained by technical and economic challenges. The transition from flask-level experiments to pilot and industrial-scale bioreactors requires stable operational parameters, particularly aeration, substrate feeding, and enzyme stability under non-sterile conditions [242]. Mass transfer limitations often restrict oxygen and nutrient diffusion in dense fungal biomass, which can reduce the degradation rates [243]. Engineering strategies such as immobilized mycelial reactors, fluidized beds, and continuous fed-batch operations can be explored to mitigate these issues.

From a techno-economic perspective, fungal degradation is currently less cost-effective than mechanical recycling or pyrolysis. The process demands extensive energy inputs for microplastic recovery, pretreatment, and controlled incubation, which increases the operational costs [244]. Integrating fungal bioprocesses into hybrid treatment chains, for example, coupling oxidative pretreatment or photodegradation with fungal enzymatic conversion, could significantly improve the overall efficiency and reduce costs [245]. Furthermore, valorizing degradation intermediates (e.g., monomers, organic acids, or biosurfactants) as feedstocks for green chemistry may transform fungal biodegradation into a viable circular bioeconomy platform rather than a mere remediation step. Future research should prioritize LCA and TEA to identify critical cost drivers and optimize scalability parameters.

Synthetic biology approaches, such as designing novel metabolic pathways or creating entirely synthetic fungal strains, hold immense potential for biodegradation. Leveraging genomic, transcriptomic, proteomic, and metabolomic data can also provide deeper insights into fungal capabilities and inform genetic engineering strategies [32,85,217]. However, a multidisciplinary integration of systems biology, environmental microbiology, and process engineering is urgently needed to translate omics knowledge into deployable biodegradation platforms.

Although the fungal degradation of microplastics has garnered significant attention due to its ecological compatibility and enzymatic versatility, it must be emphasized that this approach is still in its infancy when considering the scale of global plastic pollution. To date, most findings remain at the proof-of-concept or laboratory scale, with only limited translation into field applications. As such, fungal biodegradation should not be viewed as a definitive or exclusive solution, but rather as a promising and complementary avenue that requires further research, optimization, and validation. Nevertheless, the growing interest in this field is justified by the emerging evidence of fungal capacity to colonize synthetic surfaces, secrete oxidative and hydrolytic enzymes, and operate under diverse environmental conditions.

9. Conclusions

The increasing awareness of microplastic pollution’s environmental and health impacts has highlighted the need for realistic and sustainable strategies to mitigate plastic pollution and limit the formation and persistence of microplastics. This review highlights the potential role of filamentous fungi as a complementary biotechnological tool for the biodegradation of synthetic polymers. These fungi exhibit unique metabolic versatility, producing enzymes such as laccases, peroxidases, esterases, and cutinases that break down recalcitrant plastics. Additionally, their ability to secrete biosurfactants, adapt to diverse ecological conditions, and form consortia with other microorganisms further enhances their degradation potential. Despite these advantages, critical challenges persist in translating fungal biodegradation from laboratory success to real-world applications. In particular, the direct application of fungal biodegradation in environmental systems remains limited, as microplastics are widely dispersed in water and soil matrices, and degradation rates are strongly influenced by environmental conditions and polymer properties. The process remains polymer-specific and slow under environmental conditions, necessitating advancements in genetic engineering, enzyme optimization, and bioprocess design to enhance degradation efficiency and scalability. Nevertheless, the metabolic diversity and ecological adaptability of filamentous fungi position them as a relevant component of integrated plastic management and biodegradation strategies. In addition to technological development, regulatory frameworks must evolve to accommodate the safe use of fungi and genetically modified organisms in environmental applications. This includes clear biosafety protocols, risk assessment strategies, and international alignment on environmental release standards.

Policymakers may consider supporting research and pilot-scale applications of fungal biodegradation within circular economy frameworks and plastic waste management strategies, including funding incentives, public–private partnerships, and infrastructure support for bioreactor systems. Future research should focus on integrating fungi into comprehensive bioremediation strategies, leveraging their capabilities in tandem with physical and chemical methods. Innovations in bioreactor technology, synthetic biology, and the development of hybrid consortia could pave the way for sustainable and economically viable solutions to microplastic pollution. By addressing scientific, technical, and policy barriers, filamentous fungi can move from conceptual potential to applied reality, contributing to the development of scalable and eco-friendly strategies within integrated plastic pollution mitigation frameworks. Overall, filamentous fungi should be viewed not as a standalone solution for environmental microplastic removal, but as part of a broader strategy that prioritizes plastic reduction, controlled biodegradation systems, and integrated waste management approaches.