The widespread use of petroleum-based plastics usually named “plastics” leads to some major environmental issues. Indeed, their accumulation in the environment constitutes a major source of pollution since plastics take a lot of years to degrade and thus affect the terrestrial and marine ecosystems. Geyer et al. (2017) estimated that amongst the 8.3 × 10⁹ tons of plastics that have been produced, 5 × 10⁹ tons have been released into the environment. However, the accumulation of plastics which continues to increase due to the difficulty of their end-of-life treatment drastically reduces the use of plastics. Thus, if there is no reduction in the production of the latter and improvement in the management of plastic waste, it is estimated that in 2030, the quantity of these plastics entering the aquatic ecosystem will reach 90 × 10⁶ tons per year (Borrelle et al., 2020). Moreover, micro- and nanoplastics (MNPs) pose a risk to human health because they accumulate in the human brain and the concentrations are rising over time (Campen et al., 2024). Several studies have shown a relationship among the increase of MNP concentrations in the environment, the presence of MNP concentrations in the human brain and the increase of the global rate of Alzheimer’s disease (van Bussel et al., 2017; Zhu et al., 2023).

Consequently, one of the challenges is to develop a better management of plastics from their production to their end of life (Hopewell et al., 2009). As the recycling of plastics is very limited given the cost generated, one of the solutions is to produce plastics that are more rapidly biodegraded by adding additives to improve the oxidation of the polymer at the end of their life. For example, pro-oxidants allow the more rapid cleavage of macromolecules induced by different environmental factors, such as oxygen, heat, visible light or ultraviolet (Sivan, 2011). Both fragmentation and biodegradation would reduce the persistence of plastics in the environment. Another attractive alternative to these petroleum-based plastics is to produce bio-based plastics from agricultural resources, such as starch, cellulose or molasses. These bio-based plastics are already used in the manufacturing of many cosmetic and industrial products and could limit the environmental consequences linked to the overconsumption of plastics. Nevertheless, only a part of bio-based plastics is biodegradable. A biodegradable plastic can be degraded by microorganisms (bacteria, fungi, algae, etc.) which are present in the environment. These microorganisms have the ability to use biodegradable plastics as a source of carbon and energy, which leads to the production of organic carbon (biomass) and inorganic carbon (CO₂). Amongst biodegradable bio-based plastics, polylactic acid (PLA) that is one of the most produced bio-based plastics, is particularly interesting.

The objective of this article is to provide a state of the art on the biodegradation of bio-based plastics containing PLA. The research in the field is constantly evolving to the development of more biodegradable bioplastics. For example, Tosakul et al. (2024) developed packaging films by incorporating Para rubber (NR) and thermoplastic starch (TPS) into PLA to improve their mechanical properties (tensile toughness) and to significantly increase their degradation rate in a home-compost environment. There have been many review papers on the degradation of PLA in the literature (Anderson and Shive, 1997; Elsawy et al., 2017; Kervran et al., 2022; Zaaba and Jaafar, 2020; Pesaranhajiabbas et al., 2023; Rosli et al., 2021; Shalem et al., 2024). Different mechanisms of (bio) degradation of PLA have been described in these references. In the current work, all the biodegradation mechanisms are classified into two categories according to the degradation conditions, namely, aerobic and anaerobic conditions. In addition, the experimental methods for the quantitative assessment of biodegradation are presented with their advantages and disadvantages.

A plastic material consists of polymer which has a structural unit repeating many times. The repeating units are monomers which are bonded together via covalent bonds to form a high molecular weight polymer consisting of long carbon chains. These polymers have varied structures, sizes and compositions, leading to specific properties. Owing to their bio-based nature, bio-based plastics can reduce the carbon footprint during their production, whereas the origin of most of plastics is petroleum, which significantly increases energy consumption and greenhouse gas emission during the materials production. Bio-based plastics can have different origins: a direct extraction from biomass (starch, cellulose, etc.) or a chemical synthesis from bio-based monomers, such as polylactic acid (PLA), polyethylene (bio-PE), polyethylene terephthalate (bio-PET) and polypropylene (bio-PP). Alternatively, they can be produced by microorganisms, as is the case of polyhydroxyalkanoates (PHAs). Bio-based plastics can have the same properties as their petroleum-based counterparts and contribute to reduce the dependence on fossil resources, since they are partially produced from renewable carbon. Nevertheless, some bio-based plastics are non-biodegradable (bio-PE, bio-PET, bio-PP), whereas others are biodegradable (mixtures of starch, PLA and PHAs). PHAs belong to the family of aliphatic polyesters with, as the most common compounds, poly-βhydroxybutyrate (PHB) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) (Bugnicourt et al., 2014). Biodegradable bio-based plastics have the benefit of reducing the carbon footprint and energy consumption during the whole life cycle.

PLA is a linear aliphatic polyester synthesized by polycondensation of lactic acid produced from renewable resources by fermentation using facultative anaerobic bacteria (Lactococcus, Lactobacillus) from agricultural co-products rich in carbohydrates, such as starch, corn, wheat, sugar cane, beet, cellulose or molasses (Murariu and Dubois, 2016). So, it can be used without concern of its depletion. It consists of L and/or D monomeric units of lactic acid (C₃H₄O₂) (Figure 1). The natural isomer is L-lactic acid, while D-lactic acid is a synthetic isomer (Penu and Helou, 2017). L-lactic acid can dimerize to form lactide (a cyclic dimer of lactic acid). Thus, PLA can be obtained by the direct polycondensation of L- or D-lactic acid or by the “ring opening polymerization” reaction of lactide. The “ring opening polymerization” reaction produces high molecular mass PLA (>100,000 Da) with different ratios of L-lactide or D-lactide (Boey et al., 2021). Lim et al. (2008) demonstrated that polymers consisting of more than 8% of D-lactide isomer tend to remain amorphous and that the crystallinity of the polymer tends to decrease even more as the stereoisomeric purity of the polymer decreases. It is also possible to obtain a high molecular weight PLA polymer directly by the condensation reaction of lactic acid following dehydration.

PLA is a compostable polymer which has excellent mechanical, structural, thermal and physical properties (Binti Taib et al., 2023). It is one of the best-selling bio-based plastics in the world, since it represents a very good alternative to polypropylene and polystyrene. The possibility of being extruded, injection molded and thermoformed allows it to be used for the manufacture of various products used in the agriculture, packaging and automotive industries. Ideally, PLA packaging is durable during its service life to optimize its application and has biodegradation properties at the end of life. PLA is also used in the medical industry for the manufacture of resorbable threads owing to its biocompatibility properties (da Silva et al., 2018). In fact, it has no toxic or carcinogenic effects on the human body (Abd Alsaheb et al., 2015). Another advantage of using PLA-based plastics is that its production requires less energy (25%–55%) than conventional petroleum-based plastics (Farah et al., 2016).

As PLA have some drawbacks (such as low elongation) that limit their applications, many researches have been conducted to develop a new material by blending PLA with other polymers, such as polybutylene adipate-co-terephtalate (PBAT) (Gonçalves et al., 2017). Various kinds of inorganic and organic fillers, such as mineral clays and natural fibers, have been used in PLA/PBAT biocomposite as reinforcement to improve their performance. The addition of nanofilers can improve the rheological and mechanical performances of these materials. Thanks to new scientific data in the synthesis and properties of these bioplastics, PBAT and PLA represent nearly 35% of the total production capacity of biodegradable polymers in 2023, with PLA having the highest production (31%) (Table 1) (Bioplastic Market Development update 2023, 2024). Given their growing use in different industrial applications and manufacturing technologies (e.g., injection molding, extrusion, film blowing, 3D printing, etc.), especially in the packaging industry, PBAT, PLA and their blends will remain among the most demanded biodegradable polymers in the market. According to the European Bioplastic, it was estimated that in 2028, the production of biodegradable bioplastics would increase from 52% to 62% to the detriment of biobased/non-biodegradable bioplastics (Table 1) (Bioplastic Market Development update 2023, 2024). Among biodegradable bioplastics, PLA remains the most produced biopolymer and its use is increasing significantly (44%) (Table 1) (Bioplastic Market Development update 2023, 2024).

Bio-based plastics degrade in various environments, such as aerobic compost, soils or marine sediments (Kalita et al., 2019; Kalita et al., 2021a; Delacuvellerie et al., 2021; Weng et al., 2013). Currently, industrial composts constitute the main treatment for bio-based plastics. However, the mechanisms involved are not fully understood even in assays or tests performed in controlled conditions, since biodegradation is strongly linked to the environmental conditions. The microorganisms used in biodegradation assays are mainly collection microorganisms isolated from different environments. In most cases, they come from domestic or industrial composts (Sakai et al., 2001; Sedničkova et al., 2018), activated sludge from wastewater treatment plants recovering municipal or industrial wastes as well as digestates from anaerobic digestion plants (Pattanasuttichonlakul et al., 2018; Yagi et al., 2009). Some studies have also been carried out using soil samples (Karamanlioglu et al., 2014), sea water (Curto et al., 2021) or river water (Chiellini et al., 2007).

Standards have been developed by various international standardization organizations (e.g., ASTM International, JIS, ECN, OECD) (Eubeler et al., 2009), in order to assess the impact of the environmental conditions, such as soil, fresh water or sea water, domestic or industrial compost, digester (Table 2). These standards are based on reliable, repeatable and reproducible standardized tests (in solid and liquid phases), in relation to the microbial inoculum and the conditions (temperature, relative humidity) to carry out the tests. Analytical and testing standards define the methodology used to measure biodegradation. Conversely, specification standards set the biodegradation threshold to be achieved for a defined duration depending on the environment. The biodegradation yield must be greater than or equal to 90% in 6, 12 or 24 months for an industrial compost, a domestic compost or a soil, respectively.

The biodegradation of PLA-based polymers is a complex mechanism involving successive or concomitant physical, chemical and biological steps. The degradation of PLA-based polymers can occur by abiotic phenomena (photo-degradation, thermolysis, hydrolysis, oxidation) and/or biotic (enzymatic, biochemical) phenomena. Then, when the assimilation of degraded PLA-based polymers occurs, it corresponds to the integration by microorganisms of low molecular mass degradation residues (Figure 2). These mechanisms can be carried out not only in aerobic conditions, but also in anaerobic conditions with transformation of the polymer into biomass, water, and CO₂ and CH₄.

The biodegradation of PLA has been studied mainly in industrial compost under aerobic conditions (Kale et al., 2007b; Castro-Aguirre et al., 2018; Stloukal et al., 2015) or under anaerobic (Bracciale et al., 2024; Gómez and Michel, 2013) conditions at mesophilic or thermophilic temperatures. Most of the isolated microbial strains, capable of degrading PLA, were obtained from consortia from soils, composts, sludge from wastewater treatment plants and/or methanizers (Table 3). They mainly belong to bacteria of the Actinomycetes class (filamentous bacteria), but some fungal strains also have this capacity. The first strain of Actinomycetes isolated from soil and capable of degrading PLA is Amycolatopsis HT-32 (Pranamuda et al., 1997). Identification based on 16S rRNA sequencing revealed the presence of other genera belonging to the Pseudonocardiaceae family, such as Saccharothrix, Lentzea, Kibdelosporangium and Streptoalloteichus (Jarerat et al., 2002). Then, the development of molecular biology techniques made it possible to reference other strains of Actinomycetes degrading PLA, such as Streptomyces, Laceyella, Pseudonocardia (Table 3). Other bacterial genera, such as Geobacillus, Bacillus, Pseudomonas and Stenotrophomonas, can also be involved in the degradation of PLA under aerobic conditions (Table 3). Moreover, some bacteria are involved in the digestion of PLA under anaerobic conditions (Table 3): Xanthomonadaceae bacterium, Mesorhizobium sp. and Ureibacillus sp. (Yagi et al., 2014). The importance of the genus Tepidimicrobium, and more particularly Tepidimicrobium xylanilyticum, was reported in the digestion of PLA (Tseng et al., 2020). In fact, several bacterial strains of the genus Tepidimicrobium were involved in the anaerobic digestion (at high temperatures) of coffee capsules made of biodegradable polymers (Cazaudehore et al., 2021). As these capsules contain a mixture of polymers, genus Tepidimicrobium is probably able to biodegrade polymers other than PLA. Tepidimicrobium is therefore the dominant genus capable of degrading PLA under anaerobic conditions. On the other hand, the degradation of PLA by fungal strains has been reported in the literature. Some researchers explored the fungal degradation of PLA in soil and compost under aerobic conditions and isolated fungal strains belonging to the genera Aspergillus, Thermomyces, Cladosporium, Penicillium and Rhodotorula (Karamanlioglu et al., 2014; Nair et al., 2016). Strains degrading PLA are mainly effective at a temperature range between 50°C and 70°C, but some strains can be active at lower temperatures close to 30°C (Table 3).

Microbial communities develop on the surface of PLA-based biopolymers, mostly in the form of a biofilm. The latter is a viscous layer made up of microorganisms coated with extracellular polymeric substances (EPS), excreted by microbial cells (Walczak et al., 2015). The composition of the microbial communities evolves with the time of immersion of the biopolymer (De Tender et al., 2017). It also depends on the environmental conditions in which the biopolymer samples are placed. In the marine environment, the microbial communities of the biofilm present on floating plastics are made up of a much higher proportion of Rhodobacteraceae and Actinomycetaceae compared to those that are developed on the surface of sedimented plastics. On the other hand, the communities of the biofilm at the surface of sedimented plastics contain a greater proportion of Pseudoalteromonadaceae and Shewanellaceae (Delacuvellerie et al., 2021). Some specific tests have been performed in order to accelerate the biodegradation of PLA or to improve the rate of PLA biodegradation (Castro-Aguirre et al., 2018). For example, the compost containing PLA coupons (1 × 1 cm²) was inoculated with one or more specific microbial strains capable of degrading PLA. This requires re-culturing the pure strains in a liquid nutrient medium (such as R2B medium) under the optimal conditions for growth of these bacteria (stirring at 58°C), in order to obtain a bacterial culture at the end of growth phase (beginning of the plateau of the number of bacterial cells after the exponential growth phase). This culture is then used to inoculate the bioreactor containing the compost with the PLA films, by adding bacterial cells equivalent to 1% of the total bacterial community (10⁸ bacterial cells/g of compost) present in the bioreactor. The results indicated that the biodegradation of PLA films occurred in times comparable to organic waste.

Composting is one of the most efficient biodegradation methods for biopolymers. However, its efficiency depends on the environmental conditions (temperature, humidity, etc.), because these parameters strongly impact the microbial activity into the compost. So, the relevant parameters must be controlled to avoid a variability in the biodegradation yield. Moreover, the composition of compost also plays an important role in the biodegradation process, in particular its content in degrading microorganisms and the presence of carbon sources easily assimilated by microorganisms. Therefore, further research should be conducted to determine the composition of the compost and the environmental conditions that allow for optimized biodegradation of a given bioplastic.

Much research has been conducted to develop PLA-based biopolymers by mixing them with other constituents to improve their specific properties and increase their biodegradation rate. However, few studies have collected data on the effects of these additives to explain the improvement in the biodegradation rate. To progress in the development of new eco-friendly plastics, it is necessary to better understand the mechanisms involved by studying in more detail the morphology, water absorption, hydrophily and structural properties of PLA blends to predict their biodegradation in a given environment.

Significant advances have been made on the biodegradation of PLA, highlighting the vital role of enzymes from microorganisms in biodegradation of PLA. The use of enzymes instead of microbial cells would allow to avoid the assimilation phase of lactic acid products by microorganisms. The released lactic acid could be directed towards repolymerization to manufacture new PLA or towards other applications, thus ensuring a circular economy (Acosta and Alper, 2023). The use of enzymes for PLA degradation is still emerging. Enzyme engineering technology would improve the performance of enzymatic degradation. In addition, the isotopic tracing and fluorescent labelling method would allow new advances to better understand the catalytic mechanism.

Bio-based plastics offer many environmental benefits as a replacement for petroleum-based plastics. However, the sole reduction of greenhouse gas emissions (mainly carbon dioxide) is not the major characteristic for putting an end to the accumulation of plastics in ecosystems leading to major pollution of the nature. Indeed, plastics of biological origin should have a much higher rate of biodegradation than that of plastics from petroleum. Plastics based on PLA, which represents around a quarter of the production of bio-based plastics, have this property of biodegradability. The reported bibliographic review dedicated to PLA-based plastics highlights that there is a good understanding of the mechanisms involved in their biodegradation both in aerobic and anaerobic environments. Thus, using PLA-degrading enzymes is very effective process in order to increase the biodegradation rate. Moreover, several analytic techniques make it possible to properly characterize the effect of biodegradation on their properties. It has been demonstrated that the biodegradation rate appears to be higher for the amorphous part of the polymer compared to the crystalline part. Blends of PLA with other polymers or sources of carbon also impact the biodegradation rate. Generally, a source of carbon easily assimilated by microorganisms will amplify the biodegradation rate whereas the incorporation of compatibilizers could significantly decrease the biodegradation rate in the case of PLA/PBAT-based materials.

In addition, there is a whole normative context which makes it possible to frame validation tests according to the environment in which bio-based plastics could be found. It has thus been demonstrated that PLA-based plastics have good biodegradability in industrial compost under aerobic conditions but that biodegradation in a marine environment or in domestic compost is less effective. In this regard, the biodegradation of PLA in marine or domestic environments should be the focus on the future research, as microplastics in such environments are a big issue to address nowadays. On the other hand, anaerobic digestion, i.e., a thermophilic and anaerobic process, also seems to be a promising alternative to the biodegradation of bio-based plastics leading to the production of biogas that can be converted into energy. However, a deeper knowledge of the PLA-degrading microorganisms found in different environments and of the corresponding biodegradation mechanisms involved is still necessary to develop even more effective strategies leading to the reduction of the duration of the biodegradation process. Nevertheless, despite the existence of standards for plastic degradation testing, a common testing procedure should be developed to collect as much data as possible on the biodegradability of bioplastics and to make cross-comparisons between studies.