The light condition is one of the important abiotic factors affecting the production of microalgae EPS, which mainly includes light quality, light intensity and wavelength (Fig. 4).

(ⅰ) Different light qualities have significant effects on the EPS production and composition of microalgae. Some studies have shown that the EPS production of Chlorella vulgaris under green light and blue light was significantly higher than that under white light. For example, Zhang et al. found that after four growth cycles culturing Chlorella vulgaris, the highest EPS yield of 20.48 mg/g was achieved under green light and the EPS protein and polysaccharide contents reached 126.26 mg/g and 214.72 mg/g, respectively. Cells cultured under blue and green light secreted higher amounts of PN and PS in their EPS compared with secretion under white light. A 3D-EEM analysis showed that EPS, which are mainly composed of soluble protein-like compounds and humic acid-like compounds, had the lowest accumulation of these two type of compounds under white light and the highest under green light [40]. In addition, light quality determines the relative abundance of specific functional groups in EPS. For example, blue and green light irradiation was shown to promote the production of O=C-OH and O=C-OR functional groups, which are associated with PS, carboxylates, and glucuronides [40]. Thus, light quality affects the yield of EPS, and has a regulatory effect on their chemical composition.

(ⅱ) Light intensity is an important determinant of EPS synthesis by photosynthetic autotrophic microalgae. In general, a higher light intensity can significantly increase EPS production before reaching the light saturation point. However, there are differences in the adaptability of different microalgal species to light intensity [41]. For example, Porphyridium sordidum had the highest EPS production at a light intensity of 80 µmol m^-2 s^-1, whereas Chlorella EPS production decreased with increasing light intensity (50, 120, 180 µmol m^-2 s^-1). Similarly, Cyanobacterium aponinum, Anabena spp, and Nostoc sp, showed different patterns of change in EPS production under specific light intensities. Light intensity affects both the total yield of EPS and their composition. Increased light intensity was found to increase the protein content of EPS [41]. It was reported that, the effect of glycine on the yield of bound EPS was regulated by light intensity. For example, Shen et al. found that at 250 µmol m^-2 s^-1 light intensity (mixing time of 3 days, glycine concentration of 0.1 g/L), the B-EPS yield reached 47.3 mg/g, while at a 125 µmol m^-2 s^-1 light intensity (mixing time 3 days, glycine concentration 1 g/L), the B-EPS yield was 44.2 mg/g [42]. Light intensity regulates the amount and composition of EPS by affecting the charge distribution of extracellular PS. For example, as cyanobacterial cells were exposed to increasingly stronger light, the amount of polysaccharides (RPSs), podoplanar polysaccharide (CPS), and total EPS increases; whereas the weaker the light, the higher the ratio of podoplanar polysaccharide (CPS) to extracellular polymer (EPS), and the lower the ratio of released polysaccharide (RPS) to EPS [43].

(ⅲ) A mix of wavelengths also has a significant effect on EPS productivity. For example, EPS production increased with time when flagellates were cultured under different light conditions, and there were differences in EPS production when cells were cultured under different mixed wavelengths of light. The EPS production of cells cultured under light at a red/blue ratio of 15:7 and a red/blue/green ratio of 12:5:5 was 1.71 and 1.68 times higher than that of cells cultured under white light, respectively [44]. Correspondingly, the wavelength-shifting method also had a significant effect on EPS yield. For example, under (9 days of exposure to white light followed by 9 days of exposure to red/blue/green light at a ratio of 12:5:5, the EPS content reached its highest value, which increased by 85.61% compared to the control, suggesting that fine tuning the light conditions could increase the total EPS yield [44]. In summary, light conditions (including light quality, light intensity and wavelength mix) have important effects on the production and composition of microalgal EPS. Green and blue light have been found to significantly increase EPS production and its protein and polysaccharide contents, while light intensity regulated EPS synthesis by affecting photosynthesis and the charge distribution of EPS components. Also, the use of mixed wavelengths and wavelength shift methods further indicate that EPS productively may be effectively improved by optimizing light conditions.

Temperature is a key abiotic factor in the regulation of EPS synthesis in microalgae, and strongly species-specific, as evidenced by the differential regulation of EPS production, composition and synthesis pathway (Fig. 4). The EPS synthesis response to temperature in different microalgae is diverse and can be classified into positive temperature-responsive, optimal temperature-matched and extreme temperature-induced. In some positive temperature responsive microalgae. EPS production increased significantly with increasing temperature, For example, Nahla Mezhoud et al. found that the highest EPS production at the highest temperature (40 ℃) compared to the lowest light intensity (20 µmol m^-2 s^-1) in their study of a thermophilic Graesislla strain. Elevated temperature (20 ℃ to 40 ℃) was shown to be a major driver of EPS synthesis, indicating that high temperature may promote EPS secretion by activating protective metabolic mechanisms [45]. Some microalgae are optimal temperature-matched, with optimal temperature profiles for EPS secretion that overlap with their optimal growth temperatures. For example, B. braunii had its highest EPS production rate at 25–30 ℃, which was consistent with the optimal temperature range for algal growth [41]; Other microalgae are extreme temperature-inducing types, with the potential to produce more EPS at extreme temperatures due to the presence of enzymes that have different optimal temperatures for microbial growth during EPS production. For example, the EPS production of Anabaena sp. ATCC 33047 at 40–45 ℃ was significantly higher than at 30–35 ℃ [46]. Temperature affects the mechanism of action of EPS, probably on intracellular enzymes. During EPS production, certain enzymes are active, and unlike the temperature at which microorganisms optimally grow, extreme temperatures stimulate these intracellular enzymes to produce EPS to cope with extreme environments. Temperature regulation of EPS in microalgae is characterized by a complex dosage effect and species specificity. In industrial production, culture conditions should be optimized according to the temperature response of the target algal species. For positive temperature-responsive species, a gradient warming strategy can be used, whereas for extreme temperature-inducing species, a balance between EPS production and cell survival is required.

pH is a key environmental factor regulating the synthesis and structure of EPS, with an influence throughout the dynamic processes that affect EPS morphology and conformation, polysaccharide secretion efficiency, and protein secondary structure (Fig. 4). The chain structure of EPS exhibits significant conformational changes (such as stretching and curling) under different pH conditions, which are mainly regulated by electrostatic interactions and the state of dissociation of functional groups. For example, Wang et al. confirmed that the slight stretching of the chain structure of EPS under alkaline conditions (pH 7.0–10.5) facilitates diffusion of the extracellular matrix and exposure of functional sites by exploring EPS secretion by Bacillus megaterium TF10. Under acidic conditions (pH 1.3–7.0) protonation enhanced intramolecular hydrogen bond formation, which caused the EPS chain structure to undergo random curling and incomplete chain extension. Greater disruption of carboxyl groups and amide groups; and a disordered, randomly curled conformation were present at a higher pH [47]. PS are preferentially secreted under weakly alkaline conditions. Most algae produce higher levels of EPS under alkaline conditions. For example Scytonema javanicum PS production was significantly higher at pH 7.5 than under acidic conditions [46]. The highest EPS production reported in one study was observed in Tetradesmus obliquus and Anabaena flos-aquae at pH 8.0 and 9.0, respectively [46]. Structural changes in proteins vary according to pH conditions. A pH change affects the secondary structure of EPS proteins by reconfiguring the charge distribution and disrupting hydrogen bond networks. For example, Zhang et al. reported that the FTIR spectra of TB-EPS at pH 4.5 and 10.5 showed that the amide Ⅰ vibration shifted to 1637 cm⁻¹, which was identified as a β-fold. The amide Ⅲ vibration at pH 10.5 was attributed to random curling, while it changed to an α-helix (1304 cm⁻¹) at pH 4.5. The results showed that a lower or higher pH altered the secondary structure of proteins [48]. Overall, the optimal pH range for microalgal EPS synthesis was 7.0–10.5, within which EPS showed extended conformations, efficient polysaccharide secretion, and stabilized functional protein structures. Too high or too low a pH affects the morphology of EPS and disrupts the protein structure of EPS.

As a structural matrix for microalgal-fungal aggregates, EPS enable physical adhesion between microorganisms through unique N—H, C═O, and C—O-C functional groups in its PN and PS fractions. This intermolecular force induces microbes to form a stable three-dimensional aggregate structure, which significantly enhances pollutant binding efficiency through electrostatic interactions, ion exchange, and chelation mechanisms, and provides a microenvironmental system conducive to nutrient delivery and storage [49]. Table 1 demonstrates the removal efficiency of MFCS for conventional nutrient elements including C, N, and P.

Mechanism of EPS action on carbon metabolism in MFCS. The ability of the MFCS to remove C is flanked by changes in the amount of COD in the effluent (Fig. 5). Carbonic anhydrase produced by microalgae is able to absorb carbonate or convert it to CO₂ while generating O₂ and biomass for microbial metabolism and growth via the Calvin–Benson–Bassham cycle. Fungi produce laccase to degrade macromolecular organic matter into a soluble carbon source [1,5]. The O₂ and proteins produced in MFCS are then consumed in a complementary manner for the conversion of carbon consumption. Fungi utilize O₂ and proteins secreted by microalgae to break down large organic molecules into small soluble substances. Microalgae utilize small molecular dissolved substances produced by fungi to generate O₂ and proteins through photosynthesis. Throughout the process, O₂ and protein are constantly consumed and produced. Wu et al. found that the co-culture system of S. dimorphus and Aspergillus oryzae exhibited the highest TOC fixation rate (85.8%), suggesting that the fungus promoted carbon uptake by microalgae [49]. Gas exchange between fungi and microalgae facilitate the growth of each other and to some extent promotes the utilization of C in wastewater. Yang et al. treated molasses wastewater with microalgae, fungi and MFCS with COD removal efficiencies of 25.96%, 59.00% and 70.68%, which suggested that co-cultures of algae and fungi were superior to monocultures in terms of nutrient removal [7]. In MFCS, fungi may secrete enzymes to degrade organic matter and release carbon sources for microalgae to use. EPS, as substances secreted by microorganisms, may transfer and store carbon sources in the secretion process. EPS facilitate microbial contact with carbon sources and promote their uptake and utilization (Fig. 5). For example, in the treatment of swine wastewater, microalgae and fungi converted organic carbon in the wastewater into biomass through the action of EPS to achieve carbon removal and conversion [49].

(ⅰ) Mechanism of EPS action on N metabolism in MFCS. The removal of N by MFCS is mainly reflected in the transformation of different states of N by microalgae (Fig. 5). Wu et al. in their investigation of MFCS for the treatment of pig wastewater, reported that microalgae were key in the uptake and metabolism of inorganic nitrogen sources such as NH₄⁺-N and NO₃^–-N [49]. NH₄⁺ was first taken up by microalgal cells and subsequently converted into amino acids through a series of enzymatic reactions. In this process, glutamine synthetase (GS) catalyzed ammonia and glutamic acid to form glutamine, which was then converted back to glutamic acid by glutamine 2-oxoglutarate aminotransferase (GOGAT), which also released ammonia. The ammonia can be reused to synthesize other amino acids, which are in turn used in protein or DNA synthesis. In contrast, NO₃^--N conversion required a stepwise reduction process, first to NO₂^--N and then to NH₄⁺-N, a process that relied on the corresponding reductases, such as nitrate reductase (NR) and nitrite reductase (NiR) [49]. In MFCS, EPS may facilitate the process of nitrogen assimilation and transformation through their own functional groups. EPS enhance nitrogen metabolism through dual mechanisms mediated by their protein-polysaccharide components. First, these components provide microbial attachment sites, thus directly facilitating nitrogen uptake and assimilation. Second, the components establish inter-microbial signaling networks and material exchange channels that synergistically optimize nitrogen metabolic pathways in MFCS. This coordinated action drives the conversion of ammonia-nitrogen pollutants into either immobilized microbial biomass or gaseous nitrogen, achieving efficient nitrogen removal [5].

(ⅱ) Mechanism of EPS action on P metabolism in MFCS. MFCS remove P through assimilation by fungi (Fig. 5). For example, Mi et al. constructed a co-culture system of C. militaris and N. seminulum with microalgae to treat wastewater containing PO₄^3- pollutants. C. militaris alone and the co-culture group of C. militaris and N. seminulum significantly affected P uptake, and the greatest P uptake was achieved by the MFCS [5]. The intrinsic mechanism of this phenomenon is a result of the EPS on the surface of microalgae that provide oxygen, amino acids, PN, PS, and other substances for fungal growth, thus achieving a greater P removal effect. In an MFCS, the presence of fungi can provide a larger specific surface area for microalgae attachment, which facilitates microalgae proliferation [16]. In their study of total phosphorus removal by S. dimorphus, Wu et al. pointed out that functional groups such as quaternary and tertiary ammonium were carried by PN in EPS. These complexed with negatively charged phosphates, leading to the rapid adsorption of phosphorus. Moreover, EPS are key in the formation and stabilization of MFCS. MFCS increases the contact area between microorganisms and phosphorus, which contributes to microbial uptake and assimilation of phosphorus, resulting in wastewater phosphorus removal [49].

EPS in MFCS remove heavy metals through adsorption. Microalgae cell walls contain cellulose, hemicellulose, proteins, and other polymers [1]. A variety of anionic groups and binding sites are also present in the cell wall, which effectively adsorb and chelate cationic metals (Fig. 6). In addition, some functional fungi are able to capture and store metal ions through mycelium, and convert inorganic and methylated metal ions into volatile states [5,50]. EPS use electrostatic interactions, ion exchange chelation, and ligand mechanisms in the treatment of heavy metal pollutants, which ultimately leads to the adsorption of heavy metal substances [1]. For example, Mi et al. found that MFCS compared to a single system had a larger surface area, a more stable morphological structure, and stronger metal binding based on the degradation of mercury, cadmium, arsenic, and chromium [5]. Wang et al. further revealed through adsorption experiments that cadmium exposure specifically induced extracellular protein enrichment in TB-EPS, with distinct protein subtype variations observed under stress by other metals. This metal-responsive modulation directly validated the critical role of EPS-associated proteins in heavy metal ion adsorption, in which the proteins effectively reduced heavy metal toxicity through physicochemical sequestration [6]. In summary, the heavy metals are adsorbed by EPS through electrostatic interaction and ion exchange, while the microalgae cell wall provides binding sites that work together with the fungal hyphae to capture and efficiently remove heavy metals (Fig. 6). Table 1 demonstrates the removal efficiency of heavy metal pollutants by MFCS.

(ⅰ) Antibiotic removal mechanisms. In MFCS, microalgae and fungi are able to adsorb and accumulate pollutants through their cell walls and EPS. The adsorption of antibiotics by microalgae EPS is essentially achieved through targeted binding between the various functional groups and antibiotic molecules via specific forces, including electrostatic attraction and hydrophobic effects [51]. Electrostatic adsorption originates from the complementary charges between functional groups and pollutants. The amino groups (-NH₃⁺) in proteins attract negatively charged antibiotics, while the phosphate groups (-O-PO₃^2–) in polysaccharides bind to positively charged antibiotics [51]. Han et al. found that negatively charged groups (carboxyl, hydroxyl, phosphate groups) on the surface of Chlamydomonas reinhardtii FACHB-265 provide adsorption sites for tetracycline [52]. Wu et al. further confirmed that the protein content in EPS from Chlorella pyrenoidosa FACHB-9 was positively correlated with the degradation efficiency of enrofloxacin (EFX) and ciprofloxacin (CFX), both of which validated the electrostatic enrichment mechanism [51]. The hydrophobic effect of EPS achieves specific adsorption through van der Waals forces. The amino (-NH₂) and sulfhydryl (-SH) groups of proteins (PN) bind to tetracycline through hydrophobic interactions, while the dense network formed by PS and PN encapsulates the antibiotic, significantly inhibiting the internal diffusion of tetracycline [53]. Components such as PS, PN and lipids on the surface of EPS provide abundant adsorption sites that help capture and immobilize contaminants (Fig. 6). For example, Mi et al. found that the symbiotic system of C. militaris and N. seminulum was a highly efficient in degrader of four antibiotics, triclosan (TCS), lincomycin (LIN), sulfamethoxazole (SMX) and trimethoprim (TMP), in which EPS were critical in pollutant adsorption, and fungi secreted laccase to contribute to antibiotic degradation [5]. Additionally, EPS acted as a photosensitizer in the treatment of tetracycline (TC) through photocatalytic degradation of the antibiotic [54]. For example, Bai et al. found the presence of a photosensitizer in EPS in experiments evaluating the degradation of tetracycline hydrochloride by Chlorella vulgaris, with directional coupling of photosynthetic electron extraction with TC induction [54]. During the photodegradation process, the algal EPS content was significantly increased by the effects of the antibiotics. Furthermore, protein-like tryptophan, fulvic acid-like substances, and humic acid-like substances in EPS were shown to induce photocatalytic degradation of antibiotics [54]. Han et al. also reported that in the treatment of TC by Chlamydomonas reinhardtii FACHB-265, negatively charged groups on the surface of the microalgae provided adsorbate sites for tetracycline binding. As the main functional groups of EPS are carboxyl, hydroxyl, and phosphorylated groups, EPS adsorbed positively charged tetracycline through electrostatic interactions [52]. In summary, the PS, PN and lipids of EPS act as adsorption accumulators in antibiotic treatment, and as a photosensitizer for the photocatalytic degradation of antibiotics.

(ⅱ) Microplastic removal mechanisms. Microalgae show a significant particle size and concentration dependence in microplastic treatment. It is generally believed that microplastics at a high concentration and with a small particle size are more toxic to microalgae. However, the actual situation needs to consider the specific mechanism of action of algae species on microplastics. Studies have shown that small particle sized microplastics promote microalgal growth, while microplastics with a large particle size inhibit microalgal growth. Song et al. (2023) found that microalgal growth was promoted when exposed to 20-nm and 100-nm polystyrene microplastics (PS MPs), but was significantly inhibited when exposed to 1 μm PS-MPs [55]. Zhou et al.) indicated that 1 μm PS-MPs with a concentration of 5 mg/L and a particle size range of 0.2–5 μm were the most toxic to Microcystis aeruginosa at an exposure of 12 h [56]. Analyzed from the perspective of microplastic concentration, large particle sizes and high concentrations of microplastics can have a shading effect on microalgae and reduce the photosynthetic activity of microalgal cells. High concentrations of PS-MPs of a specific particle size altered the transcription of genes involved in cell division [56]. Aromatic proteins and humic acids in EPS directly mediate the adsorption and aggregation of MPs through hydrophobic functional groups (benzene rings, alkyl chains) [55,57]. The hydrophobic domains of aromatic proteins (such as benzene rings) can adsorb PS microplastics and promote cell self-aggregation. Scanning electron microscope (SEM) images show 100 nm PS particles encapsulated between microalgal cell aggregates, providing visual confirmation of the aromatic protein-mediated hydrophobic encapsulation process [55]. Humic acid induces microplastic aggregation through alkyl chains. Shiu et al. also found that EPS increases hydrophobicity and viscosity through aromatic residues [57]. Humic acid in EPS induces aggregation of MPs through its alkyl chains [58]. Its dilute water level points include long-chain fatty acids (such as 11-amino-undecanoic acid) identified in EPS [58]. Shiu et al. also found that aromatic residues in EPS can enhance the hydrophobicity and viscosity of the system [57]. In addition, Li et al. confirmed through 3D-EEM that 3 μm PS promotes the secretion of more aromatic proteins (tryptophan, tyrosine) by Chlorella, and EPS acts as a flocculant to promote the aggregation of MPs-microalgae complexes [59]. Song et al. also reported a significant increase in humic acid (Region V) after exposure to 20 nm PS, which mediated microplastic aggregation through alkyl chains [55]. Interest is the ability of microalgal EPS to mitigate the toxicity of PS-MPs. For example, Microcystis aeruginosa cells stimulated by 20 nm PS-MPs, secreted large quantities of PS, humic acids, aromatic proteins, and fulvic acids into the EPS. Among them, humic acid contributed to enveloping the microplastic surface, aromatic proteins induced self-aggregation, and fulvic acid accelerated algal growth [55]. These EPS components worked together to enhance the oxidative stress and antioxidant capacity of the algal cells, thereby mitigating the adverse effects of PS-MPs on algal cells. At the same time, aromatic proteins mediate the encapsulation and aggregation of microplastics by algal cells, while humic acid induces the self-aggregation of microplastics through alkyl chains. The EPS acted as a flocculant in this process, promoting the aggregation of microalgae with PS-MPs [59]. Similarly, Jiao et al. concluded in their experiments using Microcystis aeruginosa PCC7806 and PS-MPs that small particle size microplastics may increase the concentration of humic-like substances and enhance the oxidative stress capacity of microalgae [60]. Furthermore, microalgae were shown to develop an integrated defense against MP stress to mitigate small-sized MP toxicity, which is consistent with the conclusion of Song et al. [55]. Using 3D-EEM fluorescence spectroscopy, humic acid-like substances were found to be produced from TB-EPS of Microcystis aeruginosa PCC7806 [60]. In summary, microalgae EPS can reduce the toxicity of polystyrene microplastics. When microalgae are stimulated by a large number of secretion of proteins of EPS, together with the enhancement of algal cell stress resistance. EPS acts as a flocculant to promote aggregation during the treatment of microplastics. Small particle size microplastics can also add humic acid-like substances that enhance the oxidative stress of microalgae to reduce their toxicity (Fig. 7).

(ⅲ) Persistent Organic Pollutants (POPs) removal mechanisms. POPs, such as those found in cyanide, pesticide, dye, and antibiotic industrial waste streams, are generally characterized by persistence, high toxicity, and complex compositions. Biosorption is an important means of treating difficult-to-degrade organic compounds, particularly POPs. The adsorption of POPs by EPS is primarily driven by hydrophobic interactions and the complexation of polar functional groups. Concurrently, the protein and polysaccharide networks encapsulate POPs, and enzymatic reactions both inside and outside the cells degrade them. Hydrophobic interactions are commonly observed in the interactions between EPS and POPs, mediated by the hydrophobic regions (alkyl chains, aromatic rings) of proteins and the nonpolar segments of polysaccharides in EPS [61,62]. Chen et al. used 3D-EEM analysis to show that at high (ibuprofen) IBU concentrations, the signal from small aromatic proteins (Region I) in EPS was enhanced, indicating that the benzene ring structure of IBU was integrated into the EPS components [61]. Similarly, Satpati et al. also pointed out that the hydrophobic components of proteins in EPS bind to the aromatic rings of PAHs through van der Waals forces [63]. The polar functional groups in EPS, such as carboxyl groups (-COOH), hydroxyl groups (-OH), and phosphate groups (-O-PO₃^2–), can adsorb and aggregate with PAH molecules through hydrogen bonds and electrostatic interactions. Proteins and polysaccharides form gel-like networks that encapsulate PAHs within their structures, limiting their migration. Ultimately, PAHs are hydroxylated through enzyme-catalyzed reactions (such as monooxygenases and dioxygenases), increasing their water solubility and bioavailability [63]. Not only that, EPS also participates in biotoxicity regulation. Under PBDEs stress, microalgae (Chlorella sp.) rapidly secrete EPS, with proteins increasing by 51%−169% and polysaccharides increasing by 169% within 6 h, forming a physical barrier to reduce the entry of pollutants into cells [64]. Extracellular proteins are the main components of PBDEs adsorption. Lv et al. found that within 6 h of exposure to BDE-47, extracellular proteins increased by 51%, while the bioaccumulation of BDE-47 reached its peak. Similarly, exposure to high concentrations of BDE-209 resulted in a significant increase in extracellular proteins that were positively correlated with BDE-209 accumulation [64]. On the one hand, PBDEs (such as BDE-47 and BDE-209) are hydrophobic organic compounds that bind to the hydrophobic regions of proteins in EPS (such as alkyl chains and aromatic rings) through van der Waals forces. On the other hand, the polar functional groups of proteins (-NH₂, -COOH) chelate and adsorb PBDEs [64].

(ⅳ) Removal mechanisms of endocrine disrupting chemicals (EDCs). The removal of EDCs mainly relies on the dual mechanisms of adsorption and degradation of EPS. EPS adsorbs EDCs through the functional groups (-OH, -COOH, -NH₂) of polysaccharides and proteins, and achieves degradation through photocatalytic oxidation and enzymatic reactions [65]. During the adsorption process, hydrophobic forces play a dominant role. Sole et al. found that the hydrophobic properties of the alginate matrix significantly increased the BPAF adsorption rate to 80% [66]. Zhou et al. also pointed out that OPE exposure causes an increase in the PN/PS ratio of EPS, and that the hydrophobic groups (alkyl/aromatic rings) and charged groups provided by PN enhance the adsorption of EDCs [67]. Yang et al. found that under NP and BPA stress, the content of hexoses (MAN, GAL, GLC) in EPS of Tetrahymena increased, and arabinose in tightly bound EPS enhanced pollutant binding through a hydrophobic furan conformation [68]. The degradation process is mainly achieved through photocatalysis and enzymatic hydrolysis working together. EPS produces reactive oxygen species (ROS, such as ^•OH) under light exposure, oxidizing and degrading EDCs (such as E₂ and EE₂) [69]. Among these, furanoic acid-like substances (C1) act as photosensitive components, with their fluorescence intensity increasing with rising OPE concentrations. They degrade 41.53%−54.50% of OPEs via ROS [67]. In general, high concentration of EDCs can damage cells, negatively affect microalgae growth, and even cause cell death. However, low concentrations of EDCs can be used as a source of nutrients by microalgae and enhance the activity of certain enzymes, and may be removed through photolysis, biodegradation, biosorption, and accumulation [65,66]. PN, PS, and lipids in the microalgae EPS were shown to promote partial photolysis of EDCs [65], similar to a study by Bai et al. [54]. The hydroxyl, phosphate, amino acid, and carboxyl groups of EPS can promote biosorption and bioaccumulation. Proteins of EPS were found to help form an external digestive system for processing EDCs, promoting their degradation by microalgae, while PS contributed to pollutant adsorption [68]. Microalgae metabolism was shown to be accelerated during photosynthesis and respiration when stimulated by NP and BPA, which increased EPS production to cope with EDCs stimulation. Therefore, microalgae enhanced the biosorption and accumulation of EDCs through EPS functional groups such as hydroxyl and carboxyl groups (Fig. 7), thereby promoting the EDCs degradation process.

Summarizing, the mechanisms of action of EPS in MFCS include providing active sites to increase biological adsorption and accumulation, reducing the toxic effects of pollutants on microorganisms, and ultimately promoting the removal of POPs. Table 1 demonstrates the removal efficiency of MFCS for emerging pollutants.