Though its precise classification is still under discussion, Jean-Baptiste Lamarck initially identified the genus Azolla in 1783 (Bujak and Bujak, 2024). Initially grouped with Salviniaceae, phylogenetic studies later confirmed its distinct evolutionary lineage (Saunders and Fowler, 1993).

The classification of Azolla proves difficult because the genus shows significant morphological variability, vegetative reproduction, and environmental adaptability, which make species identification challenging (Lydia et al., 2023). Azolla is divided into two subgenera: EuAzolla (A. filiculoides, A. rubra, A. microphylla, A. mexicana, A. caroliniana) and Rhizosperma (A. pinnata, A. nilotica), differentiated by morphology and reproduction. There are seven extinct and twenty-five fossil species of Azolla. The distribution, characteristic features, and uses of different Azolla species have been discussed in detail by Kour et al (Kour et al., 2024). Native to America, Africa, Asia, and Australia, Azolla has expanded globally due to its invasive nature, though no species are native to Europe. While fossil evidence shows that Azolla existed in Europe at one time, it was reintroduced to the continent in 1880 (Korsa et al., 2024; Kour et al., 2024).

Several species have become invasive outside their native ranges, forming dense mats that disrupt ecosystems and economic activities. Examples include Azolla cristata (syn. A. caroliniana) originated from North and Central America and is now growing in Africa, Asia, and Europe (Korsa et al., 2024; Kour et al., 2024). The native South and Central American A. microphylla has been introduced throughout the world (Kour et al., 2024). In contrast, A. mexicana remains primarily confined to North and Central America (Kour et al., 2024). Azolla pinnata, native to Asia, Africa, and Australia, has been introduced to the USA and South America. Azolla filiculoides, tolerant of cold climates, was introduced to China from East Germany in 1977 (Madeira et al., 2019; Kour et al., 2024). Through the introduction, Egypt received A. caroliniana, A. filiculoides, and A. pinnata (Serag et al., 2000b). Azolla caroliniana developed into an invasive species in the Danube Delta of Ukraine by 1978 (Prokopuk, 2016). Reflecting evolutionary adaptations, phylogenetic studies utilizing rbcL gene sequences confirm the split of Azolla into Euazolla and Rhizosperma (Mahmood et al., 2020). Species like A. pinnata and A. filiculoides are widely used in agriculture, while others remain underexplored for potential applications (Kour et al., 2024).

The aquatic fern Azolla doubles its biomass roughly every 2 to 5 days, producing 3–9 tons of dry matter per hectare annually (Lumpkin and Plucknett, 1980; Wagner, 1997). Critical factors affecting Azolla growth and nutrient composition have been discussed in detail previously (Marzouk et al., 2023). Briefly, growth depends on temperature, light, nutrients, and water pH, with an optimum of 18–28°C; growth slows below 15°C and stops above 35°C (Sadeghi et al., 2013). Its symbiosis with Anabaena azollae enables survival in low-nitrogen conditions, though it thrives in nutrient-rich waters (Lechno-Yossef and Nierzwicki-Bauer, 2002).

Reproduction occurs mainly through vegetative propagation, via detachment of rhizome branches, which allows rapid spread. Sexual reproduction is less common, involving heterosporous sporocarps containing microspores and megaspores (Sebastian et al., 2021; Schluepmann et al., 2022). The life cycle of Azolla varies by species. The process of sexual reproduction starts when paired sporocarps develop from shoot apical meristems, including both a megasporocarp with one megasporangium and a microsporocarp with several microsporangia (Dijkhuizen et al., 2021; Schluepmann et al., 2022). During sporocarp formation, A. azollae is recruited into the indusium cap near root-forming branches. While microsporocarps discharge massulae, including microspores, megasporocarps develop into megagametophytes, creating archegonia ( Figure 1 ). Fertilization occurs when flagellate gametes reach the archegonia, leading to diploid growth, though the timing of microgametophyte and gamete development remains unclear (Schluepmann et al., 2022).

Sporocarps in A. filiculoides can remain viable for up to four years at 4°C or indefinitely if dried and cryopreserved at -80°C, whereas fragile, water-rich sporophytes cannot be stored (Li et al., 2018). The shift to the haploid phase happens during the start of sporangial development, which depends on light conditions, temperature, and nutrient levels (White, 1971). Unlike seed plants, Azolla exhibits high plasticity in sporangial meristem formation, occurring in both sporophytes and gametophytes. Different Azolla species demonstrate variable sporangial responses when exposed to distinct environmental stimuli. For example, A. filiculoides produces sporocarps when exposed to far-red light, but this formation stops under open-field red light conditions (Dijkhuizen et al., 2021). Sporocarp formation is likely controlled by a conserved phase transition network involving regulatory elements known from seed plants, such as MIKCC, AP2, and GAMYB-microRNA319 interactions (Ambrose and Vasco, 2016). The processes controlling spore germination and gametophyte growth are probably controlled by the sporocarp itself, given the protected nature of Azolla gametophytes. During periods of environmental stress, sporocarps descend to the depths of aquatic environments and stay dormant until the conditions improve (Sood and Ahluwalia, 2009). Different Azolla species thrive in diverse habitats. Azolla pinnata, for instance, likes higher temperatures; A. filiculoides may survive in colder temperatures (Metzgar et al., 2007). However, other factors, such as high salinity, UV radiation, and heavy metals, can affect their growth (Korsa et al., 2024). Azolla plants in cold regions submerge during the winter and then emerge when the temperature increases. It can change their color from grey green to red-purple when exposed to intense sunlight. Azolla thrives in freshwater bodies like ditches, swamps, lakes, and rivers and is also called duckweed, mosquito, or water fern (Kour et al., 2024). While modern species are free-floating, fossils suggest that extinct species had suberect growth (Watanabe and Berja, 1983). Molecular research highlights genetic traits that enhance stress resistance, offering potential for selective breeding. Its sporophyte phase features a floating rhizome with leaf-like fronds and submerged roots.

The nitrogen-fixing cyanobacterium A. azollae resides in specialized cavities of Azolla leaves, forming a mutualistic symbiosis first observed by Strasburger in 1873 and later described by De Bary (Carrapiço, 2010; Kour et al., 2024). This relationship enables Azolla to thrive in nitrogen-poor waters and function as an effective organic fertilizer ( Figure 2 ) (Peters and Meeks, 1989).

Molecular studies confirm the long-term coevolution of Azolla and its cyanobiont (Qiu and Yu, 2003; Papaefthimiou et al., 2008; Pereira and Vasconcelos, 2014). Phylogenetic analyses indicate a single evolutionary origin of the symbiosis, which has remained stable for over 100 million years (Bujak and Bujak, 2024). Vertical transmission through megasporocarps ensures that each new generation inherits its cyanobiont without external inoculation, maintaining high nitrogen-fixation efficiency (Ran et al., 2010).

The system functions without requiring external inoculation while preserving strong nitrogen-fixing efficiency (Carrapiço, 2010). The propagation of A. azollae within Azolla ferns depends on its apical colony in the shoot apex and the movement of its motile filaments (hormogonia) to organ initials like leaf cavities and sporocarps. The regulation of hormogonia movement and cell differentiation in Azolla is mostly unknown. Some evidence suggests that secretory trichomes and deoxyanthocyanins might affect this process (Cohen et al., 2002). The leaf cavity functions as a microhabitat that controls oxygen levels to protect nitrogenase from deactivation, thereby enabling nitrogen fixation. Azolla’s leaf cavities and sporocarps host a diverse microbial (Rai et al., 2002). Some studies suggested that some bacteria synthesize plant hormones like indole-3-acetic acid, which can improve the growth of Azolla (Kumar et al., 2022). Therefore, the Azolla-Anabaena relationship forms a complex microbial network that functions as a superorganism beyond its initial binary symbiosis (Carrapiço, 2017). Azolla maintains association with one cyanobacterial species, which contrasts with legumes hosting multiple symbiotic partners and prompts further investigation into its coevolution and metabolic interactions. Genetic research has identified regulatory differences in nitrogen fixation, which may lead to agricultural improvements (Pabby et al., 2003; Devaprakash et al., 2024).

Through its symbiosis with A. azollae, Azolla contributes 30–60 kg N ha^-¹ per season via biological nitrogen fixation, improving soil fertility and raising nitrogen-use efficiency by up to 70% compared with synthetic fertilizers (Kollah et al., 2016; Kour et al., 2024) ( Table 1 ). With a rapid growth rate that doubles biomass in 3–5 days, Azolla fixes 1.1–3.5 kg N ha^-¹ day^-¹, exceeding many legumes (Pillai, 2001; Vijayan et al., 2024). When used as a dual crop in rice paddies, it supplies 40–60 kg N ha^-¹ per cycle (Adhikari et al., 2020). Azolla inoculation with 16.5–17.5 t fresh weight ha^-¹ fixes 52.5–55.1 kg N ha^-¹, while 12.2 t dry matter ha^-¹ provides 33.8 kg N ha^-¹ (Raja et al., 2012). The efficiency of nitrogen fixation varies depending on climatic conditions, floodwater nutrient levels, Azolla species, and rice growth stages (Kour et al., 2024).

Beyond nitrogen, after incorporation, Azolla enhances soil organic matter, microbial activity, and physical structure. Its decomposition increases aggregate stability, porosity, water retention, and permeability while reducing bulk density (Marzouk et al., 2023; Sun et al., 2024; Ansabayeva et al., 2025). These changes support higher crop yields. Humus derived from Azolla improves cation exchange capacity and nutrient availability (Ca²⁺, Mg²⁺, K⁺, P) (Kour et al., 2024). The breakdown of Azolla in soil helps various nitrogen-fixing bacteria and fungi to flourish, which in turn enhances nutrient cycling and crop nutrition (Samarajeewa et al., 2005; Adhikari et al., 2020).

Compared to synthetic nitrogen sources, Azolla-derived nitrogen is more efficient in terms of plant uptake and fertilizer use efficiency (Seleiman et al., 2022; Marzouk et al., 2023). Integrating organic and inorganic fertilizers sustains crop productivity and enhances soil health (Pushpanathan et al., 2004). Several studies demonstrate that mixing Azolla into soil helps improve nitrogen mineralization and its usage. The efficiency of fertilizers is enhanced when Azolla is added to the soil (Bhuvaneshwari and Singh, 2015; Adhikari et al., 2020). A previous study reported that the application of 86 kg N ha^-¹+1000 kg Azolla ha^-¹ increased rice growth by 15.54%, yield by 25.49%, and nitrogen-use efficiency (Safriyani et al., 2020).

Beyond nitrogen, Azolla increases phosphorous availability by 20–30%, hence very helpful for soils lacking phosphorus (Raja et al., 2012). With 3–5% nitrogen and 3–6% potassium in its biomass, it exceeds traditional green manures in nutrient value (Kour et al., 2024). Azolla breakdown increases urease and phosphatase activity, encouraging mineralization of nutrients (Herath et al., 2023). Moreover, Azolla is essential for the control of soil pH since it reduces acidification in acidic soils and increases phosphorus solubility in alkaline soils, thus boosting the availability of nutrients in several agroecosystems (Herath et al., 2023; Marzouk et al., 2024).

Azolla reduces runoff, prevents erosion, and improves aggregation, particularly when cultivated along contour lines or irrigation channels (Adhikari et al., 2020; Herath et al., 2023). Floating mats in rice paddies protect against sediment loss, while fine rootlets deposit silt in wetlands and channels, limiting nutrient depletion (Raja et al., 2012; Kollah et al., 2016). These processes enhance root development and water-use efficiency in rice fields (Razavipour et al., 2018).

Azolla contributes to abiotic stress management. It tolerates moderate salinity, removing excess salts from soil and water (Sadeghi et al., 2014). Its mats limit evaporation and salt buildup, reducing crop salinity stress (Serag et al., 2000a). Compost from Azolla enhances rice growth on saline soils by releasing organic acids that improve nutrient availability while aiding salt removal (Razavipour et al., 2018). Collectively, these properties establish Azolla as a cost-effective alternative to conventional soil amendments.

Azolla forms a dense floating mat on the water surface, reducing evaporation by up to 60% and conserving soil moisture (Kour et al., 2024). By covering the water surface, Azolla limits sunlight penetration, thereby lowering water temperature and evaporation rates, which is particularly beneficial in regions with water scarcity or irregular rainfall (Marzouk et al., 2023). Additionally, In non-flooded cropping systems, Azolla can also be applied as a living mulch, improving soil water retention and reducing moisture loss (Raja et al., 2012).

The thick Azolla mat prevents sunlight from reaching submerged weed seeds, inhibiting germination and growth. This eco-friendly method provides an alternative to herbicides (Adhikari et al., 2020). Previous studies show that Azolla can reduce weed biomass by up to 50% (Herath et al., 2023). Since weeds compete with rice for nutrients, light, and water, infestations can cause yield losses of 16–100% depending on severity (Geetha et al., 2022). Unlike herbicides such as 2,4-D, glyphosate, and propanil, which harm non-target organisms and degrade soil and water quality (Shekhawat et al., 2022), Azolla offers a safer alternative. Its potential in smallholder systems, first recognized in 1927, remains underexploited (Herath et al., 2023).

Azolla effectively suppresses multiple weeds, including Echinochloa crus-galli, Cyperus serotinus, Monochoria vaginalis, Eclipta prostrata, Fimbristylis miliacea, and Cyperus rotundus (Herath et al., 2023). By blocking light, altering microclimate (reducing evaporation and soil temperature), and improving soil structure when incorporated as green manure, Azolla reduces weed competition and enhances rice growth (Marzouk et al., 2023). Azolla also exhibits allelopathic properties that inhibit weed germination and growth. It releases secondary metabolites such as phenolic compounds, flavonoids, and tannins, which suppress invasive weed species (Bahadur et al., 2015; Ameena et al., 2024). Some studies have demonstrated that Azolla extracts negatively affect root elongation in weeds, further reinforcing its potential as a natural weed management tool (Herath et al., 2023).

The floating Azolla mat also disrupts pest life cycles. It prevents mosquitoes from laying eggs on water surfaces, lowering mosquito populations and reducing vector-borne disease risk (Kour et al., 2024). It interferes with the breeding of rice stem borers and leafhoppers, which require open water for egg-laying (Marzouk et al., 2023). Furthermore, Azolla supports beneficial organisms such as predatory insects, frogs, and fish, which feed on pest larvae, thereby enhancing natural pest control mechanisms and reducing the need for chemical pesticides (Raja et al., 2012). In integrated rice–fish systems, Azolla not only acts as biofertilizer and fish feed but also suppresses weeds and pests, reducing chemical inputs and maintaining ecological balance (Pabby et al., 2003). Azolla extracts possess antifungal and antibacterial properties, helping to mitigate plant diseases such as rice blast and sheath blight (Phukon et al., 2017). Azolla bioactive compounds have also demonstrated effectiveness against fungal infections in other crops, highlighting its potential in sustainable disease management (Pereira et al., 2015).

Azolla is a nutrient-rich, high-protein feed supplement for livestock, poultry, and fish. It provides an excellent amino acid profile, high digestibility, and essential micronutrients, serving as a sustainable alternative to conventional protein feeds such as soybean meal and fish meal, which are costly and environmentally intensive (Kour et al., 2024).

The nutrient composition of Azolla varies by species, geography, production methods, and soil conditions (Shaltout et al., 2012; Bhavyasree, 2015). Several studies have reported different compositions of different species of Azolla (Katole et al., 2017; Khursheed et al., 2019). Azolla microphylla and A. filiculoides, which are most commonly used, contain 91.77%–92.25% moisture, 3.9%–5.2% crude protein, 0.6%–1.8% crude fat, and 2% ash (Bhaskaran and Kannapan, 2015). On a dry matter basis, Azolla has 25–35% crude protein, 10–15% minerals, and up to 10% amino acids, making it comparable to commercial protein feeds (Marzouk et al., 2023). Despite its high nutritional value, inclusion in animal diets is generally limited to 25% due to anti-nutritional compounds.

Among species, A. pinnata, native to warm regions, has higher polyphenolic tannins, reducing digestibility, while A. filiculoides, found in the Americas and Europe, contains lower polyphenols and higher protein, making it a better protein source (Brouwer et al., 2018)). Azolla is also rich in lysine, methionine, and arginine, essential for muscle growth, as well as key minerals like calcium, phosphorus, magnesium, iron, and potassium, which support bone health and metabolism (Herath et al., 2023; Yohana et al., 2023). High beta-carotene and vitamin A levels enhance vision, immunity, and reproduction (Adhikari et al., 2020). Its low lignin content (<5%) ensures high digestibility for both ruminants and non-ruminants (Raja et al., 2012), while bioactive compounds such as flavonoids and phenolics improve gut health, feed efficiency, and disease resistance (Kour et al., 2024).

Azolla is an efficient natural sink for atmospheric CO₂ due to its rapid growth, high biomass accumulation, and symbiosis with A. azollae, which enables continuous nitrogen fixation without external inputs ( (Vroom et al., 2024) (Kour et al., 2024). Its integration into wetlands and rice paddies enhances carbon cycling, soil carbon storage, and long-term ecosystem stability (Sadeghi et al., 2014).

Evidence shows that Azolla can sequester CO₂ at rates comparable to, or greater than, terrestrial plants. The Eocene “Azolla bloom” contributed significantly to global cooling, highlighting its historic efficiency as a CO₂ sink (Yuan et al., 2024). Modern studies estimate that a 1-ha Azolla Pond captures 21,266 kg CO₂ annually, and that 1,018,023 km² (one-fifth the Amazon) could offset current global CO₂ increases. Compared to terrestrial ecosystems, which absorb 20–30% of anthropogenic emissions, Azolla ponds remove CO₂ 18 times more efficiently than an equivalent Amazon forest area (Hamdan and Houri, 2022).

Practical applications extend beyond sequestration. As a biofertilizer, Azolla improves soil quality and reduces the need for inorganic fertilizers that contribute to GHG emissions (Hamdan and Houri, 2022). In poultry farming, replacing 50% of feed with Azolla reduced CO₂ by 35%, N₂O by 22.3%, and CH₄ by 4.7%, corresponding to a 28.5% reduction in global warming potential per 1,000 birds (Espino and Bellotindos, 2020). Similarly, cultivation trials reported annual fixation of 1.86 t CO₂ and 0.33 t N ha^-¹, providing dual climate and agronomic benefits (Brinkhuis and Bijl, 2014). In Sri Lanka, expanded Azolla use in paddy fields could mitigate 509,422 t of CO₂ annually (Surenthiran and Loganathan, 2012), while A. filiculoides sequesters 32.54 metric tons CO₂ ha^-¹ year^-¹, surpassing grassland, forest, and algae (Dawson and Smith, 2007).

CO₂ enrichment experiments (380–680 ppm) further demonstrated enhanced Azolla biomass, confirming its scalability as a mitigation strategy (Cheng et al., 2010). During the Azolla interval, mean sea surface temperatures dropped from 13°C to 10°C, demonstrating its historical role in climate regulation (Brinkhuis et al., 2006). Sensitivity analyses indicate that optimal Azolla cultivation could require sequestration areas between 763,518 and 1,527,036 km² to significantly counteract atmospheric CO₂ rise. Given its historical impact on climate stabilization and its efficiency in CO₂ capture, Azolla-based strategies could contribute significantly to mitigating global warming and enhancing carbon management in agroecosystems (Hamdan and Houri, 2022).

Agriculture is a major contributor to greenhouse gas (GHG) emissions, particularly CO₂, CH₄, and N₂O (Chataut et al., 2023). CO₂ results from microbial decay and organic matter oxidation, while CH₄, a potent GHG with 20–60 times the global warming potential of CO₂, is produced under anaerobic conditions in flooded rice paddies, livestock digestion, and manure storage (Smith et al., 2008). N₂O arises from nitrogen transformations in soil, particularly under excessive fertilization (Xiao et al., 2024). Rice paddies contribute ~20% of global CH₄ emissions, necessitating mitigation strategies. Azolla offers a natural means of reducing CH₄ emissions in rice systems. By releasing oxygen, absorbing excess nutrients, and altering soil redox potential, Azolla suppresses methanogenesis while maintaining or improving rice yields (Razavipour et al., 2018). Field studies consistently report 30–60% reductions in CH₄ emissions when Azolla is used as green manure or a floating cover (Serag et al., 2000a). In a three-year double rice cropping study, Azolla integration lowered CH₄ emissions, reduced nitrogen fertilizer requirements, and maintained yields, largely due to improved soil oxygenation (Xu et al., 2017).

Synergistic practices further enhance these benefits. In Japan, combining Azolla with poultry-litter biochar increased rice yields by 27–75% while cutting CH₄ by ~25% and N₂O by up to 98% (Kimani et al., 2020). In India, dual cropping with Azolla reduced CH₄ flux by 40% compared to urea fertilization alone, confirming the role of oxygen release in lowering emissions (Bharati et al., 2000). Laboratory studies also show that soils treated with Azolla and urea exhibit higher CH₄ oxidation than urea alone, due to oxygen supplied by cyanobacteria (Adhya et al., 2000; Sood et al., 2012). Comparisons with other organic amendments reveal that CH₄ efflux per grain yield is lowest in Azolla+urea systems, outperforming Sesbania, farmyard manure, and urea alone (Adhya et al., 2000). Although some Chinese studies reported higher CH₄ emissions under Azolla dual cropping (Ying et al., 2000), soil type and nutrient status strongly influence outcomes. For example, Indian soils tend to produce lower CH₄ flux under similar management (Gollany et al., 2015). Alternative integrated models, such as rice–fish culture, also reduce CH₄ by improving soil aeration, lowering emissions by ~35% compared to conventional paddies (Kollah et al., 2016). Combining Azolla with such climate-smart practices could provide scalable, site-specific solutions for mitigating CH₄ emissions while enhancing rice productivity and sustainability.

Azolla is an efficient phytoremediator with the ability to absorb and accumulate heavy metals from contaminated water. Both living and dead biomass are effective in removing pollutants, owing to mechanisms that include passive adsorption onto cell walls and active metabolic uptake (Sood et al., 2012). Its rapid growth and high bioaccumulation potential enable the uptake of Pb, Cd, As, Cr, and Hg from industrial and agricultural effluents, with removal efficiencies reported up to 80% (Vroom et al., 2024). The mechanism of heavy metal uptake involves passive adsorption onto cell walls as well as active metabolic absorption, making Azolla a suitable candidate for the remediation of polluted wetlands, rivers, and agricultural runoff areas (Sadeghi et al., 2014).

Field studies show that A. pinnata removes 70–94% of heavy metals from effluents, with tissue concentrations up to 740 mg/kg (Rai, 2008). Azolla filiculoides efficiently absorbs Cr, Pb, Zn, Hg, Cu, Cd, Ag, and Ti from wetland environments, demonstrating its potential for metal removal in natural water bodies (Hassanzadeh et al., 2021). Hydroponic studies indicate species-specific differences: A. caroliniana accumulated up to 284 mg/kg of As, while A. filiculoides accumulated only 54 mg/kg (Zhang et al., 2008). Another study reported that A. caroliniana also bioaccumulates Hg and Cr (III and VI), with tissue concentrations between 71 and 964 mg/kg dry weight (Bennicelli et al., 2004).

The tolerance of Azolla to heavy metals varies among species. For example, A. filiculoides demonstrated the highest tolerance to Cr exposure, retaining 72% of its control biomass under contamination (Arora et al., 2006). Nonetheless, heavy metals reduce growth, chlorophyll, and protein content, with Cd and Pb showing the greatest toxicity (Sarkar and Jana, 1986; Guo-Xin et al., 2003; Sood et al., 2012). Stress responses include reduced photosynthesis, O₂ evolution, and enzyme activity, while detoxification is supported by increased phenolics and PAL activity (Dai et al., 2006). Similarly, Hg toxicity in A. pinnata reduces chlorophyll a, protein, RNA, DNA, and nutrient uptake, further compromising growth and metabolic functions (Rai and Tripathi, 2009). Copper particularly affects photosystem II efficiency (Sanchez-Viveros et al., 2010).

Heavy metal exposure also induces ultrastructural damage in Azolla, affecting organelles at the cellular level. Structural disruptions include chloroplast swelling, mitochondrial deformation, chromatin condensation, and nuclear membrane disintegration (Sela et al., 1988, 1990; Sood et al., 2012). Copper accumulates preferentially in roots, whereas Cd is evenly distributed in plant tissues, forming detoxification aggregates with PO₄ and Ca (Sela et al., 1988). Cadmium localizes in the epidermis, cortex, and bundle cell walls within 77 hours, leading to the formation of electron-dense granules (Sela et al., 1990). Lead precipitation in A. filiculoides was primarily localized in vacuoles, with higher accumulation in mature leaves (Benaroya et al., 2004). In A. pinnata, Pb exposure caused frond compactness, stomatal closure, and epicuticular wax deposition, though these effects were mitigated by Fe supplementation (Gaumat et al., 2008).

Beyond metal accumulation, Azolla has demonstrated potential for biosorption. Studies show that dead or pretreated Azolla biomass effectively removes Cs, Sr, Pb, Zn, Ni, Cu, Au, Cd, and Cr from contaminated water sources (Sood et al., 2012). The bioaccumulation capacity of Azolla is influenced by metal concentration and environmental conditions (Dai et al., 2006; Sood et al., 2012). Hydroponic experiments have shown that A. filiculoides can remove Cr⁶⁺ with a maximum adsorption capacity of 20.2 mg/g at pH 2 and 32°C, while Ni uptake reached 27.9 mg/g at 60% saturation (Zhao and Duncan, 1998). Pb removal efficiency by A. filiculoides remained at approximately 90% between 10 and 50°C, with minimal influence from biomass concentration (Sanyahumbi et al., 1998). Furthermore, A. filiculoides has demonstrated a 99.9% efficiency in gold biosorption at pH 2 (Sood et al., 2012).

Apart from heavy metal removal, Azolla effectively removes excess nutrients from wastewater, reducing eutrophication risks. Azolla filiculoides has been reported to extract up to 122 kg of phosphorus per hectare annually (Vroom et al., 2024). Sequential treatment using Landoltia punctata followed by A. filiculoides achieved complete NH₄ and NO₃ removal and a 93% reduction in PO₄, significantly lowering wastewater toxicity (Miranda et al., 2020). In addition, Azolla plays a crucial role in domestic wastewater treatment by removing nitrogen and phosphorus, thereby improving water quality for irrigation (Muradov et al., 2014). Laboratory studies further indicate that A. filiculoides effectively removes textile dyes such as Congo Red, Acid Red 88, Acid Green 3, Acid Orange 7, and Basic Orange from industrial effluents (Tan et al., 2010; Sood et al., 2012). The biosorption potential of Azolla is enhanced by its rapid growth and high surface area, making it an eco-friendly alternative to conventional wastewater treatment methods (Sood et al., 2012).