2. Mycorrhizal symbiosis: biological and functional foundations
Mycorrhiza is a mutualistic association between soil fungi and the root system of most plants. This interaction, which evolved over millions of years, is crucial for plant nutrition and resilience in difficult environmental conditions. The fungus, through its network of hyphae, explores a volume of soil significantly greater than that accessible to the roots alone, facilitating the absorption of water and low-mobility nutrients, such as phosphorus (P) and nitrogen (N). In return, the plant provides the fungus with carbohydrates produced through photosynthesis.
There are different types of mycorrhizae; the most studied for bioremediation are ectomycorrhizae and endomycorrhizae (particularly arbuscular mycorrhizae, or AM). Ectomycorrhizae form an external mantle on the roots and an intercellular network (Hartig net), while AM develop inside the cortical cells of the roots, forming specialized structures like arbuscules and vesicles.

3. Remediation mechanisms activated by mycorrhizal symbiosis
The ability of mycorrhizae to mitigate the toxicity of heavy metals is based on a complex interaction of physical, chemical, and biological processes, which can be classified as follows:
3.1. Immobilization (Biosorption)
This is the primary mechanism. The fungal biomass, particularly the cell walls of the hyphae, acts as an efficient biosorbent. The cell walls are rich in polysaccharides (like chitin and chitosan), proteins, and melanins that offer numerous binding sites for heavy metal ions. Through ion exchange and surface complexation processes, the fungi sequester and immobilize the heavy metals within their extracellular biomass. This significantly reduces the concentration of free and bioavailable metal ions in the rhizosphere, protecting the host plant from toxic uptake.
3.2. Chelation and secretion of organic acids
Mycorrhizal fungi can secrete a variety of compounds into the soil, including low-molecular-weight organic acids (e.g., citric acid, oxalic acid) and phytochelatins (in specific cases). These compounds bind to soluble metal ions, forming stable complexes that reduce their mobility and bioavailability. The chelation process helps prevent metal leaching into groundwater and keeps contaminants in a non-toxic form.
3.3. Modification of soil pH and enzyme release
The metabolic activity of mycorrhizal fungi can induce changes in the pH of the rhizosphere microenvironment. The secretion of protons or other acidic or alkaline compounds can alter the solubility of heavy metals; for example, increasing the pH in acidic soils can promote the precipitation of some metals, reducing their availability. Some studies also suggest that fungi can release specific enzymes that contribute to the transformation of toxic forms of metals or metalloids (e.g., arsenic) into less harmful forms.
3.4. Increased host plant tolerance
Mycorrhizal symbiosis not only acts on soil remediation but also confers greater tolerance to heavy metal-induced stress on the plant. Mycorrhizae improve the plant's water and nutritional status, making it more robust and resilient. In some cases, fungi can even regulate the transport of metals within the plant, sequestering them in less sensitive cellular compartments (e.g., vacuoles) to minimize damage.

4. Applications and future potential
Mycoremediation represents a promising approach for the remediation of contaminated sites, particularly for phytostabilization, where the goal is to immobilize contaminants in the soil to prevent their dispersion. Although research has already demonstrated the effectiveness of this technique under laboratory and pilot-scale conditions, its large-scale application requires a deeper understanding of the specific interactions between fungi, plants, and contaminants in different environmental contexts. The identification of highly efficient fungal and plant species and their targeted inoculation at contaminated sites represent future challenges. Mycoremediation stands as a key component of an integrated and sustainable approach to environmental recovery, capable of restoring not only the chemical quality of the soil but also its ecological functionality.

5. Bibliography
1. Joner, E. J., Briones, R., & Leyval, C. (2000). Role of mycorrhizal hyphae on the regulation of heavy metal mobility in soil. Soil Biology and Biochemistry, 32(11), 1651-1655.
2. Khan, A. G. (2006). Mycoremediation of toxic metals: The potential of fungi. In: Soil Remediation with Mycorrhizal Symbioses: A Global Perspective. Springer, Dordrecht.
3. Garbisu, C., & Alkorta, A. (2001). Phytoextraction: A technology for cleaning contaminated soils? Reviews in Environmental Science and Bio/Technology, 1(2), 163-181.
4. Cairney, J. W. G., & Meharg, A. A. (2003). Mycorrhizae, heavy metal-tolerant fungi, and phytoremediation. Journal of the British Mycological Society, 107(1), 165-177.
5. Arias, A. J., & Valdés, C. J. (2010). Mycorrhizae in phytoremediation: A review. Journal of Environmental Science and Engineering, 4(2), 123-134.
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