Adaptation and remediation strategies of mangroves against heavy metal contamination in global coastal ecosystems: A review

Coastal areas are facing increasing heavy metal contamination from various human activities, posing a severe threat to aquatic and coastal biodiversity. Mangroves, abundant in coastal and marine ecosystems, are particularly vulnerable to the accumulation of toxic inorganic pollutants and heavy metals (HMs). Mangroves have proved to be effective in mitigating heavy metal pollution through a range of mechanisms, including avoidance, chelation, sequestration, tolerance, excretion, and filtration. Studies highlighted a higher metal bioconcentration of metals in mangrove roots compared to leaves, indicating their remarkable endurance capability towards high levels of HMs. The uptake of HMs by mangrove roots occurs through passive diffusion across cell plasma membranes or active transport, regulated by metal ion transporters and influenced by electrochemical and/or concentration potential gradients. In phytoremediation strategies, mangrove plants employ several physiochemical mechanisms to avoid HMs uptake, such as cell wall immobilization, complexation with biomolecules like phytochelatins (PCs), and extracellular barriers at the root endodermis. Moreover, mangroves can develop iron-plaques on their root surface, acting as reservoirs for immobilized HMs and significantly influencing their sequestration, particularly in wastewater containing Cr, Pb, and Ni ions. Mangroves would also trigger antioxidant defenses to cope with the absorbed HMs and normalize the excessive production of reactive oxygen species (ROS)-mediated oxidative stress. However, the antioxidant response in mangroves varies depending on the concentration and type of toxic HMs, plant species, and exposure duration. Additionally, mangroves excrete excessive amounts of HMs through salt glands via adaxial trichomes, allowing them to mitigate phytotoxicity. Although mangroves demonstrate a high tolerance capacity towards HMs, hydrological alteration, deforestation, marine construction, and marine anthropogenic activities pose significant threats to several global mangrove species. In conclusion, conducting multiple toxicity tests using early mangrove seedlings are imperative that threshold effect concentrations of various HMs be determined under natural environmental conditions encompassing nutrients, salinity, temperature, and the presence of several contaminants. Besides, long-term, large-scale mesocosm experiments in real-field settings should be conducted in future to obtain more realistic and reliable insights into the uptake and tolerance of HMs in mangroves.