Heavy metal transformation in mangrove ecosystems: a multi-scale perspective from intertidal dynamics to plant adaptive responses

Mangrove ecosystems function as vital biogeochemical interfaces between terrestrial and marine environments, playing a crucial role in transforming heavy metals (HMs). However, this ecosystem is heavily impacted by climate change and anthropogenic activity, including an increase in HM toxicity. The current review synthesizes understanding of HM transformation across three interconnected levels: tidal dynamics, rhizosphere processes, and plant adaptation strategies. Initially, tidal inundation affects the distribution, speciation, and mobility of HMs by altering sediment biogeochemical properties, including pH, redox potential, salinity, and microbial activity. Further, tidal effects influence metal immobilization and remobilization, thereby impacting HM behavior within the rhizosphere, which serves as a secondary barrier to metal transport. Activities in the rhizosphere, including the presence of microbes, generate redox micro-gradients, and release organic ligands that facilitate metal complexation, precipitation, and detoxification. The synergistic interactions between roots and microbes support rhizoremediation in mangrove systems, lowering HM toxicity, and enhancing sediment stability. Additionally, mangroves employ various structural, physiological, and biochemical strategies, including selective metal uptake, excretion, internal detoxification systems, and the activation of antioxidant enzymes, to reduce HMs-induced stress. However, adaptation mechanisms differ among species and are influenced by interactions between tidal regimes, rhizosphere conditions, and plant traits. Integrating the three hierarchical levels-tide, root, and plant-highlights that mangrove ecosystems function as self-regulating biogeochemical systems capable of stabilizing and transforming HMs under dynamic environmental conditions. Such integrative mechanisms advance nature-based remediation strategies and reinforce mangroves' role as effective natural barriers against HM pollution, thereby contributing to sustainable coastal management and ecosystem resilience in a changing global environment.