Heavy Metals Fate in Mangrove Wetlands: Integrated Insights into Sediment-Microbe-Plant Interactions and Fraction Control

To unravel the complex biogeochemical fate of heavy metals (HMs) in mangrove ecosystems, this study disentangled the environmental drivers of their total distribution versus their ecological bioavailability. We employed a comparative Partial Least Squares Path Modeling (PLS-PM) approach across four mangrove areas under a distinct anthropogenic pressure gradient in the Beibu Gulf. The models revealed a critical divergence: while sediment physical structure was the primary factor controlling the total distribution of HMs, their bioavailability was more intricately governed by a combination of sediment chemistry (salinity) and nutrient cycling (N, P). This fraction-controlled bioavailability, in turn, was a significantly better predictor of biological responses. For phytoaccumulation, while root uptake correlated with sediment bioavailability, the metal loads in aboveground tissues were largely decoupled from sediment conditions, challenging risk assessment paradigms based solely on sediment data. For microbial communities, their structure was more strongly associated with metal bioavailability than total concentrations, with these bioavailable fractions appearing to exert selective pressure on key taxonomic groups. This study demonstrates that a fraction-centric approach, which separates the drivers of distribution from those of bioavailability, is essential for mechanistically understanding and accurately managing HMs risks in coastal wetlands.