Site-specific associations between soil methylmercury accumulation and plant, soil, and hgcA-associated microbial modules across Chinese coastal wetlands.

Coastal wetlands retain mercury (Hg), but factors associated with methylmercury (MeHg) accumulation may differ among systems. We sampled 105 plots across seven Chinese coastal wetlands and measured total Hg (THg), MeHg, soil properties, plant attributes, and hgcA-associated microbial communities. We calculated the apparent Hg methylation ratio (%MeHg = 100 × MeHg/THg) and examined site-specific associations using ordination, PERMANOVA, Bayesian hierarchical models, variation partitioning, and within-site regressions. THg, MeHg, and %MeHg showed different spatial patterns. Minjiangkou had the highest THg and lowest %MeHg; Yancheng had low THg and MeHg but high %MeHg. At Yancheng, the high ratio partly reflected low THg and did not indicate high absolute MeHg or an elevated methylation rate. After accounting for site, Plant and Soil each retained an independent adjusted fraction of 8.9% for %MeHg; Plant retained 14.4% for MeHg. The hgcA-associated module added no positive independent adjusted fraction (-0.5% for %MeHg; -2.0% for MeHg). Within-site correlates differed among wetlands and responses, and only some were FDR-supported. THg alone was not a surrogate for MeHg accumulation or relative enrichment. These associations should be tested in process studies; they do not establish causal controls or quantify ecological risk.