Mangroves increased the mercury methylation potential in the sediment by producing organic matters and altering microbial methylators community

Mangrove ecosystem has attracted global attention as a hotspot for mercury (Hg) methylation. Although numerous biotic and abiotic parameters have been reported to influence methylmercury (MeHg) production in sediments, the key factors determining the elevated MeHg levels in mangrove wetlands have not been well addressed. In this study, Hg levels in the sediments from different habitats (mudflats, mangrove fringe, and mangrove interior) in the Futian mangrove wetland were investigated, aiming to characterize the predominant factors affecting the MeHg production and distinguish the key microbial taxa responsible for Hg methylation. MeHg concentrations in the sediments from the mangrove interior (1.03 ± 0.34 ng g^-1 dw) were significantly higher than those in mudflats (0.26 ± 0.08 ng g^-1 dw) and mangrove fringe (0.45 ± 0.10 ng g^-1 dw). Mangrove vegetation also promoted the accumulation of organic matters in sediments, which stimulated the growth of methylators, ultimately leading to an elevated MeHg level in the sediment. The data from 16S sequencing and random forest analysis further indicated that the increased abundances of Desulfococcus and Desulfosarcina, which belong to complete-oxidizing microbes with acetyl-CoA pathway and are favored by mangrove vegetation, were the primary contributors to MeHg production. Besides, syntrophic partners of methylators (e.g. Syntrophus) also play a considerable role in MeHg production. The present findings provide a deep understanding of Hg-methylation in mangrove wetlands, and offers valuable insights into of the interactions between mangrove plants and soil microbiome in the presence of Hg contamination.