Vegetation of exotic fast-growing species Sonneratia apetala increases the potential of methylmercury production: Insights from carbon bioavailability, microbial metabolism and mercury methylators.

Mangrove sediments are hotspots for neurotoxic methylmercury (MeHg) production, with litter-derived organic carbon strongly affecting mercury (Hg) methylation. However, the specific role of carbon bioavailability in regulating net MeHg production remains unclear. This study investigated sediments vegetated by exotic fast-growing Sonneratia apetala (SA) and native Kandelia obovata (KO) in southern China. Contrary to the expectation that larger carbon pools enhance methylation, MeHg levels were 2.1-2.6 times higher in SA sediments despite KO containing 1.2-4.2 times more total organic carbon. This disparity was driven by carbon bioavailability: SA sediments exhibited a significantly higher proportion of available carbon (34-50%) compared to KO (28-36%), which stimulated microbial activity and enriched Hg-methylating microbes (1.4-3.3 times higher in hgcAB gene abundance). Metagenomics showed that SA not only promoted key Hg-methylating taxa (e.g., Desulfobacterales, Syntrophobacteria) but also upregulated their metabolic pathways for labile carbon use and methyl transfer to Hg. Our results demonstrate that carbon bioavailability, governed by species-specific litter chemistry, is the key driver of net MeHg production. The findings provide an in-depth understanding of Hg biogeochemistry by linking soil carbon quality to microbial metabolic networks, and offer novel insights for evaluating the ecological risks associated with exotic species in mangrove restoration.