Cross-habitat interactions drive methylmercury contamination in a disturbed river ecosystem: novel metagenomic and biogeochemical insights.

Understanding contaminant dynamics in ecosystems requires considering interactions between habitats-an aspect often overlooked in research. Mercury (Hg) studies typically focus on methylmercury (MeHg) production in sediments, often neglecting the role of biofilms such as periphyton. This study analyzes sediments and periphyton in a disturbed river using biogeochemical and metagenomic approaches. We found that microbial communities differed between habitats, but Hg-methylating microbes were taxonomically similar, with higher abundance in sediments. Organic matter (OM), a key Hg vector, likely affects Hg dynamics differently across habitats: MeHg concentrations increased with increasing terrigenous OM in sediments, whereas in periphyton, MeHg increased with greater contributions of aquatic-derived OM. Surprisingly, periphyton showed higher MeHg concentrations than sediments, despite lower hgcA abundance, the gene associated with MeHg production. Our multi-indicator analysis provides a conceptual model suggesting that MeHg is primarily produced in active sediments (indicated by elevated carbon dioxide and methane), diffuses into the water column (supported by carbon dioxide-MeHg correlations), and accumulates in protein-rich periphyton in shallow, low-flow waters where prolonged exposure can enhance MeHg retention. While some MeHg production occurs in periphyton, especially at a wetland site with thick growth, periphyton at a hydroelectric-impacted site showed the highest MeHg levels despite absent hgcA and methylation activity, pointing towards MeHg retention from the water. As a major food source for primary consumers, periphyton likely redistributes accumulated MeHg through the food web. This study highlights the importance of considering MeHg transfer between habitats and the need to examine entire aquatic ecosystems to fully understand MeHg dynamics.