Autochthonous DOM governs the occurrence and methylation of mercury-containing nanoparticles in urban river water: An integrated study of field investigations and laboratory experiments.

The methylation of mercury-containing nanoparticles (Hg-NPs) into neurotoxic methylmercury (MeHg) poses threats to wildlife and humans. However, how autochthonous dissolved organic matter (DOM) affects the occurrence and MeHg production of Hg-NPs remains poorly understood. In this study, the distribution of Hg-NPs and their interactions with autochthonous DOM in the Nanfei River, an urban river in eastern China, were systematically explored. Field investigations revealed that mass concentrations of Hg-NPs increased along the flow path, rising from 0.10 ± 0.06 to 0.91 ± 0.21 ng·L, and were positively correlated with MeHg levels (p < 0.001), indicating that Hg-NPs (predominantly < 60 nm in diameter) are bioavailable substrates for Hg methylation in the river. The spatial distribution of Hg-NPs was primarily driven by the increasing level and changing composition of DOM along the river, particularly by fractions with a higher degree of humification and higher molecular weight. Laboratory experiments showed that algae-derived DOM, which has a higher content of aromatic protein, was more effective in promoting Hg-NP formation and maintaining their net MeHg production than other DOM sources. These findings highlight the critical role of autochthonous DOM, particularly algae-derived DOM, in regulating the occurrence and methylation of Hg-NPs, providing new insights into MeHg risk prediction under algal blooms.