Divergent molecular transformation networks converge on enhanced halogenated DOM formation in lakes under anthropogenic pressure: Insights across China and the United States.

Halogenated dissolved organic matter (HDOM) is considered one of the most toxic fractions of DOM, as it potentially contributes to the formation of disinfection byproducts, which are documented to pose serious risks to water safety. Previous studies mainly focused on bulk DOM transformation and characterized its molecular diversity, while less attention has been given to the formation mechanisms and environmental drivers of HDOM that plays a key role in toxicity outcomes, particularly in lake systems relied upon as human water sources. Here, we elucidated molecular HDOM transformation dynamics in selected lakes in China and the U.S. using FT-ICR MS, interpretable machine learning, and PageRank molecular transformation networks. Results suggest that DOM halogenation in selected lakes from both countries is consistently dominated by oxidation-mediated pathways, enhancing the pool of halogenated precursors relevant to potential disinfection byproducts formation. Molecular networks reveal that dehalogenation and halogenation in U.S. lakes exhibit selective localized transformations, while Chinese lakes follow interconnected pathways, revealing region-specific biogeochemical behaviors of HDOM under anthropogenic pressures and molecular diversity. However, despite divergent molecular networks and pathways, lakes subjected to inevitably intense anthropogenic pressures in both countries exhibit a consistent tendency toward enhanced halogenation (increased toxicity) and accelerated carbon cycling. DOM structure (aromaticity, aliphaticity, and unsaturation) strongly regulates halogenation reactivity in lakes. Specifically, iodination preferentially targets electron-rich aromatic functional groups, whereas chlorination and bromination exhibit broader reactivity across both aromatic and aliphatic structures. This study provides cross-regional insights into how anthropogenic pressures regulate molecular pathways underlying HDOM transformation.