Source-dependent DOM transformation regulates methylmercury photodegradation in urban waters.

Methylmercury (MeHg) photodegradation is a major natural attenuation process in surface waters, yet how dissolved organic matter (DOM) source and transformation regulate this process remains unclear. Here, watershed-scale field investigations combined with laboratory decomposition experiments were conducted to elucidate the role of DOM heterogeneity in MeHg photodegradation. Surface waters from the Nanfei River exhibited pronounced spatial differences in DOM composition, with humic-rich macrophyte-derived and protein-rich algae-derived DOM (M- and A-DOM) showing contrasting photochemical behaviors. MeHg photodegradation rate constants ranged from 0.054 to 0.112 h-1 and were positively correlated with DOM aromaticity, humic-like components, and ·OH production, but negatively correlated with protein-like fractions. During decomposition, A-DOM initially suppressed MeHg photodegradation by up to 87% because of protein-rich components and photon-screening effects, whereas M-DOM consistently promoted photodegradation. Progressive humification enhanced MeHg photodegradation in both DOM types by stimulating photochemically produced reactive intermediates (PPRIs) and photosensitized reactions. These results demonstrate that DOM quality and transformation state, rather than bulk DOM concentration, govern MeHg photochemical fate. This study establishes a mechanistic framework linking eutrophication-driven DOM transformation with Hg photogeochemical cycling and improves predictions of MeHg persistence in productive freshwater ecosystems.