Dissolved organic matters with low molecular weight fractions exhibit high photochemical potential for reactive oxygen formation

The photochemical properties of dissolved organic matter (DOM) were highly related to the molecular weight (MW) and organic compositions. In this study, the bulk algae- and macrophyte-derived DOM (ADOM and MDOM, respectively) and Suwannee River humic acid (SRHA) were applied and fractionated into low MW- (LMW, <1 kDa) and high MW-(HMW-, 1 kDã0.45 μm) fractions. The formation and mechanisms of photochemically produced reactive intermediates (e.g., HO•, 1O2, and 3CDOM*) for these bulk and MW-fractionated samples were compared via the irradiation experiment, fluorescence and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). Results showed that humic-/fulvic-like substances were mainly distributed in the LMW fraction which occupied about 44–60% of total organic carbon for ADOM and MDOM and 13% for SRHA. Photochemical experiments showed that the autochthonous DOMs (e.g., ADOM and MDOM) were characterized with comparable formation rates and quantum yields of reactive oxygens with the allochthonous SRHA, suggesting the high photochemical formation potential. Further analysis showed obvious MW-dependent heterogeneities that, irrespective of DOM types, the LMW-fraction exhibited higher formation rates and quantum yields, followed by the bulk- and then the HMW-fractions. The fluorescence and FT-ICR-MS results indicated that the unique biochemical classes, i.e., humic-/fulvic-like moieties and protein-/lipid-derived compounds in the LMW fractions may be responsible for the high apparent quantum yields. This study highlighted the importance of simultaneous characterization of MW and organic compositions for evaluating the photochemical potential and other behaviors and effects of aquatic DOMs.