Photochemical reactivity of dissolved lignin in river and ocean waters

The photochemical reactivity of dissolved lignin and photobleaching of dissolved organic matter (DOM) were examined in riverine and open-ocean water samples. Approximately 75% of the total dissolved lignin in Mississippi River water was lost during 28 d of incubation in sunlight, mostly due to photooxidation. The remaining fraction of dissolved lignin was much less susceptible to photooxidation. About 90% of the dissolved lignin in river water was present as high-molecular-weight (HMW; >1,000 Dalton) DOM. However, after exposure to sunlight, about 80% of the remaining lignin was present as low-molecular-weight (LMW; <1,000 Dalton) DOM. An absolute increase in concentration of LMW lignin provided direct evidence for the phototransformation of macromolecular DOM to smaller molecules. The composition of riverine dissolved lignin also changed dramatically during photooxidation. The abundance of syringyl relative to vanillyl phenols decreased twofold, while concentrations of vanillic acid relative to vanillin, (Ad/Al)v, increased fourfold. The increase in (Ad/Al)v was found exclusively in the photooxidized LMW DOM. The photochemically altered composition of riverine dissolved lignin was consistent with the composition of oceanic dissolved lignin. The dissolved lignin in HMW DOM from the equatorial Pacific Ocean was highly resistant to photooxidation. These findings suggest that photochemical reactions play a prominent role in determining the composition and reactivity of terrigenous DOM in the ocean.