Photochemical mineralization of synthetic lignin in lake water indicates enhanced turnover of aromatic organic matter under solar radiation

The degradation of C-14-[ring]-labelled synthetic lignin (C-14-DHP) and dissolved organic carbon (DOC) from lake water were studied simultaneously. C-14-DHP was incubated in humic lake water (colour 173 mg Pt l(-1)) for 7 d in the dark or under solar radiation. In the dark < 0.4% of the introduced C-14-DHP label and 4% of the indigenous DOC were mineralized, indicating that the C-14-labelled aromatic rings of DHP and the humic DOC were microbiologically recalcitrant. Under solar radiation (116 MJ m(-2)), 17-21% of the C-14-labelled carbons in DHP and 18-23% of the indigenous DOC were mineralized in 7 d. Simultaneously the water solubility of C-14-DHP increased. Solar radiation converted the aromatic cores of synthetic lignin to CO2 and soluble organic photoproducts. The results suggest that solar radiation plays a key role in the decomposition of natural polyaromatic matter.