Effects of Photoirradiation on Natural Organic Matter: Importance for Studies of Stream Hydrobiogeochemistry

Natural organic matter (NOM) is a key component of lakes, streams, and wetlands, and it plays an important role in many biogeochemical processes. We investigated changes in molecular weight (by size exclusion chromatography, SEC), fluorescence, and UV/Vis absorbance of bulk filtered surface water NOM (SW) and XAD-8 and XAD-4 isolates upon irradiation in a solar simulator to 48 hours. Upon irradiation of SW and XAD-8 samples, the higher molecular weight NOM components were initially degraded to form more fluorescent intermediate molecular weight components, along with lower molecular weight components. After several hours of irradiation, the intermediate molecular weight fluorescent components degraded; little or no production of lower molecular weight components was detected by SEC. Hence, the overall sample fluorescence increased within the first few hours of irradiation and then decreased. The XAD-4 isolate did not contain high molecular weight components that could break down to form additional fluorescent components; therefore, irradiation resulted in a more simple and continuous decrease in molecular weight and fluorescence. Ongoing studies are focusing on the potential role of Fe in the observed changes to the NOM properties. These results demonstrate that photo-induced changes in NOM properties are a complex function of sample composition as well as amount of exposure to sunlight. This research further indicates that studies of NOM-related processes in surface waters need to consider the potential for a complex sequence of photo-induced changes to the NOM properties that are likely to affect reactivity.