Photogeneration of singlet oxygen and free radicals in dissolved organic matter isolated from the Mississippi and Atchafalaya River plumes

Abstract

The photoreactivity to UV light of ultrafiltered dissolved organic matter (DOM) collected during cruises along salinity transects in the Mississippi and Atchafalaya River plumes was examined by measuring photogenerated free radicals and singlet molecular oxygen (1O2) photosensitization. Singlet oxygen was detected by its infrared phosphorescence at 1270 nm using both steady-state and time-resolved techniques. The 1O2 quantum yields were corrected for self-quenching of 1O2 by the DOM substrates. Photogenerated free radicals were monitored by electron paramagnetic resonance (EPR). Two size fractions of the dissolved organic matter were examined: material retained with a 3 kDa cut-off filter and material retained with a 1 kDa cut-off filter. The highest 1O2 quantum yields were found in the lower molecular mass material. There was little change in the 1O2 quantum yields with increasing salinity, indicating that the photosensitizing ability of the estuarine DOM does not decrease as terrestrial DOM is transported to sea and mixes with marine DOM. In contrast to 1O2 formation, the steady-state levels of photoproduced free radicals did not significantly differ between high and low molecular mass DOM, and the levels were substantially higher in riverine DOM than along plume salinity transects. This rapid transition in free radical level suggests that terrestrially-derived DOM experiences significant changes in this aspect of its photoreactivity in low (<10 ppt) salinity waters.