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

The photoreactivity to UV light of ultrafiltered dissolved organic matter ( DOM) collected during cruises along salinity transects in the Mississippi a nd Atchafalaya River plumes was examined by measuring photogenerated free r adicals and singlet molecular oxygen (O-1(2)) photosensitization. Singlet o xygen was detected by its infrared phosphorescence at 1270 nm using both st eady-state and time-resolved techniques. The O-1(2) quantum yields were cor rected for self-quenching of O-1(2) by the DOM substrates. Photogenerated f ree radicals were monitored dy electron paramagnetic resonance (EPR). Two s ize fractions of the dissolved organic matter were examined: material retai ned with a 3 kDa cut-off filter and material retained with a 1 kDa cut-off filter. The highest O-1(2) quantum yields were found in the lower molecular mass material. Then was little change in the O-1(2) quantum yields with in creasing salinity, indicating that the photosensitizing ability of the estu arine DOM does not decrease as terrestrial DOM is transported to sea and mi xes with marine DOM. In contrast to O-1(2) formation, the steady-state leve ls of photoproduced free radicals did not significantly differ between high and low molecular mass DOM, and the levels were substantially higher in ri verine DOM than along plume salinity transects. This rapid transition in fr ee radical level suggests that terrestrially-derived DOM experiences signif icant changes in this aspect of its photoreactivity in low (< 10 ppt) salin ity waters. Published by Elsevier Science B.V.