Examining superoxide dynamics in irradiated natural waters

Superoxide (O2-$$ {\mathrm{O}}_2<^>{-} $$) is a reactive oxygen species (ROS) that is primarily produced by the one-electron transfer of photooxidized chromophoric dissolved organic matter (CDOM) to O-2 in sunlit natural waters. Here we examine the environmental and chemical parameters (pH, ionic strength, buffer, and halides) that may influence O2-$$ {\mathrm{O}}_2<^>{-} $$ photochemical production rates and decay pathways in natural water. Using the enzyme superoxide dismutase and H2O2 measurements, we present results from an irradiated freshwater CDOM source indicating that O2-$$ {\mathrm{O}}_2<^>{-} $$ reductive decay pathways (P/P-SOD) dominate with increased pH and NaCl additions and maximal photoproduction rates (RO2-$$ {R}_{{\mathrm{O}}_2<^>{-}} $$) increase with carbonate compared to borate buffer. Over 2 h of irradiation, a significant decline in RO2-$$ {R}_{{\mathrm{O}}_2<^>{-}} $$ was seen for all samples along with a minor increase in oxidative pathways. These results imply shifts in O2-$$ {\mathrm{O}}_2<^>{-} $$ decay pathways and production rates that seem to vary across natural waters and as a function of irradiation history.

Automated summary

This study examined the influence of environmental and chemical parameters on the production rates and decay pathways of O2- in natural water. The results showed that pH, ionic strength, buffer, and halides all had an impact on the production and decay of O2-. The reductive decay pathways of O2- were dominant with increased pH and NaCl additions, and the maximal photoproduction rates of O2- increased with carbonate buffer. After 2 hours of irradiation, there was a significant decline in O2- production rates and a minor increase in oxidative pathways. These findings suggest variations in O2- decay pathways and production rates across natural water and as a function of irradiation history.