AUTHOR=Powers Leanne C. , Miller William L. TITLE=Apparent Quantum Efficiency Spectra for Superoxide Photoproduction and Its Formation of Hydrogen Peroxide in Natural Waters JOURNAL=Frontiers in Marine Science VOLUME=3 YEAR=2016 URL=https://www.frontiersin.org/journals/marine-science/articles/10.3389/fmars.2016.00235 DOI=10.3389/fmars.2016.00235 ISSN=2296-7745 ABSTRACT=

Superoxide (O2-) is a key intermediate in the cycling of organic matter and trace metals in natural waters but production rates are difficult to determine due to low steady-state concentrations, rapid decay rates, and unstable standards. On the other hand, superoxide's dismutation product, hydrogen peroxide (H₂O₂), is relatively stable in filtered water. Thus, if the stoichiometry between O2- and H₂O₂ is known, one can derive superoxide data from H₂O₂ measurements. The relationship between O2- and H₂O₂ remains uncertain in seawater but work by Petasne and Zika (1987) presented a method for examining the relationship between O2- and H₂O₂ during irradiations of coastal seawater using superoxide dismutase (SOD), which forces a 2:1 stoichiometry between O2- and H₂O₂. Here we report the first O2- apparent quantum yield (AQY) spectra following their approach; performing irradiations of various fresh and seawater samples and measuring H₂O₂ accumulation with and without added SOD. For all but a single riverine sample, H₂O₂ AQY spectra fell in a narrow range, but O2- AQY spectra varied such that O2-:H₂O₂ ratios were always >2 and were highest for the clear waters of the Gulf Stream (~3.4 O2- per H₂O₂ generated). Because this approach eliminates the need to measure O2- production rates directly, it represents a simple way to refine the stoichiometric relationships that would potentially allow global estimates of O2- photoproduction rates, O2- steady-state concentrations ([O2-]_ss), and related surface ocean redox reactions based on more manageable H₂O₂ photochemical studies.