Isotopic fractionation of O2 during photochemical O2 consumption: A relevant process for estimating primary production in sunlit surface waters?

Isotopic fractionation of O2 is an important tracer for estimating primary production in aquatic environments because it helps to disentangle the respective contributions from O2 production, consumption, and gas-exchange. Isotope-based methods for estimating primary productivity typically involve measurements of either only 18O/16O ratios or, in the case of triple oxygen isotope approaches, also 17O/16O ratios. Aerobic respiration is generally assumed to be the only process consuming O2, with a constant value for O-isotopic fractionation, expressed as ε or λ values, respectively. However, emerging evidence suggests that in the photic zone of lakes and oceans, photochemical O2 consumption can be of similar magnitude as microbial respiration and photosynthetic O2 production. To determine whether photochemical O2 consumption should be included in isotope-based assessments of primary productivity, we measured the O-isotopic fractionation (as 18O-ε and λ values) of two important photochemical O2 consumption reactions. First, we investigated the energy transfer from photochemically excited dissolved organic matter (DOM) to O2, leading to the reversible formation of singlet oxygen, which can irreversibly react with several functional groups within DOM. Under realistic conditions for sunlit surface waters, this photochemical O2 consumption reaction is associated with 18O-ε values of -25 ‰ to -30 ‰, which are larger than typical values for respiration (approx. -20 ‰). The second photochemical process investigated was the reaction between O2 and photochemically produced organic radicals, which yielded substantially smaller values for 18O-ε (0 ‰ to -15 ‰). 18O-ε values for photochemical O2 consumption may thus be distinguishable from those for respiration. Yet, the overall isotopic fractionation in sunlit surface water will depend on the relative contributions of the different photochemical O2 consumption reactions. Although some studies have measured the isotopic fractionation of photochemical O2 consumption in natural water samples, additional research is needed for properly implementing these processes into isotope-based estimations of primary production. Finally, results from triple oxygen isotopic fractionation measurements suggest an overlap of λ values in the range of 0.51-0.53 for photochemical O2 consumption (as determined in this study) and for respiration experiments from literature.