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.