Weak isotopic fractionation of dissolved O₂ during community respiration

The isotopic composition of dissolved O₂ (δ¹⁸O) in aquatic environments is strongly affected by the preferential uptake of the lighter isotopologue during biological consumption processes. Numerous studies have shown that during incubation experiments, the isotopic effect of microorganism respiration (ε_organism) is on the order of −20‰. However, studies of the co-variations of O₂ and δ¹⁸O in natural environments show considerably weaker in situ fractionation (ε_app). A possible explanation for this discrepancy is that a significant fraction of the O₂ consumption is diffusion-limited. Although this is a generally accepted mechanism in sediments, it cannot explain the weak fractionations observed in mid-ocean sites. Here, we analyze a time series of O₂, δ¹⁸O, and auxiliary data from the northern Gulf of Aqaba (Red Sea). Although an incubation experiment showed strong fractionation against the heavy isotopologue (ε_organism = −24.5‰), the in situ ε_app was only −14‰ in deep water isolated from the photic zone. We show that this result requires an additional O₂ consumption mechanism with weak fractionation, rather than mixing, and suggest that this mechanism is diffusion-limited respiration into aggregates of organic material. We estimate that this mechanism could be responsible for 30% of the O₂ consumption in the Gulf and suggest that it may also constitute a major O₂ consumption pathway in the world's oceans.