The precise mass dependence of respiratory O₂ consumption underpins the “oxygen triple-isotope” approach to quantifying gross primary productivity in modern and ancient environments. Yet, the physical-chemical origins of the key ¹⁸O/¹⁶O and ¹⁷O/¹⁶O covariations observed during respiration have not been tied to theory; thus, the approach remains empirical. First-principles calculations on enzyme active-site models suggest that changes in the O–O bond strength upon electron transfer strongly influence respiratory isotopic fractionation. However, molecular diffusion may also be important. Here, we use measurements of the relative abundances of rare isotopologues ¹⁷O¹⁸O and ¹⁸O¹⁸O as additional tracers of mass dependence during dark respiration experiments of lacustrine water. We then compare the experimental results to first-principles calculations of O₂ interacting with heme-oxidase analogues. We find a significantly steeper mass dependence, supported by theory, than has been previously observed. Enrichments of ¹⁷O¹⁸O and ¹⁸O¹⁸O in the O₂ residue suggest that θ values are strongly influenced by chemical processes, rather than being dominated by physical processes (i.e., by bond alteration rather than diffusion). In contrast, earlier data are inconsistent with theory, implying that analytical artifacts may have biased those results. Implications for quantifying primary productivity are discussed.

中文翻译：

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呼吸过程中什么会分解氧同位素？多种同位素同位素计量和理论的真知灼见

呼吸O ₂消耗的精确质量依赖性是“氧三同位素”方法的基础，该方法用于量化现代和古代环境中的总初级生产力。但是，在呼吸过程中观察到的关键¹⁸ O / ¹⁶ O和¹⁷ O / ¹⁶ O协变的物理化学成因尚未与理论联系在一起。因此，该方法仍然是经验性的。酶活性位点模型的第一性原理计算表明，电子转移后O-O键强度的变化会强烈影响呼吸同位素分级。但是，分子扩散也可能很重要。在这里，我们使用稀有同位素同位素¹⁷ O ¹⁸的相对丰度的测量O和¹⁸ O ¹⁸ O作为湖水暗呼吸实验中质量依赖性的附加示踪剂。然后，我们将实验结果与O ₂与血红素氧化酶类似物相互作用的第一性原理计算进行比较。我们发现，理论支持下的质量依赖性比以前观察到的要陡得多。O _2中的¹⁷ O ¹⁸ O和¹⁸ O ¹⁸ O富集残留表明θ值受化学过程的强烈影响，而不是受物理过程（即，键改变而不是扩散）支配。相反，早期的数据与理论不一致，这意味着分析伪像可能会使这些结果产生偏差。讨论了量化初级生产力的含义。