Multiple-oxygen isotope constraints on aerobic respiration and gross primary productivity

Primary production on land and in the surface ocean is a critical component of Earth's carbon and oxygen cycles, controlling the uptake of CO₂ and the release of O₂ to the atmosphere. Quantitative estimates of these O₂ and CO₂ fluxes remain challenging. Of the methods used, the triple-oxygen-isotope analysis of O₂ is considered to provide the most robust in situ estimate of gross productivity. However, this method relies on one key assumption-that the oxygen isotope fractionation of aerobic respiration is constant-and recent experimental studies have shown that it can vary significantly. When applied to the same calculation of gross productivity, the experimentally determined variation in fractionation could produce [Formula: see text]100% error. In this work, the mechanistic origin of the variation in respiratory fractionation is explored using experiments on the model organism Escherichia coli. These experiments suggest that the fractionation is strongly correlated with the cell-specific O₂ consumption rate, weakly correlated with the dissolved-O₂ concentration, and may also depend on the identity of the terminal oxidase enzyme used to reduce O₂. Modeling suggests that these influences on fractionation are likely to be important in the marine water column, and could produce up to [Formula: see text]120% error in estimates of gross productivity made using triple-oxygen-isotope analyses. This error may be significantly reduced through concurrent analyses of the "clumped" (multiply substituted) isotopologue abundances in O₂.