ACS Earth and Space Chemistry: What Fractionates Oxygen Isotopes During Respiration? Insights from Multiple Isotopologue Measurements and Theory

Jeanine L. Ash, Huanting Hu, and Laurence Y. Yeung


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.

doi: 10.1021/acsearthspacechem.9b00230

Environmental Science & Technology: In situ quantification of biological N2 production using naturally occurring 15N15N

Laurence Y. Yeung, Joshua A. Haslun, Nathaniel E. Ostrom, Tao Sun, Edward D. Young, Maartje A. H. J. van Kessel, Sebastian Lücker, and Mike S. M. Jetten


We describe an approach for determining biological N2 production in soils based on the proportions of naturally occurring 15N15N in N2. Laboratory incubation experiments reveal that biological N2 production, whether by denitrification or anaerobic ammonia oxidation, yields proportions of 15N15N in N2 that are within 1‰ of that predicted for a random distribution of 15N and 14N atoms. This relatively invariant isotopic signature contrasts with that of the atmosphere, which has 15N15N proportions in excess of the random distribution by 19.1‰. Depth profiles of gases in agricultural soils from the Kellogg Biological Station Long-Term Ecological Research site show biological N2 accumulation that accounts for up to 1.6% of the soil N2. One-dimensional reaction-diffusion modeling of these soil profiles suggests that subsurface N2 pulses leading to surface emission rates as low as 0.3 mmol N2 m-2 d-1 can be detected with current analytical precision, decoupled from N2O production.


JGR-Biogeosciences: Short-Term Changes in Physical and Chemical Properties of Soil Charcoal Support Enhanced Landscape Mobility

Combinatorial effects on clumped isotopes and their significance in biogeochemistry

A new paper from Laurence Yeung on the fundamentals of “clumped-isotope” fractionation, was recently accepted in Geochimica et Cosmochimica Acta. It shows, through simple theoretical arguments, the factors influencing the occurrence of rare-isotope pairs in molecules when they are made. One might be able to base future tracers of biogeochemistry on these principles.

One of the findings is also a convenient practical summary: When playing Craps, never bet on snake eyes if you suspect the dice are loaded―it, along with the other hard rolls (double threes, double fours, etc.) are less likely to come up when the dice are not evenly weighted.

doi: 10.1016/j.gca.2015.09.020


The arrangement of isotopes within a collection of molecules records their physical and chemical histories. Clumped-isotope analysis interrogates these arrangements, i.e., how often rare isotopes are bound together, which in many cases can be explained by equilibrium and/or kinetic isotope fractionation. However, purely combinatorial effects, rooted in the statistics of pairing atoms in a closed system, are also relevant, and not well understood. Here, I show that combinatorial isotope effects are most important when two identical atoms are neighbors on the same molecule (e.g., O2, N2, and D-D clumping in CH4). When the two halves of an atom pair are either assembled with different isotopic preferences or drawn from different reservoirs, combinatorial effects cause depletions in clumped-isotope abundance that are most likely between zero and –1‰, although they could potentially be –10‰ or larger for D-D pairs. These depletions are of similar magnitude, but of opposite sign, to low-temperature equilibrium clumped-isotope effects for many small molecules. Enzymatic isotope-pairing reactions, which can have site-specific isotopic fractionation factors and atom reservoirs, should express this class of combinatorial isotope effect, although it is not limited to biological reactions. Chemical-kinetic isotope effects, which are related to a bond-forming transition state, arise independently and express second-order combinatorial effects related to the abundance of the rare isotope. Heteronuclear moeties (e.g., C–O and C–H), are insensitive to direct combinatorial influences, but secondary combinatorial influences are evident.

In general, both combinatorial and chemical-kinetic factors are important for calculating and interpreting clumped-isotope signatures of kinetically controlled reactions. I apply this analytical framework to isotope-pairing reactions relevant to geochemical oxygen, carbon, and nitrogen cycling that may be influenced by combinatorial clumped-isotope effects. These isotopic signatures, manifest as either directly bound isotope “clumps” or as features of a molecule’s isotopic anatomy, are linked to molecular mechanisms and may eventually provide additional information about biogeochemical cycling on environmentally relevant spatial scales.

Read more about the research in the Yeung Lab at