Effects of temperature and carbon source on the isotopic fractionations associated with O2 respiration for 17O/16O and 18O/16O ratios in E. coli

Abstract 18 O/ 16 O and 17 O/ 16 O ratios of atmospheric and dissolved oceanic O 2 are used as biogeochemical tracers of photosynthesis and respiration. Critical to this approach is a quantitative understanding of the isotopic fractionations associated with production, consumption, and transport of O 2 in the ocean both at the surface and at depth. We made measurements of isotopic fractionations associated with O 2 respiration by E. coli. Our study included wild-type strains and mutants with only a single respiratory O 2 reductase in their electron transport chains (either a heme-copper oxygen reductase or a bd oxygen reductase). We tested two common assumptions made in interpretations of O 2 isotope variations and in isotope-enabled models of the O 2 cycle: ( i ) laboratory-measured respiratory 18 O/ 16 O isotopic fractionation factors ( 18 α) of microorganisms are independent of environmental and experimental conditions including temperature, carbon source, and growth rate; And ( ii ) the respiratory ‘mass law’ exponent, θ , between 18 O/ 16 O and 17 O/ 16 O, 17 α = ( 18 α) θ , is universal for aerobic respiration. Results demonstrated that experimental temperatures have an effect on both 18 α and θ for aerobic respiration. Specifically, lowering temperatures from 37 to 15 °C decreased the absolute magnitude of 18 α by 0.0025 (2.5‰), and caused the mass law slope to decrease by 0.005. We propose a possible biochemical basis for these variations using a model of O 2 reduction that incorporates two isotopically discriminating steps: the reversible binding and unbinding of O 2 to a terminal reductase, and the irreversible reduction of that O 2 to water. Finally, we cast our results in a one-dimensional isopycnal reaction-advection-diffusion model, which demonstrates that enigmatic δ 18 O and Δ 17 O variations of dissolved O 2 in the dark ocean can be understood by invoking the observed temperature dependence of these isotope effects.