Effects of temperature and carbon source on the isotopic fractionations associated with O_2 respiration for ^(17)O/^(16)O and ^(18)O/^(16)O 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.