Does metabolic water control the phosphate oxygen isotopes of microbial cells?

The oxygen isotopes ratio (δ¹⁸O) of microbial cell water strongly controls the δ¹⁸O of cell phosphate and of other oxygen-carrying moieties. Recently it was suggested that the isotopic ratio in cell water is controlled by metabolic water, which is the water produced by cellular respiration. This potentially has important implications for paleoclimate reconstruction, and for measuring microbial carbon use efficiency with the ¹⁸O-water method. Carbon use efficiency strongly controls soil organic matter preservation. Here, we directly tested the effect of metabolic water on microbial cells, by conducting experiments with varying the δ¹⁸O of headspace O₂ and the medium water, and by measuring the δ¹⁸O of cell phosphate. The latter is usually assumed to be in isotopic equilibrium with the cell’s water. Our results showed no correlation between the δ¹⁸O of O₂ and that of the cell phosphate, contradicting the hypothesis that metabolic water is an important driver of δ¹⁸O of microbial cell water. However, our labeled ¹⁸O water experiments indicated that only 43% of the oxygen in the cell’s phosphate is derived from equilibration with the medium water, during late-log to early-stationary growing phase. This could be explained by the isotopic effects of intra-and extra-cellular hydrolysis of organic compounds containing phosphate.