Hydrogen isotope fractionation during lipid biosynthesis by Tetrahymena thermophila

Abstract

Hydrogen isotope ratio values of lipids are increasingly used to reconstruct past variation in hydrological conditions. However, apart from recording the hydrogen isotope composition of ambient water, δD values of lipids also depend on specific biosynthetic pathways and growth conditions. We have evaluated the hydrogen isotope fractionation by the ciliated protozoan, Tetrahymena thermophila, grown in pure culture at three temperatures (24°C, 30°C and 36°C) and in water with a range of hydrogen isotope composition. T. thermophila synthesizes tetrahymanol, a pentacyclic triterpenoid alcohol and the diagenetic precursor of the biomarker gammacerane. We focused our attention on the isotopic controls on tetrahymanol and various fatty acids (FAs).The δD values of FAs and tetrahymanol correlated linearly with the hydrogen isotope composition of water, but growth temperature was also clearly an important factor controlling lipid D/H compositon. Hydrogen isotope fractionation during tetrahymanol biosynthesis changed with higher growth temperatures, resulting in D-depleted signatures relative to water at 24°C and 30°C and D-enriched composition at 36°C. T. thermophila grown at 36°C – a temperature above opitmum growth conditions – showed a significant change in lipid composition, with the abundance of tetrahymanol increasing relative to total FAs. We suggest the change is a response to temperature stress and a decrease in the stability of the cell membrane. The temperature effect is also presumed to impact δDlipid by altering the hydrogen isotopic composition of NADPH and potentially intracellular water.