QUANTIFYING ENVIRONMENTAL CONTROLS ON ARCHAEAL LIPID HYDROGEN ISOTOPE SIGNATURES IN AN EXTREMOPHILIC ARCHAEON

The hydrogen isotope signatures (δ²H) of lipid biomarkers track environmental processes and can remain stable over geologically relevant time scales. Lipid-H may retain its original isotopic composition for up to 108 years, enabling studies of past climate, metabolism, and/or ecology. While most research has focused on photosynthetic plants and algae from the domain Eukarya (e.g., plant waxes, long-chain alkanes), microbial lipids are of interest because of the wide diversity of environments that Bacteria and Archaea inhabit, as well as their diverse suite of metabolisms. Microbial lipid δ²H may therefore provide additional information about geobiological processes. Within the prokaryotes, bacterial lipids have been well-studied, and it is now understood that δ²H_Lipid primarily reflects the H fluxes for lipid synthesis, linking the H-isotope fractionation between lipids and water to central metabolism. As such, certain classes of bacterial lipids are unreliable proxies for paleoenvironmental conditions, but do reflect features of central catabolism, namely the cellular NADPH-flux balance. In contrast, archaeal lipid δ²H remains understudied. Here, we determine the H isotope fractionation (²ε_Lipid/Water) between the biphytane sidechains of glycerol dialkyl glycerol tetraether (GDGT) membrane lipids and water with a model archaeon, the thermoacidophile Sulfolobus acidocaldarius. This strain was grown in pure culture under varying temperature, pH, aeration rates, and electron donor fluxes. Separately, we varied the δ²H of growth water and of the electron donor and carbon source (D-glucose), which allows us to estimate the relative contributions of H from protons in water and organic substrates to the final lipid products. In all cases, lipid δ²H is strongly correlated with water δ2H and is consistently depleted relative to growth water. The impact of each environmental condition on ²ε_L/W was minor, with ²ε_L/W varying by 80 ‰ among all treatments. These experiments provide the basis for a framework to interpret archaeal lipid biomarker δ²H in ancient sediments and modern extreme environments.