Microbial D/H

Octopus Spring in Yellowstone National Park. The orange and yellow colors are bacterial mats, one of the environments where we have been studying the D/H ratios of lipids.

One of the major focuses for our research is an effort to use (and understand) the distribution of stable hydrogen isotopes (2H, known as 'deuterium', and 1H, known as 'protium') in individual organic compounds. While measurements of hydrogen isotopes go all the way back to the initial work of Harold Urey and Harmon Craig, an important distinction is that those time-tested methods measured the abundance of isotopes in bulk materials, such as water, minerals, and total organic matter. We have been conducting studies which look at hydrogen isotopes in individual compounds; the greater specificity of the measurements gives us a more detailed understanding of relevant processes, but of course is more difficult. The measurements are achieved by connecting a standard gas chromatograph (GC) to a standard isotope-ratio mass spectrometer (IRMS) via a high-temperature pyrolysis (P) oven. This hyphenation yields the acronym GC/P/IRMS. To learn more about this technique, see Sessions (2006). Because all of our analytes must be readily volatilized (we are using a gas chromatograph), it is mainly applicable to lipids, hydrocarbons, and other nonpolar moleculars.

Our main focus is currently trying to understand the variability of lipid δD values between and within different organisms, and what the biochemical basis for these variable fractionations is. While early work showed that lipid/water fractionations between plants and water are relatively constant (Sauer et al., 2001; Sachse et al., 2012), studies of bacteria have shown that there can be huge differences in δD, even in the same organism when grown on different food sources (Zhang et al., 2009). These differences can approach 500‰, i.e. a 50% difference in relative deuterium abundance. We hypothesize that these differences are related to metabolic pathways that reduce the molecule NADPH, a sort of cellular battery that stores reducing power (in the form of a hydride ion) to be used for synthesizing biomolecules. Our current work is focused primarily on growing a range of different microorganisms in culture, and manipulating their growth conditions, to see how this affects lipid D/H fractionation.

The eventual goal of this research is to understand how, when, and why different microbes acquire different lipid δD values. Once we understand these patterns, we would be able to use this isotopic proxy to say something useful about the metabolism of uncultured organisms, microbial symbionts, and ancient life on Earth. For example, we think we will eventually be able to analyze the lipids of environmental microbes and be able to understand their mode of growth (heterotroph, photoautotroph, chemoautotroph). We also hope one day to be able to extract a biomarker from an rock millions of years old and — from its D/H ratio — understand something about that ancient metabolism.

Recent papers on this subject:

Dawson KS, Osburn MR, Sessions AL, and Orphan VJ (2015) Metabolic associations with archaea drive shifts in hydrogen isotope fractionation in sulfate-reducing bacterial lipids in cocultures and methane seeps. Geobiology DOI:10.1111/gbi.12140.

Sachse D, Billault I, Bowen GJ, Chikaraishi Y, Dawson TE, Feakins SJ, Freeman KH, Magill CR, McInerney FA, van der Meer MTJ, Polissar P, Robins RJ, Sachs JP, Schmidt HL, Sessions AL, White JWC, West JB, Kahmen A (2012) Molecular paleohydrology: Interpreting the hydrogen-isotopic composition of lipid biomarkers from photosynthesizing organismsAnnual Reviews of Earth and Planetary Science 40, 221-249.

Ferreira AA, Santos Neto EV, Sessions AL, Schimmelmann A, Aquino Neto FR (2012) 2H/1H ratios of hopanes, tricyclic and tetracyclic terpanes in oils and source rocks from the Potiguar Basin, Brazil. Organic Geochemistry 51, 13-16.

Osburn MR, Sessions AL, Pepe-Ranney C, and Spear JR (2011) Hydrogen-isotopic variability in fatty acids from Yellowstone National Park hot spring microbial communities. Geochimica et Cosmochimica Acta 75, 4830-4845.

Zhang, Xinning, Gillespie, Aimee, Sessions, Alex L. (2009). Large D/H variations in bacterial lipids reflect central metabolic pathways. Proceedings of the National Academy of Science, USA (Aug 4 issue).

Li, Chao, Sessions, Alex L., Kinnamen, Frank, Valentine, David L. (2009). Hydrogen-isotopic variability in lipids from Santa Barbara Basin sediments. Geochimica et Cosmochimica Acta 73, 4803-4823.

Campbell, Brian J., Li, Chao, Sessions, Alex L., and Valentine, David L. (2009) Hydrogen isotopic fractionation in lipid biosynthesis by H2-consuming Desulfobacterium autotrophicum. Geochimica et Cosmochimica Acta 73, 2744-2757.

Jones, Ashley, Sessions, Alex L., Campbell, Brian J., Li, Chao, Valentine, David L. (2008). D/H ratios of fatty acids from marine particulate organic matter in the California Borderland BasinsOrganic Geochemistry 39, 485-500.

Sessions A.L. (2006) Seasonal changes in lipid D/H fractionation by Spartina alternifloraGeochimica et Cosmochimica Acta70, 2153-2162.

Sessions A. L. and Hayes J.M. (2005) Calculation of hydrogen isotopic fractionations in biogeochemical systemsGeochimica et Cosmochimica Acta 69, 593-597.

Valentine D.L., Sessions A.L., Tyler S.C., and Chidthaisong, A. (2004) Hydrogen isotope fractionation during H2/CO2 acetogenesis: hydrogenase efficiency and the origin of lipid-bound hydrogenGeobiology 2, 179-188.

Sessions A. L., Jahnke L. L., Schimmelmann A., and Hayes J. M. (2002) Hydrogen isotope fractionation in lipids of the methane-oxidizing bacterium Methylococcus capsulatusGeochimica et Cosmochimica Acta 66, 3955-3969.

Copyright 2015 by Alex Sessions. All rights reserved.