Isotopes as Metabolic Tracers

'Metabolism' refers to the collection of all the chemical reactions that organisms utilize to live, whether its respiring food for energy, making new molecules for growth, or even precipitating minerals. As geobiologists, we are constantly seeking new ways to understand and reconstruct the metabolism of both modern and ancient organisms. In the Sessions lab, the main tools we use for this purpose are the stable isotope compositions of organic molecules, primarily ²H, ¹³C, and ³⁴S. Our research efforts take a fairly familiar approach, even while they vary quite a bit in the details: start with living organisms that can be manipulated in the lab (like bacteria), or sampled from well-understood environments (like plants), and see how their isotopic characteristics vary with growth, environment, life stage, and so on. Once we understand how the isotopic proxy behaves in living organisms, then we are ready to go apply it to ancient sediments and rocks, ranging from a few thousand years old to billions of years. A few specific examples of our research into isotopic metabolic tracers are described below, but of course we are always on the hunt for new ones. Students working in this area have a range of backgrounds, from biochemistry and microbiology (working on modern organisms) to geology and geochemistry (working on ancient rocks).

The longest-running project in my lab involves measuring and studying hydrogen isotope fractionations in plants, microbes, and a variety of other things. In the 2000's we helped pioneer the use of leaf-wax D/H ratios as paleoclimate archives, and have published many such records. More recently, we've been focused on the biochemistry that underlies the very large range in fractionations of lipids produced by different microbes. We first hypothesized -- and later confirmed -- that the flow of reducing equivalents through NADPH was the key variable. Mostly recently, we have turned our attention to amino acids, and the variability and biochemical controls on their D/H ratios.

We have also been active in studying sulfur isotope fractionation during microbial sulfur cycling (in sulfate reduction and DMS degradation), as well as the abiotic sulfurization reactions that occur in sediments. (OK, that is technically not metabolism, but its still interesting!). We recently used the contrast between surface and porewater sulfur isotopes to probe the sources of marine dissolved organic sulfur. And we are currently interested in how plants and phytoplankton fractionate sulfur at the molecular level; we recently published a new method for isolating cysteine and methionine and measuring their d34S values. Basically, we're interested in all the different ways that S enters organic matter, and the isotopic fingerprints that it acquires along the way.

Last but certainly not least, we are pursuing the position-specific C isotope fractionations of amino acids and other biomolecules as recorders of metabolism. This has required us to invent new ways of making measurements (a longstanding collaboration with John Eiler), but promises to yield much more detailed information than previous compound-specific measurements. Applications include the identification of fermenters in complex communities; tracking photorespiration in plants and algae; and developing a proxy for C-fixation pathway in microbes. Additional details are provided here.