Developing position-specific amino acid carbon isotope analysis as a tool for geobiology

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A longstanding goal in geobiology has been to develop ways of reconstructing and understanding the metabolic activities of organisms, both past and present, in their environment. Here the investigator takes a step toward that goal by developing new methods to precisely measure the abundance of the heavy stable isotope of carbon (C-13) in different atomic positions of amino acids. His prediction is that these patterns of isotope abundance will serve as a 'fingerprint' of different metabolic pathways, and when measured in amino acids from environmental proteins will allow to infer the metabolic activities of organisms from that environment. He will employ a relatively new mass spectrometry method (Orbitrap MS) that measures the C-13/C-12 ratios of several molecular fragments very precisely. These overlapping fragments can then be mathematically recombined into a model of isotope abundance at each atomic position. As test cases for the new measurements, he will examine the following three hypotheses. 1) The pattern of isotope abundance in all amino acids will differ depending on how the host organism fixes carbon, whether by the Calvin Cycle (like all plants) or via a number of alternative pathways used by bacteria. 2) The isotope pattern in the amino acids glutamate and proline will reflect the growth efficiency of the organism, because of the specific metabolic connection between these two compounds. 3) The isotope pattern in serine and alanine will reflect the extent of a process known as photorespiration, which occurs in plants as temperatures rise and effectively reverses the work of photosynthesis.The overarching goal of this proposal is to develop position-specific delta C-13 measurements in amino acids as a tool for geobiologic research. Measurements of isotopic composition at the level of individual atomic positions will provide unprecedented ability to reconstruct metabolism, environmental processes, and other geobiologic phenomena. The investigation will employ existing mass spectrometric techniques described by Eiler et al (2017) in which the C-13/C-12 ratios of the molecular ion and various fragment ions of a particular analyte are measured via Orbitrap mass spectrometry at high mass resolution (up to 500,000 M/Delta M) to distinguish isobars. The relative contributions of each atomic position of the analyte to each fragment ion (the 'contribution matrix') must be determined through studies of labeled compounds, and then the complete position-specific delta C-13 values can be reconstructed from the fragments. This project will develop measurement protocols, including contribution matrices, for position-specific delta C-13 in six amino acids: alanine, serine, glutamate, proline, aspartate, and methionine. These collectively span all of the major families of carbon skeletons that are sampled by amino acids. The investigator will use these new tools to explore three focused hypotheses: 1) Position-specific delta 13-C in all amino acids, include Ala, Glu, and Met, will reflect the carbon-fixation pathway of the parent organism. 2) Position-specific C-13 fractionation of C-2 versus C-3 in Glu and Pro should reflect the extent to which alpha-ketoglutarate is decarboxylated versus transaminated, which he predicts is proportionate to growth efficiency. 3) He predicts that C-13 fractionation at C-2 in Ser versus Ala should covary with the extent of photorespiration, since oxidized RuBP is recycled via the serine pathway.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.