DINOTROPHY: Deuterium in Organic Biomarkers: A new tool to investigate the role of Marine Mixotrophy in the Global Carbon Cycle

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Marine microscopic organisms drifting in the oceans (plankton) produce 50% of the oxygen we breath. Their activity helps maintain the balance of oxygen and carbon dioxide in the atmosphere. Much of this production is undertaken by eukaryote (non-bacterial) microbes called protists. Science has traditionally separated these marine protists between those akin to microscopic plants (autotrophs), getting their energy directly from the sunlight, or akin to microscopic animals (heterotrophs), getting their energy by eating other microbes. Recently, however, scientists have come to appreciate that many marine protists combine both autotrophic and heterotrophic (feeding) activities in the same single cell (mixotrophy). Mixotrophy alters marine food web functioning and affects the capacity of the oceans to remove carbon dioxide (counter climate change) from the atmosphere by increasing marine photosynthesis. This revelation overturns a century of understand in marine science, but also presents a severe challenge; traditional methods are targeted at autotrophy or heterotrophy, in separate organisms and we lack methods to determine mixotrophic activity. This project will develop a new tool to tackle this critical challenge. The high interest met in developing this project has already created a wide interdisciplinary network across the United Kingdom, France and Switzerland.Our team have previously shown that hydrogen isotopic signature of compounds produced in plants and bacteria (for instance into lipids) is uniquely sensitive to the balance of heterotrophy and autotrophy. This project will extend this investigation for compounds produced by marine microorganisms using similar approaches. We will target a particularly important group of mixotrophic protist plankton, the dinoflagellates, by measuring the hydrogen isotopic composition of specific compounds produced by dinoflagellates. Specifically, we will investigate the hydrogen isotope fractionation mechanisms (at molecular level, i.e. "site-specific") during biosynthesis of organic compounds and seek to establish the hydrogen isotope signature of marine lipids biomarkers (also used as molecular fossils) as a novel tool to investigate the behaviours of dinoflagellates in modern and past oceans and the impact of mixotrophy on the global carbon cycle.The core of the project will also produce of a numerical biochemical model uniquely possible by combining microorganism cultures and cutting-edge chemical analyses (including Mass Spectrometry and Nuclear Magnetic Resonance Spectroscopy) that will identify in detail the chemical steps responsible for the isotopic signature. By doing so, the project is also fostering a new cross-disciplinary group uniquely skilled in chemistry, isotope biogeochemistry and marine ecology dedicated to extend our knowledge of the marine microorganisms and their role on the Earth System.