Toward using 2H values for investigating marine mixotrophy

Hydrogen isotope ratios (2H) measured on lipids extracted from aquatic and terrestrial organisms are widely applied for studying past hydro-climatic conditions. A growing body of evidence suggests that 2H values of lipids integrate not only climatic, but also metabolic information. Metabolic effects hinder the reconstruction of climatic conditions based on 2H values from archives and urges for a better understanding of the key drivers of 2H values in organic compounds. Once disentangled, such non-climatic information in 2H values may become key for investigating misunderstood eco-physiological behaviours. One such behaviour is marine mixotrophy. While science requires tools to measure the contributions of phototrophic and heterotrophic growth in plankton, my colleagues and I have already shown that lipid 2H values are uniquely sensitive to the expression of heterotrophy relative to photosynthesis in plants. This presentation will discuss groundwork experiments performed with chlorophytes and dinoflagellates exploring whether 2H values of diverse compounds produced by protists are similarly sensitive to their central metabolic pathway. New 2H analyses performed on fatty acids, phytols and sterols from these experiments, using an isotope ratio mass spectrometer (IRMS) coupled to a gas chromatograph (GC), will be discussed. Our data suggest that lipids 2H values are indeed sensitive to the level of heterotrophic growth in diverse protists. If this relation can be confirmed and calibrated, compound specific H isotope analyses could provide a powerful means to study the role of mixotrophy on the global carbon cycle and the occurrences of HABs.