Toward using δ2H values for investigating marine mixotrophy

Hydrogen isotope ratios (δ²H) 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 δ²H values of lipids integrate not only climatic, but also metabolic information. Metabolic effects hinder the reconstruction of climatic conditions based on δ²H values from archives and urges for a better understanding of the key drivers of δ²H values in organic compounds. Once disentangled, such non-climatic information in δ²H 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 δ²H 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 δ²H values of diverse compounds produced by protists are similarly sensitive to their central metabolic pathway. New δ²H 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 δ²H 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.