Investigating the "mixoplankton" paradigm using hydrogen isotope ratios

Since micro-organisms were first visualised by Robert Hooke and Antoni van Leeuwenhoek, bacteria and protists were quickly categorised as either primary producers or consumers and thus forming the base of the marine food web. New conceptual understanding sees this traditional dichotomy between autotrophs and heterotrophs in the marine food web replaced by one that recognises that mixotrophy is widespread. Many "phytoplankton" eat, while 50% of "microzooplankton" perform photosynthesis. This mixotrophic behaviour affects the cycling of nutrients, enhances primary production, biomass transfer to higher trophic levels, and the marine sequestration of atmospheric CO₂. Moreover, the mixotrophic behaviour of many toxic protists could also be partly responsible for their ecological success and the occurrence of Harmful Agal Blooms (HABs).

While science requires a tool to measure the contributions of phototrophy and heterotrophy in plankton to help in biogeochemical modelling, my colleagues and I have already shown that hydrogen (H) isotopic signature (i.e. d²H) of lipids is uniquely sensitive to the expression of heterotrophy relative to photosynthesis in terrestrial plants and bacteria. This presentation will discuss groundwork experiments performed with Chlorella sorokiniana, Prymnesium parvum and Emiliania huxleyi that had for objective to explore whether d²H values of diverse compounds produced by protists are similarly sensitive to their central metabolic pathway. Hydrogen isotope analyses performed on organic compounds from these experiments, using an isotope ratio mass spectrometer (IRMS) coupled to a gas chromatograph (GC), suggest that H isotopic signature of lipids is indeed sensitive to the level of heterotrophic growth in diverse protists.

If this relation can be confirmed and calibrated, compound specific hydrogen isotope analyses could provide a powerful means to study the role of mixotrophy on the global carbon cycle, the cycling of nutrients and the occurrences of HABs.