The mixotrophic behavior of many microalgae has a considerable impact on primary production, biogeochemical cycles and food webs. Preliminary mixotrophic experiments with a model species surprisingly showed the emergence of two subpopulations, one heterotrophic and the other autotrophic. Taking this trophic heterogeneity into account brings a completely new vision on mixotrophy. By coupling approaches of ecophysiology, molecular biology and modeling, the general objective of the HétéroMixo project is to characterize this trophic bi-modality, and to assess its consequences on the functioning of ecosystems.