Oceanic life plays a major role in global biogeochemical cycles and the oceans act as sink for atmospheric CO2. How an ecosystem influences the carbon cycle depends on its balance between primary production and respiration. Primary producers convert inorganic carbon into their biomass, thereby removing CO2 from the atmosphere. Conversely, consumers take up organic material to fuel their respiration and release CO2 back into the environment. The two key processes in this cycle, primary production and consumption of organic material, are usually considered as being performed by different groups of organisms, autotrophs and heterotrophs. However, some organisms, the so-called mixotrophs can perform both of these processes simultaneously and hence, can have opposing effects on the carbon cycle depending on their balance between autotrophic and heterotrophic nutrition.Mixotrophic eukaryotes have recently been found as both abundant and important for key ecosystem processes in the ocean. However, the basic biology of mixotrophs is not well understood, particularly the environmental drivers that influence their nutritional balance. The goal of this project is to gain a mechanistic understanding of the role mixotrophs play in the marine carbon cycle. To achieve this, mixotrophs will be studied at all relevant organizational levels, ranging from gene expression patterns at the molecular level to the diversity and activity patterns on the ecosystem scale. In particular (i) a molecular study using transcriptome and proteome analysis will elucidate the metabolic integration of their nutritional pathways, (ii) their contribution to biogeochemical cycles will be quantified by NanoSIMS technology, and (iii) ecological factors controlling their distribution and diversity in the ocean will be assessed. This multidisciplinary approach will provide the foundation necessary for linking molecular processes to the contribution of mixotrophs to the marine carbon cycle.