The South part of the Pacific Ocean is an under-sampled oceanic region. It harbors mesotrophic (moderately low nutrient concentration) and oligotrophic (low nutrient concentration) areas in which primary production is dominated by very small eukaryotic cells, with less than 2-3 μm (pico-phytoplankton). Despite their ecological importance, many of these organisms are still uncultured and their morphology and physiology remains unknown. If offshore tropical waters become more oligotrophic in response to global change as predicted, a shift in the size class dominance may take place within the phytoplankton, with an increased dominance of smaller photosynthetic eukaryotes. Understanding the contributions of the different groups of pico – phytoplankton to the carbon cycle, their ecology and physiology is fundamental for choosing the appropriate parameters to model primary production in the future oceans. Picophytoplankton Study of Carbon rates in Oligotrophic waters of the South Pacific Ocean aims at obtaining detailed information on the diversity and production of eukaryotic picophytoplankton in the South Pacific. We will combine a range of state of the art techniques including metabarcoding of marker genes, C14 productivity measurements of flow cytometry sorted populations, electron microscopy coupled with in situ probe hybridization and high throughput culture isolation using flow cytometry sorting.