The ocean is by far the largest carbon reservoir today and is an important carbon sink. One of the key players in its capacity to draw down atmospheric CO2 are coccolithophores. They are unicellular marine algae that contribute to this global carbon cycle not only through photosynthesis but also through calcification, as their cells are covered by calcite plates. However, they are not acting alone. The microenvironment that surrounds them is rich in nutrient dense extracellular organic matter and harbours a rich microbial community. This environment is known as the phycosphere, and the microbial interactions within it range from mutualistic to antagonistic. Such relationships significantly influence the overall dynamics of the ecosystem, for example by promoting or terminating the spread of large coccolithophore blooms. However, there are still many unknowns in this microenvironment. The Cocco Channel project aims to study the interactions between coccolithophores and their associated bacterial communities within the phycosphere and to understand how these relationships influence the ocean's ability to capture CO2. This research aims to uncover the intricate dynamics of these microscale interactions by integrating methods from microbial biology, genomics, oceanography and carbon chemistry. By exploring the exchange and the structure of microbial communities associated with different coccolithophore species, the project aims to provide deeper insights into the impact of these microorganisms on the oceanic carbon cycle. In addition to laboratory and microcosm experiments, our field studies will analyse the composition, functions and interaction dynamics of natural coccolithophore communities and their associated microbiota. This comprehensive approach will help to quantify the impact of coccolithophores on carbon sequestration and provide important data for the refinement of climate models and the development of strategies to mitigate the effects of climate change.