Marine Cyanobacteria of the genera Synechococcales and Prochlorococcales are the most abundant prokaryotic phototrophs in the world’s ocean. These Cyanobacteria are highly challenged by viral infection with up to 40 % of the bacterial population being infected by cyanophages. Infection of a bacterium by a phage transforms the host cell into a so-called virocell where both viral and host genomes are expressed in parallel. Many cyanophages genomes carry so-called auxiliary metabolic genes, often related to photosynthesis and light harvesting. Within this project we seek to understand the role of these genes and their encoded proteins in the virocell using a combination of phage genetics, biochemistry and masspectrometry based proteomics.