Marine Cyanobacteria of the genera Synechococcus and Prochlorococcus are the most abundant prokaryotic oxygenic 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 auxiliary metabolic genes (AMGs), 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. Within the second funding, the role of AMGs in phage-host interaction will be investigated exemplified by ϕcpeT from cyanophage Syn9 infection Synechococcus sp. WH8109. ϕcpeT has similarity a cyanobacterial phycobiliprotein lyase, involved in the maturation of the light harvesting phycobilisome.