There are an estimated 10 viruses in the world oceans, the majority of which are phages (viruses that infect bacteria). Extensive research has demonstrated the significant influence of marine phages on microbial abundance, community structure, genetic exchange and global biogeochemical cycles. In this thesis, we contribute to increase the knowledge about the ecological role of viruses in marine systems, but also we aimed to provide a better understanding about the interactions between phages and their hosts and the genetic pool and biogeography of some the isolated phages genomes. Firstly, we followed the seasonal variability of viral communities in a coastal marine site (Blanes Bay Microbial Observatory, BBMO) and the environmental and biological factors that could modulate them. Our results showed that viral communities did not follow any clear seasonal patterns during the 5 years studied period and that viruses were mainly negatively correlated with salinity. Secondly, given the actual concern of the climate change effects on marine ecosystems, we evaluated experimentally how increasing temperatures would affect the microbial loop via protists respect to via viruses (“viral shunt”) in two contrasting Arctic marine systems. Lytic life strategy dominated instead the lysogenic strategy when we increased the temperature. But, overall the most important factor controlling bacterial abundance was bacterivory. These two studies provide us a general overview regarding viral dynamics at the community level but without knowing who infects whom and who is doing what. To provide inputs into these relevant issues, we used the model of Pseudoalteromonas bacterial strains and its phages. Our results suggest that interactions between phages and hosts are highly complex in terms on infectivity and susceptibility at microdiversity level but also reflect that phages can infect at larger taxonomic rank reaching to the family boundaries. Pseudoalteromonas marine phages are unrepresented with only 4 genomes public available. Therefore, one of the isolated Pseudoalteromonas phage from BBMO was deeply studied; we investigated its biology, morphology, genomic and proteomic characteristics. Moreover, we carried out a genomic comparison of 3 Pseudoalteromonas phages isolated from the same bacterial specie to get insights into the genome structure and functional diversity. The genomic data analyzed not only contributed to a better understanding of phage-host interactions in marine systems but also demonstrated the complexity of their dynamics and biogeographic patterns