The majority of the biomass in the ocean is microbial and viral killing is one of the main ecological factors determining the diversity, abundance and turnover of microbial cells. Yet in spite of the known overall importance of viruses, the mechanisms and dynamics of their interactions with their specific microbial hosts remain poorly understood. This project characterized viral strategies for survival and interaction with bacterial cells in the ocean, with the major goal of enabling better understanding of the conditions under which viruses can effectively control bacterial populations. To assess the genetic diversity and temporal dynamics of interacting viruses and bacteria, this project analyzed daily samples over 93 consecutive days. Major outcomes were: (i) The discovery of a novel group of viruses that represent the morphological majority of viruses in the ocean but have escaped study in either sequence or culture collections. The work also identified three reasons that have contributed to their underrepresentation in past studies and proposes ways to address this major oversight opening the prospects of more fully understanding viral diversity in the ocean and beyond. (ii) The elucidation of mechanisms of infection and resistance among a broad diversity of viruses leading to understanding of what determines the host range of viruses and how this affects environmental dynamics. The time series revealed co-existence of very closely related bacteria that are subject to different viral infection. Genomic comparison revealed that these bacteria are, in fact, so closely related that they differ only in novel mobile genetic elements that are determinants of resistance. While these genetic elements ensure resistance to a broad array of viruses, they also all have at least one virus that can overcome the resistance mechanisms and efficiently kill cells. This new mechanism of interaction among viruses and mobile genetic elements appears to be widespread. Moreover, the interaction appears to be a major factor in determining the abundance of bacterial genotypes in the Ocean. (iii) The characterization of changes in bacterial and eukaryotic communities along the time series demonstrating that extreme boom and bust dynamics are the norm rather than the exception. Every few days, a different microbe undergoes rapid expansion and equally rapid collapse of the population. The work enabled characterization of one such bloom and established that viruses can rapidly respond and are responsible for the rapid cessation of the bloom by efficiently killing cells, demonstrating for the first time that viruses can indeed be responsible for eliminating dominant bacterial populations from the environment. This work overall provided fine-scale analysis of host and virus genomic diversity and abundance in the coastal ocean, while at the same time estimating host-range and co-infection, all of which represent important, poorly constrained parameters in virus-host interactions in the ocean and in other systems. Finally, this project provides important information on design principles for use of viruses to combat harmful bacterial populations ("phage therapy") as a viable alternative to antibiotic treatment. Last Modified: 12/05/2017 Submitted by: Martin F Polz