Viruses and cells are among the most interwoven partnerships in biology. we have information from bacterial genomics and metagenomics indicating that the structure of bacterial populations is determined by their viral predators. Populations of planktonic bacteria are multi-clonal with different lineages that carry different viral receptors, i.e. have different viral susceptibility, they are also different in metabolic and environmental interaction. Like different tissues in a multicellular eukaryote they cooperate to exploit efficiently their habitat. The genetic wealth actually present in a drop of seawater within a single bacterial species is surprisingly large, rivalling, if not exceeding their eukaryotic counterparts, but to preserve this genetic wealth viral predation is of essence. Without viruses a selective sweep in which a clone becomes dominant is hard to avoid and would lead to irretrievable loss of genetic diversity and, eventually, much poorer ecological performance. This model of prokaryotic populations will be presented and evidence supporting it from analysis of genomes and metagenomes of marine bacteria.