Microbes drive the nutrient transformations that sustain Earth's ecosystems, and the viruses that infect these microbes modulate both microbial population size and diversity. The cyanobacterium Prochlorococcus, the most abundant oxygenic phototroph on Earth, contributes a substantial fraction of global primary carbon production, and often reaches densities of over 100,000 cells per milliliter in oligotrophic and temperate oceans. Hence, viral (cyanophage) infection and lysis of Prochlorococcus represent an important component of the global carbon cycle. In addition to their ecological role in inducing host mortality, cyanophages influence the metabolism and evolution of their hosts by co-opting and exchanging genes, including core photosynthesis genes.
