Biogeography, the study of species distributions over space and time, provides valuable information on the factors controlling biodiversity. Microorganisms are now widely recognized as exhibiting biogeographic patterns similar to larger organisms. However, the biogeography of viruses has been slower to develop despite their important role in ecosystems. After first proposing a theoretical framework with which to study microbial biogeographic patterns, the goal of this dissertation was to demonstrate whether an ecologically important subset of viruses -- cyanophages, or viruses that infect the marine cyanobacterium, Synechococcus -- also exhibit such patterns. I isolated cyanophages every month for three years in Southern California, and at one point in time at two additional locations in the coastal U.S. I then taxonomically characterized nearly 4,000 isolates by amplifying and sequencing a cyanophage-specific marker gene in each isolate. To more accurately detect possible spatial patterns, I also characterized a subset of isolates for their host range phenotypes, which provided additional biological variability than could be assessed by the marker gene alone.;I found that coastal marine cyanophage communities are both temporally dynamic as well as spatially structured. Within the regional scale of Southern California, these communities were strikingly seasonal -- that is, their composition changed in tune with the seasons in an annually recurring manner. I found that this seasonal pattern over three years was primarily related to forecasted UV irradiance, with UV explaining nearly 33% of the temporal variation in cyanophage composition. Moreover, I found that at larger spatial scales, cyanophage community composition was also highly structured in space, both taxonomically and phenotypically. These results provide strong evidence that phages exhibit spatial and temporal biogeographic patterns. Because phages are susceptible to degradation by light, the results further suggest that their distributional patterns may be partially driven by selective environmental factors, namely UV radiation, acting directly on phages; rather than solely being influenced by the presence of susceptible hosts. The library of isolates collected here offers a unique opportunity to test this hypothesis. Overall, this work adds to the growing evidence that all microbes, including phages, exhibit biogeographic patterns that are in large part driven by environmental selection.
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