Viral discovery and unlocking the role of viruses in taxon-specific microbial mortality

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More than 90% of the living material in the ocean, by weight, is microscopic, and these microbes produce about half the planet's oxygen. Within this microbial milieu are 1 to 100 million viruses /mL, the most numerous "life" in the sea. As obligate pathogens, viruses can only reproduce by infecting cellular organisms; they are estimated to kill ~20 % of life in the ocean daily. Thus, viruses are major players in mortality, biogeochemical cycling and structuring marine ecosystems. Despite their importance, we still know relatively little about aquatic viruses and their impact on the mortality of specific organisms. For ~30 years, my laboratory has been a leader in the study of aquatic viruses and their ecosystem effects. The proposed research builds on this foundation, and focuses on virus discovery and quantifying the role of viruses in mortality, two areas where new knowledge has the potential for a large impact. There are still many viruses to be discovered, and many organisms for which viruses are unknown or poorly characterized. Giant Viruses (GVs) are a group of viruses where the potential for making high-impact discoveries is high. These leviathans of the viral world have the largest genomes of any viruses and surpass the size and complexity of those of some bacteria, bridging the gap between cells and viruses. These viruses are only known to infect protists, which include major groups of phytoplankton and zooplankton. Yet, only a few GVs have been isolated that infect plankton, even though sequencing data indicates they are widespread and abundant. Isolating more GVs may shed light on their origin and help elucidate their biology and function in nature. Another area that is ripe for viral discovery are groups of organisms for which no known viruses exist. Typically, many different types of viruses infect a single organism; yet, of almost 200 recognized bacterial phyla, viruses are only known for 39 of them. Hence, there is a high probability of discovering new viral lineages that have different biological properties and code for unknown proteins. The final area of viral discovery is viruses associated with copepods, crustacean zooplankton that are a food source for fish, but which can also be fish pathogens and potential vectors for disease. Identifying copepod viruses will allow us to track them in host populations, a first step in determining their relationship to disease. The other area of focus is on quantifying the impact of viruses on microbial mortality. Viruses kill about 20% of microbes each day, but are typically very host specific, so their impact will be selective. As different microbes have different ecosystem functions the effects may be broad. Newly developed methods will be used to look at the mortality of specific microbial taxa. This will allow us to test the hypothesis that the fastest growing microbes are subject to the greatest lysis, which has important implications for population dynamics.