Ecology of diatom viruses: connecting physiology and field dynamics through host transcriptional responses

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Overview: Viruses play critical roles in aquatic ecosystems. Phages infecting marine bacteria are abundant members of the plankton that contribute to cell mortality, structure population diversity and drive genome evolution though horizontal gene transfer. Viruses infecting eukaryotic phytoplankton have been demonstrated to induce both life cycle switching and programmed cell death in coccolithophorids and be significant agents of mortality in blooms of pelagophytes, haptophytes and raphidophytes. However, much less is known about viruses infecting one of the largest, most diverse and most productive groups of algae, the diatoms. Only thirteen diatom infecting viruses have been reported, and little is known about their mechanisms of infection, effects on host metabolism or diversity and dynamics in the field. This is a remarkable knowledge gap considering the ecological importance of the diatoms. Infection with a clonal virus on Pseudo-nitzschia multiseries can result in complete host lysis within 12-16 hours. The P. multiseries virus (PmDNAV) is a single stranded DNA virus with an icosahedral capsid of 50 nm. The PmDNAV infects the widest host range of any marine eukaryote-infecting virus, lysing other strains of P. multiseries, other species of Pseudo-nitzschia, and other genera of diatoms including many centric diatoms. With the recent completion of the genome of the host, P. multiseries, we now have a model system to investigate the response of the host to viral infection and the potential impacts of viruses on diatom mortality in the field. The objectives of this project are to:1. isolate and characterize additional diatom viruses utilizing established methods, using a variety of host strains and field viral concentrate combinations2. use RNA-Seq to determine the transcriptional profiles of three diatoms (P. multiseries, P. pungens and T. pseudonana) during the course of viral infection3. determine a surface water metavirome at four stations on a coastal to open ocean transect in diatom dominated waters in the Pacific Northwest (line P), with an emphasis on diversity and biogeography of ssDNA and ssRNA viruses. Viral and host genes whose expression is diagnostic of viral infection, will be identified by observing genomic responses to infection in culture. These genes, along with viruses assembled in the metaviromes, will be combined with eukaryotic metatranscriptomes already available from the same waters to assess virus activity in the field.Intellectual Merit: This project seeks to strengthen the model system initiated by the discovery of the Pseudo-nitzschia multiseries DNA virus. The host-virus transcriptomics will lay the groundwork for assessing the impact of viruses on diatom communities in the environment. In turn, the paired metaviromes and metatranscriptomes will reveal new questions about both diatom virus diversity and function that can then be further explored by controlled, culture-based experiments. This research will be the first extensive exploration of diatom virus ecology and function and will ultimately help further connect viruses and diatoms to global biogeochemical cycles, unravel complex organismal interactions, and inform ocean-related public health.Broader Impacts: Diatoms are the dominant primary producers in the productive coastal oceans and contribute significantly to carbon export across all oceanic regimes. Viral impacts on species distributions and cell abundances have implications for health of fisheries and ecosystem responses to climate change. More specifically, there are public health and economic impacts associated with Pseudo-nitzschia blooms. Another goal of this project is to broaden the pool of individuals who pursue graduate work in environmental sciences by providing students from underrepresented minority groups early research experience, sustained financial support, peer mentorship and exposure to the discipline at a national level. A graduate student and five undergraduates from underrepresented groups will be involved in the research.