Viral-phytoplankton interactions: Isolation, characterization and ecology of viruses infecting the diatom Pseudo-nitzschia Michael Carlson1*, Benjamin Johnson1, Gabrielle Rocap1 1 * School of Oceanography, University of Washington, Seattle, WA Presenting author Abstract Viruses are important regulators of phytoplankton communities, contributing to bloom termination and preventing transfer of fixed carbon to higher trophic levels. Although diatoms are one the most important marine primary producers, only a few diatom-infecting viruses have been characterized. The cosmopolitan diatom genus Pseudo-nitzschia is most noted for the ability to produce domoic acid, a neurotoxin that can reach ecologically dangerous levels during large blooms. Toxic blooms of Pseudo-nitzschia are common in coastal Washington waters but in 2005 was a toxic bloom of Pseudo-nitzschia detected inside Puget Sound. Preliminary data suggest the viral size fraction of water collected during this bloom event is lethal to some cultures of Pseudo-nitzschia. We hypothesize that there may be several viruses that can infect Pseudo-nitzschia, ranging from single stranded RNA viruses to double stranded DNA viruses from the Phycodnaviridae. Isolation and characterization of the putative Pseudo-nitzschia virus(es) is currently underway. Exponentially growing Pseudo-nitzschia cultures are challenged with environmental samples and monitored for potential lytic events via both fluorescence changes and epifluorescence microscopy to quantify virus-like particle production. We have also developed a protocol for growing lawns of Pseudo-nitzschia on solid media to enable individual plaques to be picked for characterization. Our ultimate goal is to develop a genomically characterized virus-host system to better understand bloom dynamics in the field.