The role of viruses in the life and death of phytoplankton

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While phytoplankton are the backbone to aquatic ecosystems, their life and death strategies are not well understood. With such high abundances persisting in nature, single-celled phytoplankton were once believed to be immortal, undergoing perpetual asexual division and only succumbing to death when ingested or sunk to the depths of the ocean. We are now beginning to realize that these single-celled organisms may also die by a number of other natural means. However, the processes and importance of these other modes of death among phytoplankton remain poorly understood. Although relatively few algal viruses have yet been isolated and characterized, they are now recognized to play an important role in the mortality of phytoplankton. In my lab, we have 3 unrelated algal viruses that are only partially characterized. To further understand the diversity of algal viruses our objectives are to: sequence and annotate the genome of HaNIV, examine the transmission of disease and characterize the 2 virions produced during Ha2DNAV infection, and characterize the virus infecting Pycnococcus provasolii. The novel viruses will be compared with known algal viruses to determine relatedness and better understand viral diversity. In another project of this research program we will study cellular death, which can occur by two mechanisms: programmed cell death (PCD) or necrosis. These modes of death have very different signatures, and we have developed methods for examining this in diverse species of algae. The isolation of 3 unrelated viruses (HaNIV, HaV & HaRNAV) that infect the same host (Heterosigma akashiwo), as well as 2 related viruses (HaV & MpV) that infect 2 unrelated hosts (H. akashiwo & Micromonas pusilla), provides us with a unique opportunity to compare the process of death. Our objectives are to determine the characteristics of death that are dictated by the virus and/or the host as an important step toward examining for and determining the prevalence of cell death in phytoplankton communities. By inhibiting the PCD pathway during viral infection, we will also determine if PCD is necessary for a productive viral infection and if it is elicited as a host protection mechanism.