Giant viruses are loosely defined as having a genome greater than 280,000 base pairs (bp) and currently there are less than 20 full virus genome sequences in this size range. A cursory analysis of this group immediately reveals a diverse range of viruses including poxviruses, phycodnaviruses (algal viruses), herpesviruses, a bacteriophage and nimaviruses (shrimp white spot syndrome virus). The largest to date is the Acanthamoeba polyphaga Mimivirus at 750 nm diameter with a 1.2-million bp genome. Genome sequence data from these giant viruses can help clarify their taxonomic position. These giant viruses possess novel and typically non-virus genes, many of which encode central cellular functions and metabolic pathways. Preliminary observations by flow cytometry and analysis of the Global Ocean Sampling (GOS) database suggest that giant viruses are extremely abundant in aquatic environments and have been overlooked by biased sampling techniques. Most virus studies look at what passes through a 0.2 micrometer filter. Giant viruses, up to 0.75 micrometers, will be caught on these filters and evade detection. The central hypotheses of this project by Drs. W. Wilson (Bigelow Lab for Ocean Sciences) and S. Wilhelm (Univ Tenn) are that giant marine viruses are ubiquitous, diverse and contain large amounts of novel genetic information. Four algal viruses from current collections and 16 additional giant viruses isolated from seawater are being sequenced in this project. The environmental viruses are being isolated and sequenced by adapting single cell sorting techniques. Once sorted, giant viruses are being amplified by multiple displacement amplification (MDA) to generate Giant Virus Single Amplified Genomes (GV-SAGs) for downstream sequencing. To accomplish this work, the PIs are using state-of-the-art facilities for flow cytometry (Bigelow) and sequence analysis (University of Tennessee/Oak Ridge National Laboratory) coupled with bioinformatics expertise from collaborator Professor Jean-Michel Claverie, a world-leader in giant virus research.
This research is being discussed during an informal Café Scientifique program designed to teach the public about the latest ideas in science. The PIs are also incorporating a 3-orders of magnitude (virus, lipids/proteins & DNA) module into the Keller-Bigelow Laboratory Orders Of Magnitude (BLOOM) program, designed to teach Maine high school students about marine ecosystems. The PIs are partnering with the School of Journalism at the University of Tennessee (UT) to bring undergraduate and graduate journalism students into the lab to document this research for the general public.
Viruses are considered the smallest biological entities on the planet - but even amongst them there are giants. Aureococcus anophagefferens is the causitive agents of Brown Tides on the eastern seaboard of the United States. In recent years researchers have shown that a large virus (called AaV) can infect and lyse populations. Indeed, during bloom collapse as many as 2.5 x 1020 AaV particles may be found in Great South Bay (NY) alone! DNA sequencing has been used to study this virus as well as other viral leviathans from marine surface waters as part of this study. Information from the genome of AaV tells us it is a member of the Mimiviridae family - only the second member of this group known to infect algae (all others appear to infect Amoeba). Moreover, analyses of the structure of the genome demonstrates that only a ~160 kBp region is needed for this viruses to be classified in this group (making it the smallest member of the family - hence its nickname "Mini-Mimi". The observations in this study also demonstrate how these giant viruses may grow: large numbers of repeats that have a character similar to transposable elements ("jumping genes") are found on the terminal ends of the viruses linear genome and contain genes the virus has stolen from other organisms. Along with the study of AaV, this work has also taught us how to isolate individual virus particles and sequence their genomes. This high tech approach to virology is allowing us to, for this first time, study intact and individual virus particles without knowing whom they infect. It is providing us with a great deal of information concerning the genetic potential of viruses in the ocean, and may one day lead to new discoveries concerning enzymes and other bioactive compounds.
