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Marine Virology and Microbiology
Pavilion Lake microbialites collected for a viral production experiment
Pavilion Lake microbialites collected for a viral production experiment   

Matthias Fischer



For my Ph.D., I studied a large icosahedral virus which infects the heterotrophic nanoflagellate Cafeteria roenbergensis (Fig. 1). Protist grazers like this nanoflagellate feed mainly on heterotrophic bacteria thereby transferring biomass to higher trophic levels as well as feeding organic carbon back to the microbial loop. Viruses are likely to lyse a significant fraction of these organisms, however reports on viral activity in heterotrophic nanoflagellates are sparse. This study presented the first comprehensive description of such a virus-host system, including a detailed genetic characterization of the virus (Fig. 2) which belongs to the group of Nucleocytoplasmic Large DNA Viruses (NCLDV). Cafeteria roenbergensis virus (CroV) has several unique features, including a 730 kb dsDNA genome and many genes encoding putative functions that are highly unsual for a virus. Genetic and electron microscopic studies of CroV led to the discovery of a second, smaller virus named Mavirus, which is a virus parasite (virophage) of CroV. Mavirus replicates within the CroV virion factory in co-infected C. roenbergensis cells, which leads to decreased CroV production and increased host cell survival. In contrast to the other known virophage, Sputnik, the genome of the CroV-associated Mavirus is most similar to DNA transposons of the Maverick/Polinton class. It is hypothesized that these transposons originated from the endogenization of ancestral virophages similar to Mavirus.


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