Polar marine viral diversity and dynamics

As primary producers and recyclers, microbes are a key component of the ocean's ecosystem functioning. Viruses infect them, often lysing host cells and releasing viral progeny along with cellular contents that fuel the microbial food web. They exert a top-down control that shapes population dynamics, diversity and succession. Viral seasonality, diversity and activity remain understudied, a gap even more pressing in polar environments, which experience strong seasonal variability and climate-driven shifts in food web dynamics and carbon cycling.
This thesis advances our understanding of polar marine viruses. Chapter 2 provides the first high-resolution seasonal exploration of dsDNA viral diversity in coastal Antarctica. We identified 7,957 viral species-level operational taxonomic units, of which 75% were novel. The findings expand our understanding of the diversity and dynamics of crass-like viruses, polinton-like viruses (PLVs), virophages, and Nucleocytoviricota viruses (NCVs). Chapter 3 examines a phytoplankton mortality time-series across two cold and one warm productive seasons. Warming increased gross growth and grazing, while viral lysis remained relatively independent of temperature, exerting a greater relative impact during colder seasons and accumulation phases. Chapter 4 couples flow-cytometric sorting with shotgun sequencing, putatively linking NCVs, virophages and PLVs to key phytoplankton taxa such as Micromonas, Phaeocystis and cryptophytes. Chapter 5 uses one-step infection experiments with Micromonas polaris and MpoV-45T to test the effect of light intensity and duration and temperature, on infection dynamics. Together, these findings portray polar marine viruses as reservoirs of genetic novelty and driving forces shaping host dynamics, succession, and ecosystem carbon flow.