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Abstract Viruses infecting aquatic microbes vary immensely in size, but the ecological consequences of virus size are poorly understood. Here we used a unique suite of diverse phytoplankton strains and their viruses, all isolated from waters around Hawai'i, to assess whether virus size affects the suppression of host populations. We found that small viruses of diverse genome type (3–24 kb genome size, 23–70 nm capsid diameter) have very similar effects on host populations, suppressing hosts less strongly and for a shorter period of time compared to large double‐stranded DNA viruses (214–1380 kb, 112–386 nm). Suppressive effects of larger viruses were more heterogeneous, but most isolates reduced host populations by many orders of magnitude, without recovery over the ~ 25‐d experiments. Our results suggest that disparate lineages of viruses may have ecological consequences that are predictable in part based on size, and that ecosystem impacts of viral infection may vary with the size structure of the viral community. 

Abstract Marine microbes are important in biogeochemical cycling, but the nature and magnitude of their contributions are influenced by their associated viruses. In the presence of a lytic virus, cells that have evolved resistance to infection have an obvious fitness advantage over relatives that remain susceptible. However, susceptible cells remain extant in the wild, implying that the evolution of a fitness advantage in one dimension (virus resistance) must be accompanied by a fitness cost in another dimension. Identifying costs of resistance is challenging because fitness is context‐dependent. We examined the context dependence of fitness costs in isolates of the picophytoplankton genusMicromonasand their co‐occurring dsDNA viruses using experimental evolution. After generating 88 resistant lineages from two ancestralMicromonasstrains, each challenged with one of four distinct viral strains, we found resistance led to a 46% decrease in mean growth rate under high irradiance and a 19% decrease under low. After a year in culture, the experimentally selected lines remained resistant, but fitness costs had attenuated. Our results suggest that the cost of resistance inMicromonasis dependent on environmental conditions and the duration of population adaptation, illustrating the dynamic nature of fitness costs of viral resistance among marine protists. 

Abstract In oligotrophic oceans, the smallest eukaryotic phytoplankton are both significant primary producers and predators of abundant bacteria such asProchlorococcus. However, the drivers and consequences of community dynamics among these diverse protists are not well understood. Here, we investigated how trophic strategies along the autotrophy‐mixotrophy spectrum vary in importance over time and across depths at Station ALOHA in the North Pacific Subtropical Gyre. We combined picoeukaryote community composition from a 28‐month time‐series with traits of diverse phytoplankton isolates from the same location, to examine trophic strategies across 13 operational taxonomic units and 8 taxonomic classes. We found that autotrophs and slower‐grazing mixotrophs tended to prevail deeper in the photic zone, while the most voracious mixotrophs were relatively abundant near the surface. Within the mixed layer, there was greater phagotrophy when conditions were most stratified and when Chlaconcentrations were lowest, although the greatest temporal variation in trophic strategy occurred at intermediate depths (45–100 m). Dynamics at this site are consistent with previously described spatial patterns of trophic strategies. The success of relatively phagotrophic phytoplankton at shallower depths in the most stratified waters suggests that phagotrophy is a competitive strategy for acquiring nutrients when energy from light is plentiful.