Influence of Nitrogen Source on Mixoplankton-Giant Virus Infection Dynamics

Abstract

Giant viruses are large, double-stranded DNA viruses that play an important role in marine ecosystems by killing unicellular eukaryotic plankton. Due to their size, giant viruses are hypothesized to enter and infect aquatic protists via phagocytosis, potentially disguised as bacterial prey. This study investigates the role of nitrogen source on the infection dynamics of two dictyochophyte-infecting giant virus strains, DictyXV and FloV-SA1, when infecting six mixotrophic algal strains. The viruses and algae in this study were isolated from the epipelagic zone of the North Pacific Subtropical Gyre, where mixotrophic algae consume prey to overcome nutrient limitation. We hypothesized that the algae grown under mixotrophic conditions would face increased susceptibility to viral infection, and therefore greater mortality, because they would be conditioned to graze on bacterial and viral-sized particles. First, we acclimated the parent cultures to two different conditions: dissolved N (autotrophic growth) or prey-derived N (mixotrophic growth) in seawater-based medium. Second, we infected the experimental cultures with either DictyXV or FloV-SA1, and monitored algae and virus populations using flow cytometry until ten days post-infection. These results were then analyzed to determine percent mortality and burst size, and statistical models tested whether virus strain, host strain, and N source explained the success of the viral infections. Our results indicate that FloV-SA1 infection leads to higher percent mortality than DictyXV in 11 of 12 host:medium combinations and that FloV-SA1 had a slightly larger burst size than DictyXV , suggesting it is able to replicate more efficiently inside the host cells. We found that N source variably affected the magnitude of mortality and burst size in most algae strains, and not always in the direction hypothesized. These results emphasize that N source and trophic plasticity can shape virus-mixotroph interactions, complicating our ability to predict how these interactions may contribute to marine biogeochemical cycles.