Contrasting Life Strategies of Viruses That Infect Photo- and Heterotrophic Bacteria, as Revealed by Viral Tagging

Ocean viruses are ubiquitous and abundant and play important roles in global biogeochemical cycles by means of their mortality, horizontal gene transfer, and manipulation of host metabolism. However, the obstacles involved in linking viruses to their hosts in a high-throughput manner bottlenecks our ability to understand virus-host interactions in complex communities. We have developed a method called viral tagging (VT), which combines mixtures of host cells and fluorescent viruses with flow cytometry. We investigated multiple viruses which infect each of two model marine bacteria that represent the slow-growing, photoautotrophic genus Synechococcus (Cyanobacteria) and the fast-growing, heterotrophic genus Pseudoalteromonas (Gammaproteobacteria). Overall, viral tagging results for viral infection were consistent with plaque and liquid infection assays for cyanobacterial myo-, podo- and siphoviruses and some (myo- and podoviruses) but not all (four siphoviruses) heterotrophic bacterial viruses. Virus-tagged Pseudoalteromonas organisms were proportional to the added viruses under varied infection conditions (virus-bacterium ratios), while no more than 50% of the Synechococcus organisms were virus tagged even at viral abundances that exceeded (5 to 10×) that of their hosts. Further, we found that host growth phase minimally impacts the fraction of virus-tagged Synechococcus organisms while greatly affecting phage adsorption to Pseudoalteromonas. Together these findings suggest that at least two contrasting viral life strategies exist in the oceans and that they likely reflect adaptation to their host microbes. Looking forward to the point at which the virus-tagging signature is well understood (e.g., for Synechococcus), application to natural communities should begin to provide population genomic data at the proper scale for predictively modeling two of the most abundant biological entities on Earth. Viral study suffers from an inability to link viruses to hosts en masse, and yet delineating "who infects whom" is fundamental to viral ecology and predictive modeling. This article describes viral tagging-a high-throughput method to investigate virus-host interactions by combining the fluorescent labeling of viruses for "tagging" host cells that can be analyzed and sorted using flow cytometry. Two cultivated hosts (the cyanobacterium Synechococcus and the gammaproteobacterium Pseudoalteromonas) and their viruses (podo-, myo-, and siphoviruses) were investigated to validate the method. These lab-based experiments indicate that for most virus-host pairings, VT (viral tagging) adsorption is equivalent to traditional infection by liquid and plaque assays, with the exceptions being confined to promiscuous adsorption by Pseudoalteromonas siphoviruses. These experiments also reveal variability in life strategies across these oceanic virus-host systems with respect to infection conditions and host growth status, which highlights the need for further model system characterization to break open this virus-host interaction "black box."