Project: Giant viruses in the open ocean: Is large size adaptive where cells are scarce?

NSF Award Abstract:
Viruses can infect all forms of life. Viruses are highly diverse, and one aspect of diversity is size: genomes of viruses vary more than a thousandfold in length, and the size of viral particles varies nearly a millionfold. The discovery of “giant” viruses was astounding because they can be physically larger and code for more genes than many free-living microorganisms. There is growing evidence that giant viruses are widespread and diverse in the ocean, but much about their ecology remains unknown. What critical ecological tradeoffs vary with virus size, allowing small and large viruses to coexist? Do these tradeoffs cause the distribution of virus sizes to vary across habitats? This project aims to answer these questions for viruses that infect phytoplankton, the microscopic plants that are the foundation of ocean productivity. This research can also influence a diverse array of scientific fields because virus size varies greatly in other ecosystems and host-associated microbiomes. The fundamental constraints on size may be broadly similar across systems, but the processes driving virus size have not been thoroughly investigated in any of them. This project supports the training of a postdoctoral researcher, two graduate students, and undergraduate students in integrative science that includes field, laboratory, and modeling components. National Science Foundation-supported Research Experience for Undergraduates and Tribal Colleges and Universities programs at UH Manoa that serve Pacific Islanders and other underrepresented groups are used for recruiting students. In addition, science outreach at public events in Hawai’i includes an interactive game to communicate ideas about giant viruses and their role in the ocean.

Large viruses may have four advantages over smaller viruses: i) ability to infect a greater diversity of host genotypes, ii) better control of host metabolism, iii) large enough size to enter host cells by ingestion, and iv) greater persistence in the extracellular environment. These advantages may compensate for the advantages held by smaller viruses: higher contact rates with their hosts and greater offspring number per infection. The advantages of large size may be more consequential in oligotrophic habitats, where the microbial eukaryote community is primarily small phagotrophic flagellates (mixotrophs and heterotrophs), at low population densities, with resource-limited growth. The project goals are: (1) To test whether giant viruses indeed dominate in the oligotrophic ocean compared to a productive coastal location, as suggested by initial observations of this research team; (2) To test the above four hypotheses about the advantages of large size by conducting laboratory experiments with diverse viral isolates, and (3) To use an eco-evolutionary model of eukaryotic microbes and their viruses to explain observed size patterns.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.