Demographics of lytic viral infection of coastal ocean Vibrio

Abstract

Viral predation on bacteria in the ocean liberates carbon from the particulate fraction, where it is accessible to higher trophic levels, and redirects it to the dissolved fraction, where it supports microbial growth. Although viruses are highly abundant in the ocean little is known about how their interactions with bacteria are structured. This challenge arises because the diversity of both bacteria and viruses is exceedingly high and interactions between them are mediated by specific molecular interactions. This thesis uses heterotrophic bacteria of the genus Vibrio as a model to quantify virus-host interactions in light of host population structure and ecology. The methods developed in this thesis include streamlining of standard bacteriophage protocols, such as the agar overlay, and facilitate higher throughput in the isolation and characterization of novel environmental virus-host systems. Here, >1300 newly isolated Vibrio are assayed for infection by viral predators and susceptibility is found to be common, though total concentrations of predators are highly skewed, with most present at low abundance. The largest phylogenetically-resolved host range cross test available to date is conducted, using 260 viruses and 277 bacterial strains, and highly-specific viruses are found to be prevalent, with nearly half infecting only a single host in the panel. Observations of blocks of multiple viruses with nearly identical infection profiles infecting sets of highly-similar hosts suggest that increases in abundance of particular lineages of bacteria may be important in supporting the replication of highly specific viruses. The identification of highly similar virus genomes deriving from different sampling time points also suggests that interactions for some groups of viruses and hosts may be stable and persisting. Genome sequencing reveals that members of the largest broad host-range viral group recovered in the collection have sequence homology to non-tailed viruses, which have been shown to be dominant in the surface oceans but are underrepresented in culture collections. By integrating host population structure with sequencing of over 250 viral genomes it is found that viral groups are genomically cohesive and that closely-related and co-occurring populations of bacteria are subject to distinct regimes of viral predation.