Insights into Viral-Induced Mortality of the Microbial Food Web Within the California Current Ecosystem Through a Series of Modified Dilution Experiments

The California Current Ecosystem (CCE) contains numerous, short-lived upwelling filaments that extend from the coast and transport water further offshore. These filaments are characterized by colder, nutrient-rich water, resulting in increased phytoplankton activity. The eukaryotic phytoplankton diatoms often dominate in these filaments as they can outcompete other phytoplankton and form large blooms, though there is increasing evidence for diatom susceptibility to infection by RNA viruses. As viruses are estimated to be responsible for the turnover of more than a quarter of photosynthetically fixed carbon by cell lysis, diatom infection could have a significant role in the flux of carbon. To investigate the mortality of phytoplankton across episodic upwelling events in the CCE, a series of modified dilution experiments were conducted using grazer-free and grazer/virus-free diluent. Phytoplankton predation by viruses was found to be strongest within the upwelling waters of a coastal filament. In particular, the diatom Chaetoceros had a negative correlation with its virus within the filament, and when viral predation pressure was experimentally reduced it became the predominant phytoplankton based on 18S rRNA. This upwelled water parcel was subsequently followed via Lagrangian sampling further offshore, where its microbial community composition via 16S and 18S rRNA resembled that of other offshore CCE communities, with non-diatom phytoplankton and protistan grazers dominating. However, increased relative abundance of diatom-like ssRNA viruses was observed after 24 hours, indicating offshore advection of either an active infection or chronically infecting viruses. These studies suggest that viral and grazer impacts vary depending on community structure and that the resolution obtained via the sequencing of both microbial and viral communities provides essential context for understanding estimation of lysis rates within complex communities.