Dr. Sheri Floge, The Ohio State University

Marine viruses: key players in planktonic food webs and carbon cycling

Drifting photosynthetic microbes occupying the sunlit waters of the global ocean support complex food webs that ultimately provide essential nutrition to human populations and regulate global scale biogeochemical cycling. Within planktonic food webs, marine microbes and mesozooplankton are connected by a wide range of individual biotic and abiotic interactions, the sheer complexity of which hinder accurate prediction of oceanic carbon (C) fluxes. Using laboratory- and field-based stable isotope pulse-chase multitrophic food web experiments, I sought to quantify the net impact of marine viruses on cross-trophic C and nitrogen (N) transfer in systems dominated by picophytoplankton. The data show that viruses both stimulated dissolved organic carbon (DOC) production and bacterial growth, consistent with the viral shunt hypothesis, and increased C and N transfer from picophytoplankton to the copepod Acartia tonsa. Thus, viruses not only ‘shunt’ but also ‘shuttle’ C in complex planktonic food webs. In addition, copepod fecal pellet production rates were higher in the presence of viruses, suggesting a mechanism for viral enhancement of C export. Multi-omics analyses revealed that intact, virus-infected cyanobacteria release labile compounds during early stages of infection, potentially stimulating zooplankton grazing. Taken together, the data suggest that the combined effects of viral and zooplankton predation can be greater than the sum of individual interactions, and illustrate the importance of understanding the net impact of multiple microbial interactions when linking microbial cellular physiology to global scale biogeochemical processes.