|Title||Grazer and viral impacts on microbial growth and mortality in the southern California Current Ecosystem|
|Publication Type||Journal Article|
|Year of Publication||2015|
|Authors||Pasulka A.L, Samo T.J, Landry MR|
|Journal||Journal of Plankton Research|
|Type of Article||Article|
|Keywords||aquatic environments; dilution technique; dimethylsulfoniopropionate dmsp; dissolved organic-matter; emiliania-huxleyi; flow-cytometric analysis; grazing experiments; heterotrophic bacteria; microzooplankton grazing; mortality; pelagic food webs; phytoplankton growth; picophytoplankton; viral lysis|
Protistan grazers and viruses are major agents of mortality in marine microbial communities with substantially different implications for food-web dynamics, carbon cycling and diversity maintenance. While grazers and viruses are typically studied independently, their impacts on microbial communities may be complicated by direct and indirect interactions of their mortality effects. Using a modification of the seawater dilution approach, we quantified growth and mortality rates for total phytoplankton and picophytoplankton populations (Prochlorococcus, Synechococcus, picoeukaryotes) at four contrasting sites in the California Current Ecosystem. Grazing mortality was significant in 10 of 15 cases, while viral effects were significant for 2 cases. Nonetheless, mortality estimates for the entire phytoplankton community based on chlorophyll a were 38 +/- 13% higher when viral effects were included, relative to grazing alone. Mortality estimates for picophytoplankton varied in space and among groups. We also explored a potential methodological constraint of this method and hypothesize that heterotrophic bacteria may be affected by the dilution of their growth-sustaining substrates. For all picophytoplankton, estimates of grazing and viral mortality were inversely related within and across experiments. Indirect interactions among grazers and viruses may be important to consider if there are tradeoffs in the grazing and virus resistance strategies of prey/host cells.
"Perhaps the most striking result of the present study is that despite high variability in the responses of individual populations, there emerge consistent and significant inverse relationships between grazing and viral lysis both within individual experiments and across all experiments spanning a range of environmental conditions and grazing pressures."