Abstract
Mortality mechanisms play an important role in how oceanic microorganisms contribute to global biogeochemical cycles. Salps are widespread pelagic tunicates known to remove phytoplankton from coastal and high-latitude waters, but their interaction with microorganisms in the vast tropical and subtropical gyres is not well quantified. Using quantitative measurements of six major marine microorganisms in the guts of six distinct but co-occurring salp species from the North Pacific Subtropical Gyre, we examined the impact and dynamics of salp feeding on marine microorganisms in a vast open ocean region. All salps preferentially removed prey greater than 1 μm in diameter, including marine Synechococcus, diatoms, Crocosphaera, and Chrysochromulina, while the smaller Prochlorococcus and SAR11 were not a major source of prey biomass. We also found that salp feeding varied between salp taxa with some salp guts dominated by both Crocosphaera and Chrysochromulina while others were dominated by Crocosphaera alone. Together, these results suggest that salp impacts are not uniform across taxa and their patterns of selective feeding among marine microbes requires consideration of species-specific feeding strategies and environmental context. Further, this work suggests that the mortality pressure of salp feeding on marine microorganisms may shape microbial community structure and that this pressure varies with the diversity and dynamics of macrozooplankton predators.