Highly reduced and thermally charged venting fluids from the subseafloor mix with surrounding seawater, creating a sharp geochemical gradient which promotes a hub of biological diversity at the Loihi Seamount hydrothermal vents. While studies of prokaryotic diversity at hydrothermal vent sites have highlighted the important roles microorganisms play in deep sea carbon cycling and offered a unique window into subseafloor microbial communities, depictions of deep-sea marine ecology and food webs are incomplete without characterization of single-celled microbial eukaryotes (protists). I propose to use culture-independent techniques (tag-sequencing and metatranscriptomics) to provide a thorough understanding of protistan biogeography in and near venting fluids, focusing on the vent fluid-seawater interface. Additionally, RNP-SIP paired with metatranscriptomics will be used to further our understanding of the role that protistan grazing plays on the population dynamics of dominant Fe-oxidizing bacteria found at Loihi Seamount. Understanding trophic interactions within the protistan community is incredibly important, as these processes form the foundation of deep-sea marine food webs and mediate a significant amount of carbon transferred to higher trophic levels.