Vent fluids mixing with seawater at deep-sea hydrothermal vent systems support hotspots of biological activity by microbial chemosynthetic primary producers. Single-celled microbial eukaryotes (or protists) play ecologically significant roles in marine food webs, including as heterotrophic protists that consume microbial prey. We know a diverse community of protists inhabit the deep-sea hydrothermal vent environment, but less is known regarding their role as grazers. Interactions between chemosynthetic microorganisms and protists at these sites, especially in the form of grazing, undoubtedly impact the distribution and richness of the resident chemosynthesizers and the transfer of carbon. To quantitatively assess protistan grazing on the bacteria and archaea community, analog prey (bacterial cells with a fluorescent stain) were introduced to hydrothermal vent fluid (<100°C) and incubated. Collected samples enabled the estimation of protistan grazing pressure by considering the loss of prey or rate of cell uptake among protistan grazers. Results revealed protistan grazing pressure to be higher within the hydrothermal vents compared to the background and plume environment. Paired sequence surveys targeted protist populations at the same sites revealed a diverse assemblage of putative grazers, parasites, and symbiotic hosts. This work addresses C-DEBI Theme 2: Activities, Communities, and Ecosystems, by integrating quantitative measurements of protistan grazing activity with a survey of the diversity and community structure of the protist population to provide novel insights into underexplored protist communities at hydrothermal sites and relevant trophic interactions which form the foundation of deep-sea marine food webs and mediate carbon transfer to higher trophic levels.
