We study predator-prey and host-symbiont interactions among marine protists. The protists are a conglomerate of distantly related single celled eukaryotes that dominate carbon fixation and its consumption in pelagic ocean food webs. Marine phytoplankton play a central role in many biogeochemical cycles and, along with cyanobacteria, are responsible for about half of our planets atmospheric oxygen. We study interactions among marine phytoplankton and protistan grazers in an effort to better understand how various trophic pathways shape microbial food web ecology, biogeochemical cycles, and microbial eukaryotic evolution. We also focus on the ecology and evolution of mixotrophic protists, organisms that combine phototrophy and heterotrophy, in order to understand when and why mixotrophy is important and its role in chloroplast acquisitions. Mixotrophy is a widespread phenomenon in marine microbial foodwebs, and includes algae that eat and protozoa that steal chloroplasts (i.e. kleptoplasty) or host algal endosymbionts.