Quantifying symbiont communities in Galápagos Pocillopora corals Matt R. Biermann1,2, Katrina R. Giambertone2,3, Margarita Brandt2,4*, Christopher W. Shenoy2, Cheryl A. Logan2  1San Diego Mesa College, 7250 Mesa College Dr, San Diego, CA 92111  2California State University, Monterey Bay, 100 Campus Center, Seaside, CA 93955.  3Moss Landing Marine Laboratories, 8272 Moss Landing Rd, Moss Landing, CA 95039  4Universidad San Francisco de Quito, Campus Cumbayá, Diego de Robles s/n, Quito 170901, Ecuador Abstract:  Climate change has had relentless negative impacts on tropical coral reef ecosystems. Mass coral bleaching is occurring more frequently and becoming more intense, particularly during severe El Niño-Southern Oscillation (ENSO) events. Corals in the Galápagos archipelago have experienced severe bleaching during ENSO events and are exposed to a wide range of temperatures throughout the year due to the convergence of multiple oceanographic currents. Previous work determined that Pocillopora corals from warmer northern sites had greater heat and cold tolerances in comparison to corals from the southern islands that experience cooler and more variable temperatures on average. The mechanisms underlying these thermal tolerance patterns are currently unknown. Utilizing quantitative PCR, we quantified symbiont communities in coral colonies from two islands containing comparatively stress-tolerant (Darwin) and stress-sensitive (Isabela) corals to investigate the role of symbiont community in setting thermal tolerance. We found higher proportions of Durusdinium over Cladocopium in the northern site of Darwin relative to the southern site of Isabela. Darwin colonies (n=5) contained a mix of Cladocopium and Durisdinium, whereas all Isabela colonies (n=5) contained 100% Cladocopium. Our results suggest that symbiont type may underlie relative heat tolerance of corals in the northern Darwin site compared with those from Isabela.  Understanding the role of symbiont type to coral thermal tolerance can help inform ongoing coral restoration efforts in the Galápagos.