Diel patterns of symbiont expulsion in Caribbean reef corals are coral species-dependent and driven by symbiont load and photosynthetic performance

Abstract The mutualistic relationship between scleractinian corals and dinoflagellate algae (Family Symbiodiniaceae) is critical to the success of corals on tropical reefs yet underpins their vulnerability to climate change. This symbiosis often deteriorates during marine heatwaves as corals rapidly expel their symbionts, inducing mortality unless symbiosis is restored. While symbiont loss during coral bleaching is well-documented, little is known about symbiont turnover and expulsion under non-bleaching environmental conditions. To better understand rates and patterns of symbiont turnover under ambient (non-stressful) conditions, we used a novel methodology to capture and quantify expelled symbionts of six Caribbean reef-building coral species. Replicate 5-cm 2 fragments of Acropora cervicornis , Diploria labyrinthiformis , Montastraea cavernosa, Orbicella faveolata , Pseudodiploria clivosa and Siderastrea siderea were incubated in 150 mL of 0.2 µm filtered seawater and maintained at 28 °C for 7 h during either day (06:00 h–13:00 h) (110 µmol quanta m −2 s −1 ) or night (18:00 h–01:00 h) conditions. Seawater from the incubation chambers was then concentrated and symbionts counted using a hemocytometer. Calculated expulsion rates were paired with relative symbiont densities in the coral tissue (symbiont/host cell ratios using quantitative PCR) and their photochemical efficiency (Fv/Fm, measured using chlorophyll fluorometry). We found that symbiont expulsion varied significantly by symbiont genus, abundance, performance, coral species, and time of day. Our findings offer insight into coral-Symbiodiniaceae homeostasis, mechanisms of coral-algal symbiosis and dysbiosis, and the potential symbiont availability and spread of symbionts in the environment and have implications for restoration approaches that involve provisioning or seeding corals with algal symbionts.