Symbiont dynamics and coral regulation under changing temperatures

Abstract Corals play an essential role in marine ecosystems by creating protective coastal structures and habitats for marine biodiversity. Their symbiotic relationship with various algal genera, where corals supply nitrogen in exchange for carbon products, is vital for their survival. However, with some algal genera being temperature sensitive, this vital symbiosis is increasingly threatened by global warming, causing significant symbiont losses, potentially leading to coral bleaching and fatal consequences. Here, we model the optimal regulation of algal populations by corals through nitrogen allocation. Two algal genera compete for nitrogen: one is effective in carbon supply and rapid growth, and the other is resilient to temperature increases. Our testable analytical solution identifies the optimal total algal population as a function of the current temperature and symbiont composition. The model also determines the relative abundances of the two algal genera based on current and historical temperatures. Our findings are consistent with numerous previous observations and experimental studies. The model clarifies how inter-genera competition under varying temperature patterns shapes the composition and dynamics of algal genera in coral symbiosis. It also clarifies that bleaching occurs when the relatively efficient algae fail to exchange enough carbon products at high temperatures.