04 April, 2016
Currently, it is unknown whether the thermo- and salinity-resilient dinoflagellate Symbiodinium thermophilum originated elsewhere or emerged from unexpectedly fast evolution catalyzed by the extreme environment of the Persian/Arabian gulf (PAG). Analyzing genetic diversity of symbiotic algae across >5,000 km of the PAG, the Gulf of Oman, and the Red Sea coastline, we show that S. thermophilum is a member of a highly diverse, ancient group of symbionts cryptically distributed outside the PAG. We argue that the adjustment to temperature extremes by PAG corals was facilitated by the positive selection of preadapted symbionts. Our findings suggest that maintaining the largest possible pool of potentially stress-tolerant genotypes by protecting existing biodiversity is crucial to promote rapid adaptation to present-day climate change, not only for coral reefs, but for ecosystems in general.