Effects of Climate Change/Global Warming on Coral Reefs: Adaptation/Exaptation in Corals, Evolution in Zooxanthellae, and Biogeographic Shifts

Increased sea-surface temperatures (SSTs) associated with climate change/global warming have caused bleaching in scleractinian corals (the loss of obligate symbiotic zooxanthellae) on a global basis, resulting in mass mortality of corals and decimation of reefs. This symbiotic relationship makes these corals an excellent bioindicator of climate change. It has been hypothesized that bleaching is a mechanism by which corals can adapt to changing environmental conditions via the “shuffling” of symbiont clades and acquisition of better-adapted symbionts. Experimental research has confirmed that zooxanthellae are sensitive to increases in seawater temperatures, exhibiting apoptosis (a form of programmed cell death) at temperatures of ≥30οC while in situ. The coral hosts, however, tolerate experimental temperatures up to 34οC, not showing signs of apoptosis and necrosis until 36οC. Thus, zooxanthellae currently appear to be poorly adapted to temperature increases, while the corals are resistant to higher temper...