Corals are ecosystem engineers in that the growth of their calcium carbonate skeleton creates the biogenic structure on which the entire ecosystem depends. The calcification and growth of reef corals depends on a mutualism between corals and their intracellular, photosynthetic dinoflagellates, Symbiodinium spp., also known as zoo-xanthellae. Both corals and zooxanthellae benefit from this relationship, as corals can receive up to 95% of their carbon from the zooxanthellae's photosynthesis, while the zooxanthellae acquire the nitrogen and other inorganic nutrients in coral excretion products for their growth. Photosynthesis by zooxanthellae enhances calcification in corals and increases coral growth rates, ultimately leading to reef accretion and the massive reef framework found in many tropical seas. Thus, the physical structure of live and dead corals created by the coral-zooxanthellae mutualism provides heterogeneity and habitat complexity, facilitating the coexistence of diverse plant and animal assemblages.
Although zooxanthellae were initially assumed to represent one species, recent molecular evidence shows that there are at least seven distinct types or clades (referred to as clades A-G). Many corals house multiple clades of zooxanthellae, setting the stage for possible competition among symbionts and for symbiont selectivity by the host. Clades of zooxanthellae differ in their photosyn-thetic capacity and their tolerance of light, temperature, and other stressors, making them differentially useful to their hosts under changing environmental conditions. When corals are stressed by increasing light levels or temperatures, they often expel their zooxanthellae and become pale in color (called coral bleaching). This process of bleaching may allow corals to take up new clades of zooxanthellae that are better adapted to the new environmental conditions. However, corals that fail to re-acquire zooxanthellae or acquire the wrong clades may ultimately die from the stress, suggesting that a failure of corals to acquire appropriate symbionts can be fatal under changing environmental conditions. Such alterations in the coral-zooxanthellae mutualism may allow corals greater flexibility in adapting to global climate change, which is a major threat to the health of coral reefs and the integrity of the coral-zooxanthellae mutualism.
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