Effects of Climate Change/Global Warmingon Coral Reefs: Adaptation/Exaptationin Corals, Evolution in Zooxanthellae,and Biogeographic Shifts

摘要

Increased sea-surface temperatures (SSTs) associated with climate change/globalwarming have caused bleaching in scleractinian corals (the loss of obligate symbioticzooxanthellae) on a global basis, resulting in mass mortality of corals and decimationof reefs. This symbiotic relationship makes these corals an excellent bioindicator ofclimate change. It has been hypothesized that bleaching is a mechanism by which corals can adaptto changing environmental conditions via the "shuffling" of symbiont clades andacquisition of better-adapted symbionts. Experimental research has confirmed thatzooxanthellae are sensitive to increases in seawater temperatures, exhibiting apoptosis(a form of programmed cell death) at temperatures of ≥30°C while in situ. The coralhosts, however, tolerate experimental temperatures up to 34°C, not showing signs ofapoptosis and necrosis until 36°C. Thus, zooxanthellae currently appear to be poorlyadapted to temperature increases, while the corals are resistant to higher tempera-tures, indicating that they are either already adapted or exapted (previously termedpreadapted) to such. Coral hosts are experiencing no mortality from increased temper-atures but a great deal of mortality from the death of their zooxanthellar symbionts.Symbiodinium sp. has a short generation time of one to several days, vs. that of its hostcoral—one to several years, affording the symbionts a distinct advantage for adapta-tion. Mutation rates are low in micro-organisms and higher in larger organisms.Smaller organisms have a disproportionately higher mutation rate than larger organ-isms, however, when compared on a per-generation basis. Thus, zooxanthellae arepositioned better to adapt more rapidly than their coral hosts. Zooxanthellae havenumerous clades, some of which are suspected to be better-adapted to higher seawatertemperatures than others. The discovery of new Symbiodinium clades may be due toour ability to detect them via better technology or due to mutation, adaptation, anddirectional selection. Changes in the frequency of occurrence of different clades better-adapted to higher temperatures may be due to the emergence of newly mutated strains(i.e., sub-clades or sub-types) or re-distribution of existing clades. Predictions for thefuture of coral reefs include the following: a) Some tropical corals may be lost to localor global extinction, based on species-specific susceptibility to temperature increases;b) it is unlikely that other symbionts such as cyanobacteria will be able to replaceSymbiodinium in the host-symbiont relationship; c) the tropics and sub-tropics willexpand poleward and the other climatic zones will shift poleward, at the expense of thepolar and sub-polar zones; d) a new "hyper-tropical zone" may appear near the equa-tor, within which corals may be less species diverse, ill represented, or absent; and e)the depth distribution of corals may be extended, but probably only nominally so.