Lewis, C.L. and Coffroth, M.A. 2004. The acquisition of exogenous algal symbionts by an octocoral after bleaching. Science 304: 1490-1492.
Little, A.F., van Oppen, M.J.H. and Willis, B.L. 2004. Flexibility in algal endosymbioses shapes growth in reef corals. Science 304: 1492-1494.
The authors of these back-to-back but independent papers in Science investigated the nature of symbiont recruitment and growth following bleaching in reef corals in order to further our understanding of the adaptive bleaching hypothesis, which in the words of Little et al. "postulates that hosts may be repopulated by better-adapted algal endosymbionts after bleaching."
What was done
In a controlled experiment, Lewis and Coffroth induced bleaching in a Caribbean octocoral (Briareum sp.) and then exposed it to exogenous Symbiodinium sp. containing rare variants of the chloroplast 23S ribosomal DNA (rDNA) domain V region (cp23S-genotype), after which they documented the symbionts' repopulation of the coral, whose symbiont density was reduced to less than 1% of its original level by the bleaching. In a somewhat analogous study, Little et al. investigated the acquisition of symbionts by juvenile Acropora tenuis corals growing on tiles they attached to different portions of reef at Nelly Bay, Magnetic Island (an inshore reef in the central section of Australia's Great Barrier Reef).
What was learned
Lewis and Coffroth say the results of their study show that "the repopulation of the symbiont community involved residual populations within Briareum sp., as well as symbionts from the surrounding water," noting that "recovery of coral-algal symbioses after a bleaching event is not solely dependent on the Symbiodinium complement initially acquired early in the host's ontogeny," and that "these symbioses also have the flexibility to establish new associations with symbionts from an environmental pool." Similarly, Little et al. report that "initial uptake of zooxanthellae by juvenile corals during natural infection is nonspecific (a potentially adaptive trait)," and that "the association is flexible and characterized by a change in (dominant) zooxanthella strains over time."
What it means
Lewis and Coffroth conclude that "the ability of octocorals to reestablish symbiont populations from multiple sources provides a mechanism for resilience in the face of environmental change," while Little et al. say that the "symbiont shuffling" observed by both groups "represents a mechanism for rapid acclimatization of the holobiont to environmental change." Hence, the results of both studies demonstrate the reality of a phenomenon whereby corals could indeed "grasp victory from the jaws of death" in the aftermath of a severe bleaching episode, which is also implied by the fact - cited by Lewis and Coffroth - that "corals have survived global changes since the first scleractinian coral-algal symbioses appeared during the Triassic, 225 million years ago."
Reviewed 21 July 2004