June 7, 2004

Bleached Bond

Filed under: Adaptation, Animals, Plants

Coral bleaching, long considered an indicator of demise, creates an opportunity for reefs to adapt by creating a new symbiotic relationship with different, and better-adapted, algae.

Delicate coral reefs, bleaching their way to death in the wake of a warmer ocean. Paints a vivid picture, doesn’t it?

In fact, the false paradigm of the Fragility of Nature is perhaps the greatest misconception foisted upon the public in the last 40 years. Just ask any 9th grade science student, and he’ll happily spew forth dogma about “small environmental changes upsetting the delicate balance between ecosystems.”

But any scientist who believes in basic evolutionary principles (that should cover about 99% or more of all scientists, right?) knows that uber-sensitivity in a species is the recipe for extinction. Most of the species around today have stood the test of time–they’ve adapted strategies that allow them to survive a wide variety of extreme conditions.

For environmental scaremongers, the inherent problem with global warming is that it happens far too slowly. This gives species plenty of time to adapt. In many cases, this simply involves moving to a warmer/cooler/wetter/drier locale. Of course, that’s also the reason why no one has ever produced a major non-fiction motion picture about global warming: Who wants to watch a movie about 200 years of gradual climate change?

Which brings us to our main topic for today: The supposed demise of tropical coral reefs as evidenced by coral bleaching. This is a major concern for Sen. John McCain of Arizona, no doubt because of Arizona’s extensive coastline. In his global warming floor debate on October 30, 2003, McCain worried about the destruction of “70% of the heat-sensitive coral reefs in the world due to increases in water temperatures——[that] place reef fisheries in jeopardy. I don’t know what happens when the beginning of the food chain disappears.”

Corals are a perfect candidate for global warming hyperbole: They collectively house a cute animal (tropical fish, which are the koala bear of oceania), they can’t relocate so adaptation is tougher, and they seem to respond to small increases in water temperature by a very strange adaptive strategy–death.

But as you might suspect, the real story is a lot more complicated. Corals have a symbiotic relationship with certain photosynthetic algae in the genus Symbiodinium. The algae get nutrients from the corals and the corals acquire photosynthetic products from the algae. There are different groups (or clades) of Symbiodinium that vary genetically and that can benefit corals in different ways.

When bleaching occurs, corals essentially reject the Symbiodinium and thus lose their color. One of several potential causes of coral bleaching is high water temperatures. After bleaching occurs, large numbers of corals die. So it may seem that bleaching is not a very good evolutionary strategy.

But some corals manage to survive these bleaching events, which allows them to acquire new Symbiodinium that are potentially better adapted to their new environment (the parallels to divorce and second marriage are far too obvious, so we needn’t go there).

In a paper that appeared in Nature three years ago, Andrew Baker proposed that bleaching may be an excellent strategy employed by corals that sacrifices short-term benefits for longer-term gains. That line of thinking would account for corals surviving for millions of years through much harsher climate changes than those that have been experienced over the last few decades.

Now, two new papers in Science add further evidence that corals must not be as “fragile” as certain senators might hope. Cynthia Lewis and Mary Coffroth of SUNY-Buffalo bleached Caribbean corals and exposed them to certain Symbiodinium genotypes for six weeks. The corals not only re-established symbiotic relationships with the algae, but in some cases they changed algae species, giving the corals a unique opportunity to select symbionts based upon the environmental conditions.

The second Science paper, by Angela Little and two coauthors from Townsville, Australia, looked at changing symbiotic relationships over the lifetime of the corals. They found that young juvenile corals tended to interact with different Symbiodinium strains than did adults, which “suggests that there maybe ‘active’ selection by the host to maximize symbiont effectiveness that varies with differences in physiological requirements between juvenile and adult corals.”

So death is not the corals’ only response to change. The ability of corals to shuffle symbionts is an effective adaptive strategy for dealing with environmental changes, such as changes water temperatures and light levels.

Add these two studies to a growing case file in support of the resiliency (rather than the fragility) of Nature. While coral bleaching appears to be mass suicide to uninformed senators, it could actually be an excellent adaptive strategy that has allowed the species to survive for millions of years.


Little, A.F., M.J.H. van Oppen, and B.L. Willis, 2004. Flexibility in algal endosymbioses shapes growth in reef corals. Science, 304, 1492–1494.

Lewis, C.L., and M.A. Coffroth, 2004. The acquisition of exogenous algal symbionts by an octocoral after bleaching. Science, 304, 1490–1492.

Baker, A.C., 2001. Reef corals bleach to survive change. Nature, 411, 765 –766.

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