Wednesday, June 07, 2006

"Hope for coral"

Reporting on environmental issues can sometimes seem like the situation in the Gary Larson cartoon in which a bunch of cows are grazing in a field, one of them looks up and says: "Wait, a minute! This is grass we´re eating! Grass!"

The point being that the same things can come round again and again, but they´re not always terribly new.

So it looks with ´Hope for coral´ as oceans warm (BBC online). I stand to be corrected, but it looks as if the new paper by Ray Berkelmans and Madeleine van Oppen (which I have not read yet) may add some new and perhaps important matters of detail, but is unlikely to change the essential finding that corals and their symbionts can adapt a little bit to stresses such as higher temperatures -- but probably not enough to cope with predicted changes.

The "discovery" that corals can exchange their algae for varieties which survive at higher temperatures is already well known. On the plus side, the BBC article may generate interest and attention among some people who previously knew very little.

[7 June P.S. After writing this blog post, I wrote to Richard Black, the author of the BBC article, and occasioned communication with Andrew Baker and others. Andrew Baker kindly forwarded a copy of Corals´Adaptive Response to Climate Change (Nature, Vol 430, August 2004), and added there was also a piece in the New York Times on 21 Dec 2004 (see here or comment attached to this post).
Richard Black wrote that he had changed his article a bit to reflect these findings. Tom Goreau added: each "clade" is in fact made up of very many separate species of zooxanthellae, each with different temperature and light tolerances. Most corals are very fussy about just which species, not clade, they associate with. The expert on zooxanthellae genetics is Todd La Jeunesse at Florida Intentational University.]


Anonymous Anonymous said...

As the Seas Warm, Algae Help Some Coral Stand Up to the Heat
December 21, 2004

KEY LARGO, Fla. - For some time, scientists have predicted that the world's coral reefs will be among the first ecosystems to suffer devastating damage from global warming. Some reefs, however, are proving surprisingly resilient, researchers say, not because of qualities of the corals themselves, but because of heat-tolerant algae that live with them.

It may even be possible that heat-related episodes of coral bleaching, which had been viewed as ominous previews of mass coral death to come, could allow these robust algae to spread, leaving corals better able to survive in a warmer world.

The scientists say this strength in the face of warming will not be enough to save the world's coral reefs, which are threatened by pollution, overfishing, tourism and other human activities. But if the findings hold up, ''they essentially buy us time'' to address those issues, said Dr. Andrew C. Baker of the Marine Program of the Wildlife Conservation Society, who led the new work.

Algae and corals live in a symbiotic relationship. Coral polyps absorb and shelter the algae, and as the algae photosynthesize they produce sugars that the corals use for energy. But this relationship can falter when ocean water warms, Dr. Baker said. ''Basically, algae cells are lost from the coral host,'' he said. ''We don't know if it's the algae leaving a sick coral colony or a coral ejecting a defunct algae. I suspect it's a bit of both.''

The result is coral bereft of much of its power supply. The living polyps that give coral reefs their color wither, usually leaving lifeless white skeleton reefs of calcium carbonate. It is as if the reefs have been bleached.

But the new research, led by Dr. Baker, suggests that heat-tolerant algae may move in to replace strains lost in bleaching events. If so, he said, ''some of the doom and gloom might not be as bad as had been suggested.''

The findings are ''definitely good news,'' said Dr. Nancy Knowlton, an expert on corals who directs the Center for Marine Biodiversity and Conservation at the Scripps Institution of Oceanography.

Dr. Baker and his colleagues studied corals affected by El Niño-Southern Oscillation events, natural disruptions of ocean currents that occur periodically and can cause ocean temperatures to rise in one area or another. In particular, they studied what happened to corals in relatively shallow waters off the Pacific coast of Panama, and in the Indian Ocean off Kenya and Mauritius. All of the corals they studied were colonized by Symbiodinium algae, a diverse genus whose various species look exactly alike, but react differently to heat.

Using DNA analysis, they compared the algae populations at those three sites with algae strains in coral reefs in the Persian Gulf, which are relatively tolerant of heat, and to other corals in the Red Sea, where temperatures do not normally rise as high and where algae strains are less heat-tolerant.

The scientists found that reefs in Panama and Kenya, which had experienced unusual high water temperatures, had high proportions of a heat-tolerant strain called symbiont D. By contrast, reefs off Mauritius, where waters remained cooler, had fewer symbiont D algae. They resembled the reefs of the Red Sea, Dr. Baker said.

Dr. Baker said these findings, reported in the journal Nature in August, suggest that when corals lose algae in a bleaching event, heat-tolerant algae can move in to replace them. Symbiont D ''is becoming more common on reefs that are experiencing bleaching events,'' Dr. Baker said. He said the findings were the first ''real-world evidence'' that corals might adapt to warming.

But this kind of adaptation may not be enough ''to let corals off the hook,'' as Dr. Knowlton put it in an interview.

For example, according to a report issued this month by the Global Coral Reef Monitoring Network, scientists do not know exactly what conditions must exist for corals to acquire new strains of algae. They do not even know if changes occur when corals recruit different algae from the water around them, or if something somehow changes the ratio of algae strains growing within them.

Also, Dr. Knowlton said, it is not yet known whether corals colonized with new varieties of algae will grow as well as they once did with other strains. For example, she said, it could be that heat-tolerant algae strains devote more of their energy to reproducing and less to providing sustenance to their coral hosts. And though symbiont D is widespread among the world's corals, Dr. Knowlton said, scientists do not know whether all of them can support the strain.

Answers to those questions await more research, and she said the study of corals was still developing. ''If it were birds it would be 18th- or 19th-century science,'' she said, ''who's living with whom, where.''

Based on past history, the coral bleaching and mortality associated with the severe El Niño event of 1997-98 was a thousand-year event, according to the monitoring network's coral report. That means, in theory, that there is only a 1 in 1,000 chance that similar effects would be seen in a given year. But, the report says, evidence strongly suggests that global warming will turn severe bleaching into ''a regular event'' by the middle of the century. So evidence of coral recovery since 1998 is encouraging, the report says.

''Some people would argue that global climate change is their biggest long-term threat,'' Dr. Knowlton said, ''and it's the one we have the least ability to do anything about. So good news on the global warming front is very welcome.''

About half the reefs that were left badly damaged after the 1997-98 El Niño event have bounced back, Dr. Baker said. ''So even from a major event there can be recovery,'' he said, if overfishing, habitat destruction and other threats are mitigated. ''But there can't be recovery if we triple and quadruple whammy these reefs.''

11:56 am  

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