Heat Damage To "Photosynthesis Engine" In Symbiotic Algae May Be Among Major Causes Of Coral Bleaching
ATHENS, Ga.-- Scientists worldwide have been perplexed for more than a decade by extensive bleaching in the ecologically important coral reefs that ring the globe. Suspected culprits in the damage have been everything from bacteria to pollution.
Recent studies have strongly implicated the gradual warming of ocean temperatures as a major cause of coral reef bleaching, and a new study by researchers at the University of Georgia confirms it. It turns out, however, that the higher temperatures aren't necessarily damaging the reef-building corals directly but instead are degrading the ability of symbiotic algae, upon which the survival of their hosts is dependent, to convert light into utilizable energy.
"Because coral bleaching is occurring on such a global scale, the idea that the problem was a direct effect of elevated temperature in sea water made sense," said Dr. Mark Warner, a postdoctoral researcher in botany and ecology at UGA. "The chance that we were seeing, instead, activation by heat of a pathogen everywhere was remote."
The research from the UGA team, which includes botany professor Gregory Schmidt and ecology associate professor William Fitt, in addition to Warner, was published this week in the Proceedings of the National Academy of Science. It will also be presented next Wednesday, July 14, at the meeting of the American Society for Photobiology in Washington. D. C.
Coral reefs are extremely important to ecosystems and to the tourist industry. They cover around two million square kilometers and support some 2500 species of coral and more than 5000 species of fish. But coral bleaching has been worrying researchers since the mid-1980s, and scientists have known that one reason is the reefs' loss of the corals' symbiotic algae and their photosynthetic pigments.
"Corals can be compared to a machine or an automobile in that certain components are more susceptible to stress than others," said Fitt, who is also on the board of directors of the Key Largo Marine Research Laboratory in Florida, a private, nonprofit research organization where a number of coral scientists conduct research. "However, if the algae in the corals are unhealthy or stressed, the hosts either expel or digest them."
The underlying biochemical causes for the reef bleaching have remained obscure, however. In field tests conducted in the Caribbean and replicated in the laboratory, however, the UGA researchers pioneered the use on coral symbionts of a technique known as pulse-amplitude modulation fluorometry or PAM fluorometry. The technique allows scientists to determine the efficiency of light used by the symbiotic algae, an improvement over other methods of measuring their metabolic activity. What the team discovered may give a major clue to how the problem of coral bleaching has arisen.
During the summer of 1997, seawater temperatures in the Florida Keys remained above normal for weeks on end. The team sampled colonies of the dominant Caribbean reef-building coral species, Montastrea faveolata and Montastrea franksi over a depth gradient from one to 17 meters.
"We used the PAM fluorometry analysis to discover that there was severe damage to the photosystem II [PSII] in the symbiotic algae," said Schmidt. PSII can be roughly described as the carburetor and one of the two pistons of the engine that runs photosynthesis, and is the point at which photosynthesis begins. The electron energy created by light in PSII is then passed to photosystem I, where much of the actual energy storage takes place -- a kind of two-stage electron escalator. "But the major message isn't that PSII gets damaged but that coral species are different in their ability to repair damage or avoid it."
The team also surprisingly discovered that a major protein of PSII called D1 had been damaged, impairing the ability of the algae to carry out photosynthesis. The D1 protein is common to plants, and in land plants exposed to sunlight, for example, the D1 protein engages in a continual cycle of breakdown and repair depending on how much sunlight a plant gets. The damage to the D1 protein in the bleaching algae, however, appears irreversible at temperatures just above the normal summer maximum.
The algae, or dinoflagellates as they are also called, present daunting problems to researchers. For reasons not yet understood, the algae from most corals cannot be grown in cultures in laboratories after they are isolated. The UGA researchers needed some way to discover possible differences in these algae, however, so they grew in the laboratory dinoflagellates cultured from the giant clam and a Caribbean coral. Lab tests unveiled significant differences in thermal tolerance between them.
If an increase in water temperatures is causing the algae to lose their ability to carry out photosynthesis, then do the algae die?
"That really isn't known yet," said Warner. "We know that the algae are expelled from the host coral, and perhaps it's because the host knows that the algae are no longer producing sugars the host needs." Tests in other laboratories have found that the algal cells expelled from coral appear necrotic or rotten but their ultimate fate is not yet known.
There is some evidence that in terms of coral bleaching "good" algae may remain in place in the coral while the "bad" are expelled, allowing algal natural selection to occur at increased water temperatures. Coral reefs have more than one kind of algae, however, and the extent of natural selection is yet unknown because coral bleaching is such a recent phenomenon.
"Our research also found that the deeper the water, the more susceptible these algae are to damage," said Schmidt. "They are rather like shade plants on land that can't tolerate full sun."
There's another reason why the PSII function might have been impaired in the symbiotic algae in the summer of 1997. Along with increased water temperatures, there was a sustained period of calm weather before and during the bleaching event, meaning light could have been able to penetrate farther into the water since there was less mixing by wave action.
The knowledge gained by other scientists about the link between ocean heat and bleaching has led the National Oceanic and Atmospheric Administration to develop methods of pinpointing ocean "hot spots" where coral bleaching could occur. The widespread discovery of coral bleaching has thus been clearly associated with rising ocean temperatures, enough for researchers to know this may well not be only a cyclical problem.
"This seems to be much more than background noise on a geological time scale," said Warner.
The research was supported by grants from the National Science Foundation, the National Oceanic and Atmospheric Administration National Undersea Research Program and the Department of Energy. The NSF recently extended its support to the research team with another grant of $400,000 for further studies on this problem.
Source: University Of Georgia. July 1999.
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