Coral-Killing Dust

Caribbean corals are dying. That much is certain. In the past two decades seaweed and other algae have overrun most of the delicate reefs that took stony corals (animals that secrete hard, calcium carbonate homes) thousands of years to build. Human activities such as running boats aground, dumping sewage and overfishing lead to some of the destruction, but scientists are now beginning to consider a more elusive threat¿soil from Africa.

Peer into the skies of Miami or San Juan on a late summer day, and you'll see that threat lurking as a reddish haze that virtually blots out the sun. Researchers have long known that strong winds periodically sweep clay-rich red soil off the dry surface of the Sahel--the region just south of the Saharan desert in North Africa--and send it across the Atlantic Ocean in giant plumes easily visible from space. The airborne dirt then rains down in south Florida and the Caribbean Sea as it has done every summer for hundreds of thousands of years. (In fact, the topsoil of Barbados is almost entirely African dust.) Only recently have researchers realized that the dust is loaded with potential coral killers--including disease spores, radioactive elements and overabundant nutrients.

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Marine geologist Eugene A. Shinn of the U.S. Geological Survey in St. Petersburg, Fla., became suspicious of the African dust after nearly 30 years of studying the Caribbean¿s reef-building corals and their perplexing demise. When Shinn began his work--and for at least 6,000 years before that time--branching elkhorn and staghorn corals (Acropora palmata and Acropora cervicornis) dominated Caribbean reefs.

To Shinn's dismay a type of tissue-killing bacterial slime called black band disease appeared at some of his research locations in the early 1970s and has proliferated ever since. In the summer of 1983 a common species of plant-eating spiny sea urchin (Diadema antillarum) began dying mysteriously. Within a year 90 percent of them were gone. Without these urchins to graze on them, the algae quickly overran the stony corals, many of which were already sick. In a mere two decades, most Caribbean reefs went from coral- to algae-dominated.

For many years Shinn explored how water pollution might be damaging reefs in the Florida Keys. But nothing he found could explain why the same diseases were killing corals in areas far from human activity. Then in the late 1990s he noticed that the proliferation of coral disease happened to coincide with the onset of severe drought in North Africa and more dust traveling across the Atlantic, a phenomenon studied in depth by Joseph M. Prospero of the University of Miami. What¿s more, Shinn and Prospero realized, the greatest African dust fallout on record was 1983¿the same year as the Diadema die-out.

Hitching a Ride

"It was the simple correlation between the death of these corals and sea urchins and the increase in dust that first got me interested," Shinn says. He and his colleagues suspected that the dust could be carrying spores or bacteria that were causing the diseases, but they had no evidence until recently.

In 1996 Garriet W. Smith and other researchers at the University of South Carolina identified a soil fungus (Aspergillus sydowii) that has been killing soft corals called sea fans (specifically, Gorgonia ventalina and Gorgonia flabellum), which were at one time widespread in the Caribbean. The mysterious fact is that Aspergillus cannot reproduce in seawater, and yet the sea fans have been locked in an ongoing battle against the disease for a decade. Something must be providing the Caribbean waters with a continuous supply of ammunition for the disease. African dust, perhaps?

Smith and his colleagues recently isolated Aspergillus spores from African dust samples collected in the skies over the U.S. Virgin Islands, and they reported their findings in the October 1 issue of Geophysical Research Letters. This report is the first noncircumstantial evidence that the African dust is dangerous to corals, but some researchers are still reluctant to place too much blame. "I'm not ready to jump on the bandwagon, but I don't want to discount it either," says William F. Precht, a marine geologist who has studied coral reefs near Belize since the early 1980s. "My personal feeling is that it's plausible, because these nasties do live in the dust, and we do know that the dust has been increasing over the past two decades."

Unfortunately for the corals--and perhaps for people as well--soil fungi aren't the only nasties hitching a ride with the dust. In African dust collected from the bottom of water cisterns in the Caribbean, Shinn and his colleagues discovered alarmingly high concentrations of poisonous mercury and radioactive beryllium-7. These findings were reinforced last February when scientists at the University of the Azores captured dust samples from an exceptionally large African dust plume as it drifted over their Atlantic island community some 1,500 kilometers west of Lisbon. The dust emitted gamma radiation up to 45,000 dpm per gram, roughly three times the radiation allowed in the workplace, Shinn says.

As for mercury, two particles of the dangerous metal turned up per million particles of dust. That may not sound like much, but mercury usually occurs in the atmosphere in concentrations of a few particles per billion particles of dust. Open-pit mercury mines in Algeria could be the source, Shinn speculates.

More Than a Cargo Van

Particles hitching a ride with the dust may not be the only coral killers. The dust itself could pose a threat. The dust is made up of small grains of iron, aluminum, phosphorus and other elements--all important nutrients known to support microscopic life, but an excess of nutrients can have negative consequences. Marshall L. Hayes of Duke University is investigating whether iron in the dust might encourage the growth of coral-killing bacteria. "You don't have to bring exotic pathogens into an area," Hayes says. "You can have an organism that is potentially pathogenic and give it more nutrients and enhance its ability to be virulent."

Molecules, proteins and other components of cells need iron to function, so in ocean realms where iron is limited, such as some coral reefs, the volume of bacteria and other microscopic life is held in check. A large dose of iron¿from an African dust plume, for instance¿could ignite rampant growth. To test this hypothesis, Hayes injected two species of bacteria known to be coral killers with iron and with African dust, which is some 3 percent iron.

In 1996 Garriet W. Smith and other researchers at the University of South Carolina identified a soil fungus (Aspergillus sydowii) that has been killing soft corals called sea fans (specifically, Gorgonia ventalina and Gorgonia flabellum), which were at one time widespread in the Caribbean. The mysterious fact is that Aspergillus cannot reproduce in seawater, and yet the sea fans have been locked in an ongoing battle against the disease for a decade. Something must be providing the Caribbean waters with a continuous supply of ammunition for the disease. African dust, perhaps?

Smith and his colleagues recently isolated Aspergillus spores from African dust samples collected in the skies over the U.S. Virgin Islands, and they reported their findings in the October 1 issue of Geophysical Research Letters. This report is the first noncircumstantial evidence that the African dust is dangerous to corals, but some researchers are still reluctant to place too much blame. "I'm not ready to jump on the bandwagon, but I don't want to discount it either," says William F. Precht, a marine geologist who has studied coral reefs near Belize since the early 1980s. "My personal feeling is that it's plausible, because these nasties do live in the dust, and we do know that the dust has been increasing over the past two decades."