Killer CLOD strikes Indo-Pacific reefs.

Over the next few weeks, dozens of biologists, particularly seaweed specialists, will receive a postcard of this color picture. But rather than discuss the pleasantries of a tropical vacation, the writing on the back warns these researchers to look out for this telltale orange color when they dive along reefs in Indo-Pacific. Their sightings will help researchers determine the extent of a plague that seems to have taken hold in these waters.

Though theses 4-inch-wide patches brighten the underwater landscape, they also forewarn of the spread of coralline lethal orange disease (CLOD), says Diane S. Littler, a marine botanist with the Smithsonian Institution's National Museum of Natural History in Washington, D.C.

She and her husband, Mark M. Littler, first noticed an orange coating on the seaweed in the Cook Islands last year. They then studied it more intensively in Fiji. CLOD, a bacterial infection, has spread also to Papua New Guinea and the Solomon Islands. The bacteria seem most prevalent in the Cook Islands, where they now infect up to 5 percent of the reef seaweeds, says Diane Littler.

CLOD microbes attack coralline algae, underwater plants that deposit calcium carbonate in their tissues as they grow. "Most bacteria would infect just the surface layers of [of a plant]," Diane Littler says. "But somehow these [bacteria] are killing the [plant] clear down to the calcium carbonate.... It kills everything in its path."

All that remains is a white strip, a ghostlike reminder of where algae once grew. Judging from the amount of white present a year after they first noticed the orange patches, the bacteria "did quite a lot of damage" in the Cook Islands, she reports.

When healthy, these algae form a hard surface that resists being battered by waves better than does coral itself. Because they are so tough, these seaweeds form the outer rim of many reefs and protect coral from the roughest water, says Diane Littler. Also, coralline algae help cement reefs together.

Once the algae disappear, the reef begins to disintegrate. Therefore, she worries that this newly discovered epidemic - the first documented in reef-building algae in the Indo-Pacific - could wreak havoc on many reefs.

Moreover, the microbes work quickly and do not seem too picky about their quarry. The Littlers placed 10 different species of red coralline algae next to an infected plants. By the next day, each species had developed an orange spot of its own. Also, the Smithsonian botanists have observed that the bacteria reproduce by producing 1- to 2-millimeter-wide orange droplets on the patch's surface. Waves sweep the droplets away, which can still infect a plant 2 weeks later, plenty of time to travel hundred of miles, says Littler.

Diane Littler has turned over samples of the CLOD bacteria to the University of Maryland at College Park for analysis. Meanwhile, the Littlers' initial observations have been accepted for publication in the journal Coral Reefs.