Meet the Choptank's new menace: deadly, shape-changing, tough algae

March 16, 1993|By Arthur Hirsch | Arthur Hirsch,Staff Writer

It sits stone still on the bay floor, a seed encased in a shell, waiting.

As fish swim by, it rouses and releases a toxic chemical into the water. In hours, thousands, perhaps millions of fish are dead, and the killer, sated on its victims' flesh, has returned to slumber in the sediment.

It is not a new movie starring Sigourney Weaver. It's a newly identified alga, and it has arrived at a bay near you.

This hybrid plant-animal microbe -- known to have killed more than a billion fish in six weeks in North Carolina -- has been discovered in a tributary of the Choptank River in Cambridge.

"I've come across a lot of strange things, but nothing like this," said Alan Lewitus, a scientist at the University of Maryland's Horn Point Environmental Laboratory outside Cambridge.

He calls it "bizarre" -- a fitting term for an organism that can go from seed to toxic cell to amoeba and back to seed in just a few hours. It's also one tough little creature: In its seed form, the alga has survived immersion in pure sulfuric acid and ammonium hydroxide. In each case, it rose to kill again.

At the Horn Point laboratory last month, in a 10-gallon tank stocked with sediment from Jenkins Creek, scientists witnessed the strange killing powers of this single-celled beast. In four days, Mr. Lewitus said, nine of a dozen 3-5 inch mummichogs died of suffocation, their respiratory systems attacked by the algae's lethal chemical. In a later test, the fish's skin sloughed off.

Samples taken from the tanks showed the presence of an organism first identified in 1988 by researchers in North Carolina. The algae, a new family, genus and species named Pfiesteria piscimorte, have not yet been linked to any fish kills in the Chesapeake Bay.

Researchers say the algae would not endanger people unless a swimmer ventured into the middle of a fish kill. If exposed to the chemical, an unknown neurotoxin, humans may experience asthmatic symptoms, disorientation and temporary memory loss, said JoAnn Burkholder, the North Carolina State University botanist who has led the research on p. piscimorte, which means "fish killer."

North Carolina researchers confirmed that the creature killed more than a billion menhaden in the Neuse River, near Pamlico Sound, in the fall of 1991. Since then, the algae has been blamed for at least a quarter of all major fish kills in that state, said Ms. Burkholder.

"People are looking for it in Florida," she said. "We think it's all along the southeast coast. We know it's in the Chesapeake and Delaware."

Officials in Delaware have built a "strong circumstantial case" against the algae as the culprit in the sudden deaths of more than 125,000 fish in the Indian River in 1987 and a smaller number of menhaden the following year, said the state fisheries program manager, Roy W. Miller.

He said a sample of Indian River water taken last year from the fish kill sites was sent to Ms. Burkholder. Sure enough, she found p. piscimorte, which until recently was known only as the "phantom algae."

Ms. Burkholder said she has tested the algae on 18 different species of fish, including bass, menhaden, goldfish, clownfish, even scallops and blue crabs. It killed them all.

Unlike the better-known "red tide" algae, which kill fish by massing in great numbers and sucking oxygen from the water, this microbe is deadly in smaller concentrations and does not necessarily color the water. Because the creature can kill invisibly, is known to have at least 15 life stages and is capable of changing quickly from one form to another, it is extremely difficult to detect.

An expert on phytoplankton, or the plant side of the plankton family, Mr. Lewitus said he has never before seen a microbe that can change form so quickly. And he's never seen an alga interact this way with fish.

Ms. Burkholder said she's grown accustomed to skeptical colleagues. One Florida scientist didn't believe the creature existed until she saw videotapes of its metamorphosis made by microscopic photography.

Ms. Burkholder calls it an "ambush predator," whose appetite is triggered by some chemical in fish excretions. That chemical, and the triggering mechanism, has yet to be identified, she said.

Scientists know that the algae lie dormant on the bay floor as a microscopic cyst, or thin-shelled, seed-like structure. When the fish chemical concentration is high enough, it causes the cyst to rise off the sediment, propelled by a whiplike tail. Almost immediately, the risen microbe turns toxic and attacks the fish. The algae display characteristics of both plants and animals, as they produce some of their own food by photosynthesis, but also eat the flesh of the dying fish. They may reproduce by either asexual or sexual reproduction and range in size from 10 to 180 micrometers, the largest form being the blob-like giant amoeba stage. A micrometer is a thousandth of a millimeter.

Ms. Burkholder said much remains to be learned about the organism, including how to kill it. Scientists at Horn Point say they'll continue working with Ms. Burkholder and testing sites around the Chesapeake.

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