Fish Killer Caught? Ephemeral Pfiesteria compound surfaces

A team of researchers claims to have found an elusive algal toxin implicated in massive fish kills along the Mid-Atlantic coast in the 1990s. They say that the compound’s characteristics explain why it has been so difficult to track down. Other researchers, however, remain skeptical.

The hunt for a toxic product of the single-celled alga Pfiesteria piscicida dates to the early 1990s, when researchers laid the blame for fish kills in North Carolina waters on the organism (SN: 9/27/97, p. 202: https://www.sciencenews.org/pages/sn_arc97/9_27_97/bob1.htm). Moreover, scientists who worked with the alga in the laboratory reported headaches and rashes. Public safety concerns led Maryland officials to temporarily limit access to certain Chesapeake Bay waterways in 1997 after a fish kill occurred there.

Not all scientists agreed that a toxic agent secreted by that alga was responsible for the fish deaths. Some researchers blamed a fungal disease first described in the 1980s (SN: 10/10/98, p. 231), while two research groups independently reported that they couldn’t find evidence that Pfiesteria shumwayae, the other named Pfiesteria species, makes a toxin (SN: 8/10/02, p. 84: Available to subscribers at Pfiesteria’s Bite: Microbe may kill fish by skinning, not poisoning). One of those teams proposed that the alga eats away fish skin.

A different group now reports a structure for the chemical that it says is responsible for the toxicity. Peter D. R. Moeller, an organic chemist with the National Oceanic and Atmospheric Administration’s Hollings Marine Laboratory in Charleston, S.C., and his colleagues describe their data in the Feb. 15 Environmental Science & Technology.

From work that used five analytical methods, the researchers propose that the toxin’s structure contains a carbon chain interrupted by a sulfur-copper-sulfur segment. They found that the compound, when energized by light, produces free radicals, which are highly reactive chemical species with unpaired electrons.

As the compound generates free radicals, it decomposes, Moeller says, so it remains active only for several days. This could explain why the toxin has been so elusive. “It’s destroying itself. That’s why you can’t find it after the fact,” Moeller says.

Various environmental cues are necessary to start the compound on its free radical–generating path, he adds. The researchers found that light, high heat, and changes in pH can set off the compound. Other environmental conditions could also contribute, Moeller says, adding that the researchers won’t know more “until we test this in the wild.”

But the new evidence has left others unconvinced of the compound’s structure and toxicity. Robert E. Gawley, an organic chemist at the University of Arkansas in Fayetteville who has searched for the toxin, says that the characterization report is incomplete and doesn’t meet “the standards of a rigorous structural proof.”

William B. Tolman, an inorganic chemist at the University of Minnesota in Minneapolis, says that the evidence for copper-sulfur bonds is weak. The reported distance between the copper and sulfur atoms is too long for the proposed structure, he says.

Tolman also notes that while the researchers provide proof that free radicals are present, “whether or not the free radicals are responsible for the toxicity is not clear.”