Pfiesteria piscicida—the “cell from hell”—appears to produce fish-killing toxins when exposed to certain forms of metals under just the right environmental conditions, according to new research.

The work, published by federal scientists in the February issue of the journal, Environmental Science and Technology, is the first to explain how P. piscicida and the closely related P. shumway shift from being harmless single-cell organisms to deadly toxin-producers blamed for massive fish kills in the Bay and North Carolina.

The explanation from the scientists is—it ain’t easy.

Which may explain why, after grabbing headlines in 1997, reports of pfiesteria-related fish kills have been nearly absent from the Bay.

In fact, scientists for the past decade have—sometimes heatedly—debated whether pfiesteria even produces a toxin. Some scientists contended the cell was using an unidentified toxin to stun and kill fish in the lab, while others found no evidence of toxin production, but observed the organisms physically latching onto fish and literally eating them alive.

In January, a research team, led by Peter Moeller of the National Oceanic and Atmospheric Administration’s Center for Coastal Environmental Health and Biomolecular Research in South Carolina, reported that for the first time, they had successfully isolated a lethal toxin produced by pfiesteria cells—a process that took them seven years to unravel.

Like other researchers, Moeller and his team started out able to kill fish with pfiesteria, but could not identify exactly what was causing the deaths. Compounding the issue, the deaths would come in quick bursts, with evidence of the killing mechanism seeming to vanish.

But after the fish kills, they found molecules in the water that were heavier than the researchers would have predicted, which led them to deduce that the presence of heavy metals was involved in producing the toxin.

In one early lab experiment, Moeller placed sheepshead minnows in three separate jars—one containing pfiesteria and minnows, another with minnows and metals, and the third with pfiesteria, minnows and metals. The minnows began dying within hours in the container with both pfiesteria and metals. “The other ones are probably still happily swimming today,” he said.

While it was only circumstantial evidence, Moeller said the observation “did give us, in my view, confirmation that these metals were involved in the process.”

Over time, Moeller and his team, which also included researchers from the National Institute of Standards and Technology, Medical University of South Carolina, and College of Charleston, identified a process by which pfiesteria cells can produce a copper-containing exotoxin—a toxin exuded from the cell itself.

When the right circumstances come into play, such as exposure to certain types of light, the right copper ions and other variables, the toxin rapidly breaks down into free-radicals—unstable molecules that are thought to be the lethal agent.

“The environmental cues that are necessary are very specific, I don’t know what they all are,” Moeller said.

When exposed in the lab, fish quickly behaved erratically; swimming in circles and even throwing themselves out of containers before dying.

But the free radicals are short-lived, often vanishing within hours. “These free radical mechanisms explain to us why we could have activity one day and not the next,” Moeller said. The ephemeral nature of the free radicals, he said, explains the difficulty researchers have had in identifying a specific toxic compound from pfiesteria.

Moeller cautioned that the processes observed in the lab still need to be verified in the field.

But he said the process, though not common, is likely not novel to pfiesteria. “There are other organisms non-related to pfiesteria which are probably doing the same thing based on anecdotal observations,” Moeller said.

Pfiesteria was originally identified in 1988 by North Carolina State University researchers JoAnn Burkholder and Ed Noga in association with fish kills in Pamlico Sound.