Palytoxin: The danger hidden in tropical aquariums

Nature’s deadliest chemicals often come in beautiful packages. Take, for instance, the batrachotoxin harbored in the vibrant skins of poison dart frogs, or the cardiac glycosides permeating the glossy green leaves and delicate pink petals of the oleander plant.

Palytoxin, which causes severe respiratory reactions, is no different. The massive molecule comes wrapped in the deceptively alluring package of an anemone-like creature related to coral. An undulating fringe beckons from the edges of flat, green, sunburst discs, hiding the deadly substance within.

In May 2017, a family of seven living near Adelaide, Australia, went to the hospital after one member of the household cleaned the family’s tropical saltwater aquarium. The person reportedly removed and scrubbed some rocks or corals from the tank, presumably to remove unwanted, unattractive growths. This cleanup triggered the release of aerosolized palytoxin that caused everyone to struggle to breathe.

These kinds of poisoning incidents are rare, but they are occurring more frequently now that improvements to home aquarium technology have made the care of beautiful and exotic fish, corals, and anemones easier. As more hobbyists are able to master the highly specific water quality, nutrient, and heat requirements needed to keep the tropical critters in good health, the chances of encountering a toxic species—and needing an emergency hospital visit—have edged upward.

A recent survey of the U.S. National Poison Data System revealed at least 171 cases in which people called poison control centers with reports of inhaling or coming into skin contact with palytoxin between 2000 and 2014 (Environ. Toxicol. Pharmacol. 2017, DOI: 10.1016/j.etap.2017.08.010). This boils down to nearly a dozen calls per year in the U.S. alone. Most of the incidents—86%—occurred at residences, leading the researchers who conducted the survey to suspect home aquariums as the source.

Still, home aquariums and coral reefs are not the only places where palytoxin lurks. Some species of single-celled, planktonic algae also produce the toxic natural product. Blooms of these algae in the Mediterranean Sea have been known to disperse palytoxin and palytoxin isomers in marine aerosols. Such an event sent more than 200 beach-going tourists along the coast of Italy near Genoa to the hospital in 2005, with about 10% requiring intensive care.

To prevent these harrowing, albeit nonlethal, incidents, scientists and health care professionals alike want to better identify and detect palytoxin and to continue raising awareness of the toxin, especially among home aquarists.

The Mount Everest of chemistry

The history of palytoxin is tied to a Hawaiian legend. Ancient warriors killed a fearsome shark god and scattered his ashes into a tide pool. So deadly was this god that applying “seaweed” from this pool to spear points ensured the death of the warriors’ enemies.

In 1961, a biologist at the Hawaii Institute of Marine Biology came across a mention of this deadly seaweed in a Hawaiian-English dictionary. After learning of the legend, and after a long search, he and a colleague located the tide pool on the island of Maui. There, they discovered an extremely potent toxin produced by a species of zoanthid—a reef-dwelling organism related to corals and resembling a cluster of miniature anemones. The species was later named Palythoa toxica.

The discovery of the zoanthid launched a saga of molecular discovery. For a decade, scientists tried to isolate palytoxin, then they labored to elucidate the compound’s structure—a feat that took until 1982 to accomplish. Finally, in 1994, a team led by Yoshito Kishi of Harvard University reported a total synthesis of the molecule—more than three decades after its discovery in the tide pool (J. Am. Chem. Soc., DOI: 10.1021/ja00103a065). The team initially synthesized eight pieces of the molecule separately, linking them to form palytoxin carboxylic acid (J. Am. Chem. Soc. 1989, DOI: 10.1021/ja00201a038). Then they converted the structure to its final form.

What took so long? Palytoxin is gargantuan, weighing in at 2,680 daltons, and it has a backbone 115 carbon atoms long. It is one of the longest continuous carbon chains that’s been found in nature. Containing 64 dissymmetric carbons and six olefinic bonds, palytoxin could have more than 10²¹ possible isomers. A 1989 C&EN story dubbed palytoxin the Mount Everest of chemistry for the monumental effort required to understand and synthesize the devilishly complex molecule.

Despite humans needing more than 30 years to figure out how to cobble palytoxin’s components together, a variety of aquatic organisms seem to do so readily. Aside from the subset of toxic Palythoa zoanthids, some species of the phytoplankton genus Ostreopsis as well as a type of cyanobacteria also synthesize and release compounds from the palytoxin family.

These palytoxin isomers share the same carbon chain structure but have slight variations in the methyl and hydroxyl groups dotted along their backbones. For example, the toxin responsible for the 2005 seaside poisoning in Italy, ovatoxin-a, differs from palytoxin in four spots. It’s produced by Ostreopsis cf. ovata. Ostreocins are also toxic isomers of palytoxin and are produced by Ostreopsis siamensis. The full suite of these palytoxin-like compounds has yet to be uncovered, and scientists are still working to determine which isomers pose the greatest danger.

“Deadly with an asterisk”

Although scientists still don’t know exactly how marine creatures synthesize palytoxin, they do know that it exerts its poisonous effects by binding to sodium pumps on the surface of all cell types in the body. Normally, these pumps push sodium ions out of cells and pull potassium ions in to maintain a healthy balance, or homeostasis. Palytoxin jams the pumps into an open position, flooding cells with sodium and calcium, which ultimately leads to cellular damage and death.

When inhaled, palytoxin acts fast, rapidly constricting blood vessels and triggering asthma- or flulike symptoms. But change a few functional groups on the monster molecule, and it can lose much of its devastating punch.

Modifications at a single chiral center are enough to reduce toxicity, says Aurelia Tubaro, a toxicologist at the University of Trieste. Tubaro began studying palytoxin after the 2005 seaside incident near Genoa. A team of Italian scientists identified ovatoxin-a as the compound responsible and demonstrated that both it and the algae cells producing it could have caused the poisonings by becoming aerosolized (Environ. Sci. Technol. 2014, DOI: 10.1021/es405617d). Though palytoxin is 100 times as potent as ovatoxin-a, the isomer can still reach harmful levels in seawater during an algal bloom, Tubaro says.

Despite its extreme toxicity to cells—concentrations as low as 20 pM can injure about half the cells in a petri dish—paly­toxin’s effects on people are often less harmful in comparison because they encounter the molecule primarily through inhalation or skin contact, says Lauren T. Murphy, a medical resident at Cooper University Health Care in New Jersey. Murphy was the lead author on the 2017 survey of palytoxin-related calls to U.S. poison centers. For palytoxin to exert its characteristic cellular lethality, it must be introduced directly into the bloodstream, a highly unlikely scenario, she says. So, she says, it’s “deadly with an asterisk.”

Usually, aquarium owners come into contact with palytoxin when cleaning rocks or corals or when transferring them into or out of tanks. A common way to clean rocks—to get rid of nuisance polyps and make space for prettier ones—is by boiling them in water. If a toxic zoanthid is present, palytoxin rises with the steam.

Exposure to the aerosol triggers symptoms that can initially be confused with allergies or the flu: difficulty breathing, runny nose and eyes, and fever. But palytoxin also elevates blood pressure and heart rate and causes tingling sensations in the fingers, toes, and lips. Some aquarium owners exposed to palytoxin have also reported a strong metallic taste in their mouths. Handling live rocks and corals also creates tiny cuts in people’s skin, and the presence of palytoxin can lead to skin ulcers or inflammation.

Jonathan R. Deeds, a research biologist with the U.S. Food & Drug Administration, has become the go-to scientist for U.S. agencies—and in some cases international agencies—seeking to confirm the chemical culprit of suspected aquarium palytoxin poisonings. When a request for analysis comes in, he tests water and zoanthid samples from home saltwater aquariums. “It comes in spurts probably about once a year. Sometimes there’s a couple of years where it’s been two or three times,” he says. “It’s usually somebody cleaning their aquarium and poisoning themselves.”

“We just don’t know”

Even though most people exposed to palytoxin inhale the compound, nothing is known about how much aerosolized toxin is required to elicit symptoms. “We can demonstrate exposure from home aquariums, but we can’t show what dose had to get into the human body to cause the effects,” Deeds says. “We just don’t know that yet.”

Part of the difficulty lies in the challenges of quantifying palytoxin and its isomers, especially at the low concentrations that would be found in aerosols. The molecule is large and “very sticky,” according to Tubaro, meaning it gloms on to plastic surfaces, which makes both sampling and analysis difficult. Her group has had success using an antibody- and carbon-nanotube-based electrochemiluminescent biosensor to improve sensitivity over liquid chromatography/mass spectrometry by more than two orders of magnitude (ACS Nano 2012, DOI: 10.1021/nn302573c), and the researchers are now developing cheap and rapid assays to use in monitoring seafood and algal blooms.

Deeds also points out that knowledge of how to decontaminate homes after a palytoxin poisoning is woefully thin. “Nobody has done any controlled decontamination studies,” he says. He fields calls from health departments worldwide asking about when it’s safe to allow families back into their homes, he adds. “Wipe everything down with mild bleach is all we know for sure right now.”

As for preventing future exposures, that too is up in the air. Monitoring palytoxin in zoanthids sold for home aquariums doesn’t fall into a clear jurisdiction, Deeds says. FDA regulates only food, and other regulatory agencies monitor traded corals and fish only to ensure that endangered species aren’t involved. No agency is responsible for determining whether a coral or zoanthid species is toxic.

Word of palytoxin’s danger has already spread through online forums and home aquarium hobbyist websites, where users caution against boiling rocks to clean them and recommend wearing gloves and eye protection when handling and cleaning coral and rocks. It’s unclear where to proceed from there, though. “Beyond just putting out information about the risks I’m not sure what the solution is,” Deeds says.

CORRECTION: This story was updated on Jan. 10, 2018, to correct the stereochemistry around the terminal five-membered ring in palytoxin and ovatoxin-a. On Jan. 16, the story was updated to correct a statement about the length of palytoxin’s continuous carbon chain.