The Culprit Behind South Australia’s Deadly Algal Bloom Might Be the Most Toxic Species Ever Tested, Scientists Say

Karenia cristata, a rare type of microscopic algae that produces toxins that harm nerve cells, can be lethal at very low concentrations

Sarah Kuta | Daily Correspondent

July 10, 2026 8:46 a.m.

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In March 2025, a mysterious, yellowish foam started washing ashore on Waitpinga Beach, located in the Australian state of South Australia. Swimmers began experiencing cold and flu-like symptoms, and dead or dying fish and leafy seadragons filled the beach.

The problem only got worse, leading to the deaths of an estimated one million fish, birds, shellfish and marine mammals across an area of more than 7,700 square miles—some 30 percent of South Australia’s coastline—over the past 15 months. The culprit? Karenia cristata, a rare type of microscopic algae that produces substances that harm nerve cells called brevetoxins.

Now, scientists have determined K. cristata is probably the most toxic algal species ever tested. Even in very small concentrations, it can be lethal, they report in a study published on July 6 in the journal Nature Ecology & Evolution.

“Karenia cristata is an order of magnitude more toxic than the next most toxic microalgae that has been studied to date,” study co-author Shauna Murray, a marine biologist at the University of Technology Sydney, says in a statement.

Hundreds of species of microalgae can be harmful when they accumulate in large numbers, often referred to as “algal blooms.” But before K. cristata reached South Australia, researchers knew very little about it. The persistent bloom that started in early 2025 marked the first time the species had been discovered in Australian waters; prior to that, it had only ever been identified near South Africa and Newfoundland, Canada.

So, a group of scientists—many based in Australia—set out to learn more about the deadly species. A powerful microscope helped them take a closer look at K. cristata’s cells, which they compared with those of other species in the Karenia genus. The team also developed new molecular genetic methods that allowed them to distinguish between different Karenia species in water samples.

Analyses revealed five Karenia species in water samples from the South Australia bloom, and K. cristata was the dominant one. By contrast, the only other Karenia species known to produce brevetoxins, K. brevis, was not present in the samples.

When scientists grew K. cristata in the lab and tested the species’ effect on the cells of fish and invertebrates, including plankton, they got some “startling” results, some of the study co-authors write for the Conversation.

“K. cristata killed half the invertebrates we studied, even in extremely low concentrations,” they write. “The algae had a similar effect on lab-grown fish gill cells.”

The microalgae were toxic at concentrations as low as 148 cells per fluid ounce, or roughly 19,000 cells per gallon. Much higher concentrations of K. cristata were routinely recorded in the seawater off South Australia during the bloom, with samples collected in August and September 2025 containing more than 3.8 million cells per gallon.

“For the first time, it gives us an explanation for why this bloom was so devastating out in the field,” study co-author Craig Styan, an environmental scientist at Australia’s Adelaide University, tells Sophie Holder of the Australian Broadcasting Corporation.

Until now, researchers assumed K. brevis was the most dangerous species, and that the species’ blooms were largely restricted to areas with warm water, such as the southeastern United States. However, they now know K. cristata blooms can be much worse, and that the species can proliferate in cooler temperatures.

“It makes me wonder where else we’ll start seeing this problem appear,” Don Anderson, a physiological ecologist at the Woods Hole Oceanographic Institution who was not involved with the research, told Science’s Erik Stokstad last year.

To prevent similar disasters from happening in the future, researchers hope to uncover the specific factors that likely contributed to K. cristata’s unprecedented growth in South Australia. They also want to investigate the underlying mechanisms of its potent toxicity.

“There’s an awful lot we need to still understand about the basic biology of the algae,” Styan tells the ABC.

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Sarah Kuta is a writer and editor based in Longmont, Colorado. She covers history, science, travel, food and beverage, sustainability, economics and other topics.