In 2025, trouble was brewing in a 20,000 km2 area off the coast of South Australia. For more than a year, marine animals were mysteriously dropping dead: around one million animals were wiped out and some humans even got sick.
Now, scientists have confirmed that the culprit behind these mass marine animal deaths is a species of microalgae called Karenia cristata. The algal bloom produced several powerful toxins, which the researchers say result in stronger toxic effects than any species they have studied before.
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“This can partly explain the scale of the impacts witnessed in this event, which have included the mass mortality of marine invertebrates, fish, mammals and birds, over many months,” says professor Shauna Murray, a marine biologist at the University of Technology Sydney.
Algae are simple plant-like organisms that create energy from sunlight through photosynthesis. Single-celled microalgae are so small that they’re invisible to the naked eye.
These algae might not look as vibrant as rainbow-coloured coral reefs but they are incredibly important in keeping the ocean healthy: they produce oxygen, take in carbon dioxide and provide habitats and food for lots of different marine creatures.
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But, as with anything, too much isn’t always a good thing. When algae blooms get out of control, things can turn nasty. One of many problems caused by harmful algal blooms (often shortened to HABs) is that some species of algae can produce dangerous toxins that kill animals and can even affect humans through the water or eating contaminated seafood.
Murray and colleagues conducted lab-based experiments on Karenia cristata to better understand how dangerous its toxins were. “Karenia cristata in the laboratory was highly toxic in very low cell concentrations,” she says. “We found that Karenia cristata is an order of magnitude more toxic than the next most toxic microalgae that has been studied to date.”
Their findings are published in the scientific journal Nature Ecology & Evolution.
Worryingly, the scientists showed that the species behind the South Australian bloom (Karenia cristata) “produces substantial amounts” of brevetoxins (BTXs). These potent neurotoxins are commonly associated with deadly ‘red tides’(produced by another type of algae called Karenia brevis) and can cause devastating mass strandings of whales, dolphins, manatees, sea lions and turtles among other animals.
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“Before now, the warm water Karenia brevis, which only occurs in the south-eastern United States and also produces brevetoxins, had been considered the most devastating marine HAB species in terms of its environmental, economic and social impacts,” Murray adds.
Although the harmful algal bloom in South Australia was first detected in March 2025, it hasn’t yet gone away. “The South Australian HAB still hasn't completely disappeared about 15 months after it started,” says Murray. “We now need to fully investigate the factors that cause Karenia to grow and understand its mechanisms of toxicity.”
This discovery represents “an emerging international threat with unknown consequences” they write in the paper. That’s because this new species of Karenia, which is found in cold water, “could potentially bloom anywhere with similar coastal water conditions,” explains Murray.
Now, the experts are continuing to study the Karenia cristata in the hope they might find ways to mitigate its harmful impacts to prevent humans or animals getting sick from future blooms.
However, Murray warns that even the experts won’t be able to prevent dangerous red tides entirely: “A harmful algal bloom is a natural disaster, like a cyclone,” she says, “and we cannot expect to fully control or reverse it.”
