The algae responsible for producing brevetoxins from devastating algal blooms in South Australia is the most toxic species of its kind studied, according to recently published research.
The study was conducted by an Australian-led team that last year found Karenia cristata as the dominant algae species in a bloom that resulted in mass mortality of coastal wildlife and impacted the South African coastline for more than 12 months since its discovery in March 2025.
The study included experiments on laboratory grown strains Karenia cristata evaluate its toxicology, as well as the toxicology of flower water samples.
Algae blooms were first discovered along Waitpinga Beach on South Australia’s Fleurieu Peninsula in March 2025. (ABC News: Caroline Horne)
“Powerful” algae
University of Adelaide Associate Professor Craig Stian, who was one of the researchers, said the results showed that the algae species was highly toxic in the laboratory even at very low cell concentrations.
“Just a few thousand cells per liter can have a really quite significant biological effect that is much stronger than what we have seen in other harmful algae when they have been tested in similar ways overseas,” he said.
Dr Stian said the testing helped them understand why the bloom had such a significant effect.
“We knew that this bloom was causing really large marine mortality, and we knew there would be brevetoxins, but we still didn’t have a really good scientific explanation for why the effects that we were seeing were so widespread,” he said.
“This shows Karenia cristata proved to be exceptionally harmful to marine organisms when we tested it in some standardized laboratory tests.
“For the first time, this gives us an explanation for why this bloom was so destructive in the field.”
Brevetoxins were discovered in foam on the capital’s beaches in August 2025. (ABC News: This Chorley)
Dr Styan said that in his experiments with microscopic zooplankton and brine shrimp, or sea monkeys, it was surprising to see how sensitive these organisms were to algae.
“It’s really harmful, even at relatively low concentrations, lower than we expected and actually much lower than the concentrations we saw when we were getting really dense blooms,” he said.
“This is much, much higher than the concentrations we tested in the lab,” he said.
More research to be done
Karenia cristata has not previously been detected in Australian waters and has only been found off the coast of South Africa and near Newfoundland, off the coast of Canada.
Dr. Stian said it would be alarming if it appeared elsewhere.
“It’s a concern, not just in South Australia, but an Australian and international concern: if Karenia cristata spreads and appears elsewhere and is as toxic as we’ve seen here, then we could see other really major mortality events.”
– he said.
He said much is still unknown and it is necessary to understand what caused the toxicity.
“What causes algae to produce these toxins, and whether it can be controlled under environmental conditions… we still have a lot to understand about the basic biology of algae,” he said.
Researchers say much remains to be done to try to stop such mass deaths. (ABC South East SA: Caroline Horne)
In response to questions about this latest study, a spokesperson for South Africa’s Department of Primary Industries and Regions said its own researchers were co-authors of the study and provided samples for the study.
“These results strengthen scientific understanding Karenia cristatasupporting future research and monitoring efforts,” the spokesperson said.
“This new study does not change current public health recommendations or fisheries management mechanisms, which continue to be based on current science and monitoring.”
Last October, Prime Minister Peter Malinauskas said the bloom was not toxic, and he again defended its characterization in March.
Acting Prime Minister Kyam Maher said today the comments were based on what was known at the time.
“We are basing our response on the evidence as it comes in,” he said.
South Australian Research and Development Institute chief executive Mike Steer said toxicity Karenia cristata was evident “in the widespread mortality we observed along the coast.”
“However, the important difference is that toxicology relates to fish, gill-breathing organisms,” he said.