Algae which caused brevetoxins in SA algal bloom most toxic of its kind studied

The algae responsible for the brevetoxins in South Australia's devastating algal bloom is the most toxic species studied of its kind, according to newly published research.

The research has been undertaken by an Australian-led team, which last year identified Karenia cristata as a dominant algal species in the bloom that resulted in mass deaths of coastal wildlife and impacted SA's coast for over 12 months, since it was discovered in March 2025.

The research involved conducting experiments on laboratory-grown strains of Karenia cristata to assess its toxicology, as well as the toxicology of water samples from the bloom.

'Potent' algae

Adelaide University associate professor Craig Styan, who was one of the researchers, said the results showed that in the laboratory the algae species was highly toxic, even in very low cell concentrations.

"Just a few thousands of cells per litre can have a really quite large biological effect, which is a much stronger effect than we've seen in other harmful algae when they've been tested in similar ways overseas," he said.

Dr Styan said the testing helped them to understand why there were such significant effects from the bloom.

"We knew this bloom caused really big marine mortality events, and we knew there'd be brevetoxins, but we still didn't have a really good scientific explanation for why the impacts we were seeing was so extensive," he said.

"It shows Karenia cristata was exceptionally harmful to marine organisms when we tested it in some standardised laboratory tests.

"For the first time, it gives us an explanation for why this bloom was so devastating out in the field."

Dr Styan said in his experimenting on microscopic zooplankton and brine shrimp, or sea-monkeys, it was surprising to see how sensitive the organisms were to the algae.

"It's really harmful, even in relatively low concentrations, lower than what we expected, and actually much lower than the sort of concentrations we were seeing when you got a really dense bloom," he said.

"That's much, much higher than the concentrations we were testing in the lab," he said.

More research to do

Karenia cristata has not previously been identified in Australian waters, and had only been detected off South Africa and near Newfoundland, off the coast of Canada.

Dr Styan said it would be concerning if it were to appear elsewhere.

"It's not just a South Australian concern, but an Australian concern, and an international concern, that if Karenia cristata was to spread and appear elsewhere, and be as toxic as we've seen it here, then we might see other really large mortality events,"

he said.

He said there was still a lot not known, and what caused the toxicity needed to be understood.

"What causes the algae to produce these toxins, and whether that's controllable under environmental conditions … there's an awful lot we need to still understand about the basic biology of the algae," he said.

South Australia's department of primary industries and the minister have been contacted for comment.