The marine microalgae responsible for the most devastating effects of the South Australian harmful algal bloom (HAB) has now been shown to be the most toxic species of its kind ever studied.
In a newly published paper in Nature Ecology & Evolution, the Australian-led research team, which late last year identified the microalga Karenia cristata as the source of neurotoxins in the bloom, has expanded on its findings with new toxicology data.
Lead author Professor Shauna Murray from the University of Technology Sydney (UTS) said the HAB, which lasted more than a year, has taken an immense environmental, economic, social and public health toll on South Australia.
“We’ve now established that Karenia cristata , which releases a range toxic compounds including brevetoxins, has stronger toxic effects than previously studied harmful algal bloom species,” Professor Murray said.
“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.”
Professor Murray said the team, including colleagues from the Cawthron Institute, New Zealand, (Dr Kirsty Smith, Dr Anne Rolto and Andy Selwood) and Adelaide University (Dr Craig Styan), conducted experiments on laboratory-grown strains of Karenia cristata to more thoroughly assess its toxicity .
“We used three different bioassay methods and a chemical analysis method to examine the toxicology of Karenia cristata grown in the laboratory , as well as the toxicology of water samples from the HAB,” Professor Murray said.
“ Karenia cristata in the laboratory was highly toxic in very low cell concentrations. Our results are in line with the periods of mass marine animal mortalities in 2025 and 2026 and the actual environmental concentrations of Karenia cristata there at the time.
“We found that Karenia cristata is an order of magnitude more toxic than the next most toxic microalgae that has been studied to date.
“This unprecedented event has international consequences, because we now know of a cold water brevetoxin producing Karenia that could potentially bloom anywhere with similar coastal water conditions.
“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.”
The team used several molecular genetic methods to map the distribution and abundances of the Karenia species involved in the HAB.
“The South Australian HAB still hasn't completely disappeared about 15 months after it started,” Professor Murray said. “We now need to fully investigate the factors that cause Karenia to grow and understand its mechanisms of toxicity.
“A harmful algal bloom is a natural disaster, like a cyclone, and we cannot expect to fully control or reverse it.
“Understanding the ecology, physiology and genetics of Karenia cristata will be the basis though to develop testing and possibly even future mitigation methods to help protect aquaculture and inform public health management.”
Nature Ecology & Evolution
Experimental study
Cells
A catastrophic marine mortality event caused by a complex algal bloom including the brevetoxin producer Karenia cristata
6-Jul-2026