A massive algal bloom has devastated marine life in South Australia over the past eight months, killing around one million animals across more than 550 species. Researchers have identified the culprit as Karenia cristata, a species producing a newly discovered toxin that could spread worldwide. The event, one of the most destructive on record, raises concerns about emerging threats to global oceans.
Over the past eight months, a vast algal bloom has ravaged more than 20,000 square kilometres of South Australia's marine environment, resulting in the deaths of an estimated one million animals from over 550 species and causing widespread impacts on human health. The bloom, identified as Karenia cristata, marks one of the biggest algal events ever recorded, affecting everything from seadragons to great white sharks and dolphins.
Previously known only from two locations near South Africa—where it caused fish die-offs in 1989 and the mid-1990s—and off Newfoundland, Canada, K. cristata has now revealed a new toxin in the brevetoxin class. These compounds damage nerves in sea life and fish gills, and they pose risks to humans through inhalation or ingestion. Shauna Murray at the University of Technology Sydney, part of the research team, expressed international concern: “We know that it can bloom in other countries in the world. What we didn’t know is that cristata produced brevetoxins and that it could cause these harmful algal outbreaks that are so huge and so disruptive and last for eight months. Now we know that and, because we know that it occurs in other places in the world, yes, I do think that it is an international threat.”
Among nearly 850 deadly marine algal blooms reported globally from 1985 to 2025, this South Australian event stands out as one of the most destructive and widespread. The bloom began during a severe marine heatwave with seawater temperatures up to 3°C above normal but unexpectedly expanded after May 2025, even as waters cooled. Researchers link rising global temperatures and nutrient levels to increasing bloom frequency, though the exact trigger remains unclear.
Christopher Keneally at the University of Adelaide highlighted the uncertainties: “There is a great deal unknown about how the specific toxins produced by this species affect humans.” He added that the species' broad distribution suggests it may already lurk in low abundance in many coastal areas worldwide, potentially proliferating with warming seas and urban nutrient pollution. The findings, detailed in a preprint on bioRxiv (DOI: 10.1101/2025.10.31.685766), underscore the urgent need for monitoring and research to mitigate future outbreaks.