In March 2025, people walking the beaches near Waitpinga and Parsons Beach on South Australia's Fleurieu Peninsula started finding the water foaming an odd colour, and the wildlife dying. Fish floated belly-up. Octopuses, rays, even sharks washed ashore. Over the following months the die-off didn't stop — it spread, eventually covering more than 20,000 square kilometres of coastline and killing off members of at least 550 different marine species. For a long time, nobody could say exactly why this particular bloom was so much worse than the harmful algal blooms scientists already knew about.
The newly identified algae species is roughly ten times more toxic than any microalga ever tested.
Now they can. A research team led by Professor Shauna Murray at the University of Technology Sydney has identified the organism driving the worst of the deaths: a newly recognised species called Karenia cristata, described in a paper published in Nature Ecology & Evolution on July 6, 2026. It belongs to the same genus as Karenia brevis, the microalga responsible for Florida's notorious red tides — but where K. brevis is a warm-water species confined to the southeastern United States, K. cristata thrives in cold water, which is part of why it went unrecognised for so long in a temperate Australian bloom.
An Order of Magnitude More Toxic
When Murray's team tested its toxicity directly, the results were extreme. "Karenia cristata, which releases a range of toxic compounds including brevetoxins, has stronger toxic effects than previously studied harmful algal bloom species," Murray said — in fact, by the team's measurements, it came out roughly an order of magnitude more toxic than any other microalga ever tested. Brevetoxins attack the nervous systems of fish and marine mammals, and this species appears to produce them more abundantly, and more potently, than anything on record.
Identifying the exact species matters well beyond South Australia. Harmful algal blooms are being tracked more anxiously worldwide as oceans warm, and a toxin-producing organism nobody had catalogued before — one already capable of persisting for fifteen months and wiping out life across an area larger than some countries — is exactly the kind of threat marine monitoring programs need to know how to test for. Now that K. cristata has a name and a toxicity profile, scientists elsewhere have something concrete to check their own waters against.
Image credit: A harmful algal bloom (red tide) discolouring coastal water