Deadly Algal Bloom in South Australia Linked to Most Toxic Microalgae Ever Tested

A toxic algal bloom off South Australia's coast, ongoing since early 2025, has killed an estimated one million marine animals across 7,700 square miles. Scientists identified the culprit as Karenia cristata, a rare microalgae producing brevetoxins that harm nerve cells. A study published July 6 in Nature Ecology & Evolution found K. cristata to be an order of magnitude more toxic than any previously studied microalgae, lethal even at concentrations as low as 148 cells per fluid ounce. The bloom, first noticed in March 2025 at Waitpinga Beach, caused swimmers to experience cold and flu-like symptoms. Researchers from the University of Technology Sydney and Adelaide University developed new molecular genetic methods to distinguish Karenia species, revealing K. cristata as dominant and K. brevis absent. Lab tests showed the algae killed half of tested invertebrates and damaged fish gill cells at extremely low concentrations. Previously known only from South Africa and Newfoundland, this is the first detection of K. cristata in Australian waters. Researchers now aim to understand the factors behind its unprecedented growth and its potent toxicity mechanisms.