The blue detectives

Harmful algal blooms occur when certain species of marine algae produce toxins with human and environmental health effects. This can lead to mass mortalities of marine animals and in the worst cases create aquatic “dead zones”.

The harmful algal bloom in SA very quickly turned into a natural disaster of unprecedented scale.

Over the next 15 months, the area being poisoned across the SA coast swelled to more 20,000 square kilometres – reaching 30 per cent of the state’s coastline.

More than 780 species of marine life were impacted with the mass mortality of millions of fish including hundreds of the iconic leafy seadragon. A survey in August 2026 found the numbers of endangered giant cuttlefish had fallen by 97 per cent in the waters off SA.

Beaches were closed and parts of the state’s aquaculture industry, worth more than $325 million each year, ground to a halt. Analysis from the SA Conservation Council put the economic impact on fisheries at $100 million, on the tourism industry at $46.8 million and on governments of a further $100 million.

It had quickly become one of Australia’s worst-ever marine environmental disasters.

The un-usual suspects

The pressure was on scientists and government officials to get to the bottom of the bloom, its causes and impacts. But good science involves detailed, time-consuming investigation.

In this case, the culprits behind the crime – the Karenia genus – could be identified relatively quickly through a microscope.

However, pinning it on which of the eight species in the group was most responsible proved far more challenging. This was important, because different species of Karenia produce very different types of toxins, or no toxins at all.

The foundation of this forensic testing lay in isolating individual algal cells from a sample of seawater.

“It starts with a single cell. We use super-fine pipettes to suck up a single cell of algae, one at a time,” said Murray, who quickly mobilised an international network of 25 scientists to study the bloom.

“We then take that cell and examine it under a microscope, conduct DNA testing or use it to grow a laboratory culture for further testing.”

Luckily, members of the team specialised in the extraction and analysis of DNA from a single cell.

“Being able to extract DNA this way can save months of waiting to get a large enough sample to work with,” said Dr Greta Gaiani, Chancellor’s Research Fellow at UTS.

The team sequenced the DNA using custom-made quantitative PCR tests, similar to those used by people around the world during the COVID-19 outbreak, and ultra-powerful electron microscopes to examine the cells.

The results surprised them.

“When our testing came back showing the dominant species was the little-known Karenia cristata, we were shocked," Gaiani said.

"It’s a very rare species that has never been found anywhere close to Australia before.

“We thought we might’ve been wrong so sent samples to a large lab in New Zealand to confirm the finding using other genetic methods.”

Karenia cristata had previously only been found in two places – off the coast of South Africa and at a small island off the coast of Newfoundland in Canada. It was so little known, only two scientific papers had been published about it.

The species is so rare, the team needed to invent a new quantitative PCR test to identify the species.

One thing that’s now known is it flourishes in cooler water temperatures – with the waters off SA ranging from 10 to 20 degrees Celsius – and becomes more abundant during the cooler months.

A study of DNA samples also showed Karenia cristata had been lurking in the waters off SA - at very low levels - since sampling began in 2016 before populations exploded in 2025.

Poison algae

Only one other harmful algal bloom worldwide has caused the same level of death and destruction as that faced off SA.

“A toxic algal bloom occurs regularly in the very warm waters off Florida in the United States, driven by a different species, Karenia brevis," said Murray.

"The poison produced by marine algae – brevetoxin – takes its name from that species.

“Both blooms share high toxicity and have lasted longer than a year, which is very rare for harmful algal blooms. Ominously, it has been recurring annually in Florida for almost a hundred years.”

With this in mind, the scientists needed to understand how toxic this rare species is.

The first step in doing that was to isolate enough of the brevetoxin produced by Karenia cristata to be able to test it.

That means growing a lot of algae, all from a single cell.

“It’s like a very sophisticated gardening exercise to get an algae sample growing,” said Murray.

“One cell divides into two only once every few days. You need at least a million cells in a sample to start to measure something like toxicity.

"There’s a lot that can go wrong. It’s a slow and painstaking process."