That’s your bloomin’ lot! How do toxic blue-green algal blooms dominate Australia’s freshwaters?

By Michele Burford

Everyone knows that Australia is home to venomous snakes, lethal jellyfish, and man-eating sharks and crocodiles, but did you know that our freshwaters can also harbour toxic blue-green algal species?

An algal bloom in a reservoir near Brisbane. Image: M Burford

Recent research we conducted has focussed the spotlight on one of these toxic species, Cylindrospermopsis raciborskii.

Australia is a land of extremes – with droughts and flooding rains. This variability in our climate means that animals and plants need to be very adaptable to survive. Cylindrospermopsis raciborskii is no exception to this having developed many clever tricks that allow it to bloom in freshwater systems, particularly lakes, reservoirs and rivers with low flow.

Scientists who study blue-green algae have known for a long time that nutrients (nitrogen and phosphorus) discharged into waterways in excessive amounts will cause algae to bloom. This includes nutrients washed from agricultural land during large rainfall events. But how does Cylindrospermopsis raciborskii get sufficient nutrients to bloom when it’s not raining?

Turns out it is a great scavenger for any small amounts of phosphorus in the water, which stores within the cell to see it through the times when no phosphorus is available. In the case of nitrogen, Cylindrospermopsis raciborskii can capture and use nitrogen from the atmosphere when there is no nitrogen in the water. So while environmental managers control algal blooms by reducing excess nutrients, the strategies used by Cylindrospermopsis raciborskii mean that controlling nutrients is less likely to be effective for this species, unless there is a major reduction in nutrients.

One of the other ways commonly used to manage algal blooms is to use mechanical mixers to force the blooms deeper into the water where there isn’t enough light for the algae to grow. But studies have shown that Cylindrospermopsis raciborskii still grows well with low light levels compared with other algae, so mixing can simply cause this species to outcompete others.

And what about the toxins it produces? Interestingly Cylindrospermopsis raciborskii is toxic in Australia, unlike many countries in the world. No one really knows why some blue-green algae are toxic but we do know that in large amounts, they can cause serious harm to humans and animals.

Our water authorities constantly monitor and treat water supplies to ensure that our drinking water is safe. However, the ability to predict the toxins present in Cylindrospermopsis raciborskii blooms will improve the effectiveness of management. Recent research has shown that Cylindrospermopsis raciborskii is genetically very diverse. This means that some genetic types produce lots of toxins while some produce very little. Research is now focussed on how we can promote the growth of the low-toxin genetic types to make water supplies safer.

So what is the likelihood of coming up with new methods to reduce blooms of Cylindrospermopsis? This is especially true when our warming climate is predicted to promote longer lasting and bigger blooms. Actually, our best bet is in improving our ability to predict when blooms are likely to occur and how toxic they will be. That will mean that we can plan for this to avoid contaminated water supplies and/or ramp up the treatment methods.

Further reading: Burford, M.A., Beardall, J., Willis, A., Orr, P.T., Magalhaes, V.F., Rangel, L.M., Azevedo, S.M.F.O.E., Neilan, B.A. 2016. Understanding the winning strategies used by the bloom forming cyanobacterium Cylindrospermopsis raciborskii. Harmful Algae, 54, 44-53.


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