1Certain cyanobacteria, such as Microcystis, are capable of changing water into a deadly poison. When they proliferate in fresh water, they sometimes produce a hepatotoxin, microcystin which, when ingested, can damage the liver or cause kidney tumours. Through drinking the water or via the rest of the food chain, they are therefore likely to affect human health as well as contaminate wildlife and domesticated animals. This is why, in a number of countries such as Canada and Australia, these microorganisms are constantly monitored. Their detection in water triggers health alerts or site closures… But are these measures always a necessity?

Cyanobacteria marked by molecular probe.

2The proliferation of cyanobacteria does not necessarily mean that the water has become toxic. For reasons that are still unclear, the synthesis of microcystin is not systematic and false alarms are regularly triggered. Hence the relevance of a new approach based on molecular markers, published in 2017. Easier and faster than current procedures, it helps identify potentially toxic cyanobacteria and performs a visual count of those which are in the process of secreting toxins. Detection is achieved via molecular probes designed to characterise the identity and activity of microorganisms. These fluorescent probes are attached to RNA molecules: ribosomal RNAs, in order to specify the identity of the microorganism; and messenger RNAs to determine the activity associated with the biosynthesis of toxins.

3This approach should contribute to improving monitoring systems and gaining a better understanding of the factors behind the synthesis of toxins. Since this seminal work, 25 activity probes have been designed to characterise all types of planktonic microorganisms as well as some of their noteworthy metabolic activities.