“blue algae”, this bacterium that threatens swimmers

More and more bodies of water are prohibited for swimming, because of the blue algae, which colors the surface of the lakes and ponds it colonizes. It is a cyanobacterium, with which a fisherman avoids all contact. “I brought a bucket, it’s my personal water from home, I wash my hands… I take all the precautions”, explains Bruno Arrial. Normally, these bacteria are harmless, and even useful. However, they become dangerous when they proliferate. The Lac du Verdon, in Cholet (Maine-et-Loire), has been colonized for more than a month. As a result, the fish have deserted.

The bacterium proliferates due to pollution, combined with high heat. “They like it when it’s enriched with phosphate, nitrogen. So often it’s on sites that are quite artificial, or damaged”explains Delphine Guillebault, microbiologist at Microbia Environnement. “With climate change and the warming that we see, each summer getting hotter and hotter, the light intensity is a key factor, too, in their proliferation”, she continues. Sometimes fatal for animals, these algae cause red patches, pimples or even symptoms of gastroenteritis in humans.