Holiday makers have been warned to avoid rotting seaweed on beaches in the north of France after it was linked to two deaths.
Earlier this month, an 18-year-old oyster farmer died while in the Bay of Morlaix in Brittany in what environmentalists believe was hydrogen sulphide poisoning.
At first initial tests suggested he had drowned.
However, local campaign group Halte aux Marée Vertes claimed he had actually died of a toxic gas released by decomposing sea lettuce.
A post-mortem into the cause of the teenager's death has been ordered by public prosecutors, The Times reported.
The death of a 70-year-old man in Douarnenez Bay last Tuesday raised similar concerns.
Six beaches in the area are currently closed because of the green algae.
When sea lettuce rots it releases the gas, which is colourless, highly poisonous and smells like eggs.
Concerns about killer seaweed on French beaches have been rising in recent years, as more and more nitrate pollution from intensive farming coincides with an increase in algae.
In 2011, 36 wild boar died on a beach near Saint-Brieuc.
Six years later a man collapsed and died while running along the same stretch.
Two years before the boar tragedy two horses died on a nearby beach.
Workmen tasked with cleaning up the algae are also known to have become extremely ill.