Horizon investigates what happened in the tropical aquarium of a German zoo two decades ago, where according to biologists, experiments with a tropical seaweed unleashed a hybrid algae which is now decimating marine life in the Mediterranean. It was in the late 1980s that Alexandre Meinesz, a professor of biology at France's University of Nice-Sophia Antipolis on the Mediterranean coast, first detected the spread of a new species of algae along the French coastline. Horizon follows Meinesz on a scientific detective story as he attempts to unravel the source of this alien algae, alert the authorities to the danger and find a solution. Meinesz quickly identified the invader as a gigantic version of an algae called caulerpa taxifolia. But what puzzled him was that this variety of algae normally grows thousands of miles away in the tropics and is a small, relatively rare, plant. The plant he'd found was bigger and denser than any member of the caulerpa family he'd ever seen before and was thriving in the cooler waters of the Mediterranean. As scientists studied the invader, they found that although it posed no threat to humans, it was so toxic many Mediterranean animals preferred to starve to death, rather than eat it. And it was growing so fast that it was swamping indigenous rivals; where it grew the seabed was becoming a green desert. But nobody in authority listened to Meinesz's warning and today the algae is causing the biggest upheaval in the Mediterranean's ecosystem in modern history. The film traces the plant back to a strain of algae bred unintentionally by the Wilhelmina Zoo, in Stuttgart, Germany during the 1970s. Its unique characteristic was that, alone among tropical plants, this one thrived in the artificial environment of aquaria. Wilhelmina sent it to other aquaria in western Europe; it soon became the most popular aquarium plant on earth, traded around the world. In the 1980s it 'escaped' into the Mediterranean where, unexpectedly, it thrived. In 1999 the Americans labelled caulerpa taxifolia a noxious weed and banned imports. But it was too late, the country's fish tanks were already awash with it. In 2000, some found its way into a coastal lagoon in southern California. The Americans have been working hard ever since to eradicate it before the Californian coast goes the way of the Mediterranean. But with the algae in millions of fish tanks across the world, where will it strike next? In the Mediterranean today, the French environmental authorities are looking for solutions. They've tried a variety of conventional forms of eradication; none can match the scale of the infestation. There is one hope: a tropical sea slug called elysia subornata, which naturally feeds on the plant. But this involves introducing yet another 'exotic' species into the Mediterranean and, understandably, scientists are nervous about that. Would they be merely replacing one 'exotic' invasion with another? While they ponder, the ecology of the western Mediterranean becomes ever more impoverished.moreless
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