About 4,500 years ago, a single seed, generated from two different species of seaweed, was found nestled in a favorable spot somewhere in what is now known as Shark Bay, near the west coast of Australia.
Scientists have discovered that the seed has grown to what is now believed to be the largest plant on Earth, covering about 200 km (77 square miles or about 20,000 rugby fields). just over three times the size of Manhattan Island).
The species, a Posidonia australis, also known as fiber ball weeds or ribbon weeds, is commonly found along the coasts of South Australia.
But when scientists began looking for genetic differences in ribbon grass in the bay, they came across a puzzle. Samples taken from places that were 180 km away suggested that there were not several specimens of Posidonia australis, but a single plant.
“We thought, ‘What the hell is going on here?'” Said Dr. Martin Breed, an ecologist at Flinders University. “We were completely baffled.”
Student researcher Jane Edgeloe of the University of Western Australia (UWA) said about 18,000 genetic markers were examined as they looked for variations in species that could help them select specimens for use in restoration projects.
But what they found was that the same plant had spread through rhizomes in the same way that a lawn can be spread from the edges by sending runners.
“The existing 200 square miles of ribbon weed meadows appear to have expanded from a single colonizing seedling,” he said.
The single plant now extends like a meadow, providing habitat for a wide variety of marine species, such as turtles, dolphins, dugongs, crabs and fish.
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The rhizomes of the ribbon weeds can grow up to 35 cm a year, and using this rate, the authors of the research, published in Proceedings of the Royal Society B, estimate that the plant will need at least 4,500 years to spread. how far he has come.
Dr. Elizabeth Sinclair, co-author of the research at the UWA, said they had not given the plant a nickname and that the original samples, taken from the seagrass meadow, originally had 116 different tags with GPS coordinates when stored. in a deep background. freeze ready for genetic sampling.
The plant has formed huge, dense meadows that in some areas extend as far as it can be seen in all directions. The ribbons of the plant are only 10 cm long in some places, but up to one meter in others.
The spread of the plant can be seen in this aerial view of Shark Bay. Photography: Angela Rossen
Conditions in Shark Bay are tough. The plant has found a way to survive in areas where salinity is twice as high as elsewhere in the bay, and can thrive in water temperatures as cold as 15 ° C and as hot as 30 ° C.
The survival of the seagrass plant appears to be related, Sinclair said, to how it had clung to all of its two parents’ chromosomes, giving it built-in genetic diversity.
“It simply came to our notice then [of] his mother’s genes and half of his father’s, he has preserved them all, “he said.
Sinclair and his colleagues are still working on the secrets of the giant specimen, but he said it appears to be “largely sterile” and should therefore rely on its own ability to grow, rather than disperse seeds.
Breed said the fact that the plant “has no sex” but has survived for so long was a puzzle.
“Plants that don’t have sex also tend to have reduced genetic diversity, which they normally need when faced with environmental change,” he said.
Breed said they had detected some very subtle mutations in the plant’s genetics in the places where it was growing that could also explain its extreme longevity.
The size of the Shark Bay Belt weed is about 20,000 acres (49,000 acres), making it much larger than an aspen tree stand in Asta, Utah, often known as the largest plant. of the world, covering 43 acres.
Associate Professor Kathryn McMahon of Edith Cowan University did not participate in the Shark Bay research, but is an expert on seaweed. He said the method used by the investigators gave him confidence that they had identified a single specimen, which he said was “amazing”.
Genetic studies of other species of seagrass had estimated that the plants could live between 2,000 and 100,000 years, so McMahon said the estimate that the Shark Bay specimen was 4,500 years old fits that range.
“They have a versatile growth pattern that contributes to that long shelf life,” he said. “They can grow into nutrient – rich patches to access the nutrients they need, or into the gaps in the meadow where there is room for them to grow or away from stressful places.
“All these characteristics mean that if they are in the right place they can persist for long periods of time.”