4,500 year old plant spans 200 km2 off Australian coast

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Scientists have found that what was a single seed 4,500 years ago has become the world’s biggest plant located off of the Australian west coast. The plant, in Shark Bay, spans over 200 sq. km which is about three times the size of Manhattan island.

Scientists have known of the existence of the Posidonia australis, also known as fire-ball weed or ribbon weed, across the southern coastlines of Australia. When they found it in Shark Bay, they looked for genetic differences in the ribbon weed across the bay. Scientists had taken samples 180km apart. This puzzled them as they realized that there are not multiple specimens of Posidonia australis in Shark Bay, rather it is only just one plant.

"We thought ‘what the hell is going on here?’” said Dr. Martin Breed, an ecologist at Flinders University. “We were completely stumped.” 

According to Jane Edgeloe, a student researcher at the University of Western Australia (UWA), about 18,000 genetic markers were examined while they were attempting to find various specimens for a restoration project. 

Instead, scientists discovered that the same plant had spread by rhizomes, much like a lawn might spread by sending out runners from its edges.

“The existing 200 sq. km of ribbon weed meadows appear to have expanded from a single, colonizing seedling,” she said.

Ribbon weed rhizomes can grow up to 35cm every year, and the authors of the study, which was published in Proceedings of the Royal Society B, calculated that the plant would have taken at least 4,500 years to spread as far as it has.

The conditions in Shark Bay are harsh. The plant has evolved to survive in locations where the salinity is double that of the rest of the bay, and it can grow in water temperatures as low as 15 ° C and as high as 30 ° C.

The seagrass plant's survivability appears to be linked to the fact that it retained all of its chromosomes from its two parents, giving it intrinsic genetic variety, according to Sinclair. 

“Instead of getting half [of] its genes from mum and half from dad, it’s kept all of them,” said Dr. Elizabeth Sinclair, a co-author of the research at UWA.

Breed believed that this could be the secret to the plant’s survival. He also stated that scientists had detected very subtle mutations in the plant’s genetics across the places it was growing, possibly explaining its extreme longevity.