World’s largest plant discovered in Australia

By Katie Hunt, CNN

The world’s largest living plant has been identified in the shallow waters off the coast of Western Australia, according to scientists.

The sprawling seagrass, a marine flowering plant known as Posidonia australis, stretches for more than 112 miles (180 kilometers) in Shark Bay, a wilderness area protected as a World Heritage site, said Elizabeth Sinclair, a senior research fellow at the School of Biological Sciences and Oceans Institute at The University of Western Australia.

That’s about the distance between San Diego and Los Angeles.

The plant is so large because it clones itself, creating genetically identical offshoots. This process is a way of reproducing that is rare in the animal kingdom although it happens in certain environmental conditions and occurs more often among some plants, fungi and bacteria.

“We often get asked how many different plants are growing in a seagrass meadow. Here we used genetic tools to answer it,” said Sinclair, the author of a study on the seagrass that published late Tuesday in the journal Proceedings of the Royal Society B.

“The answer definitely surprised us — just ONE! That’s it, just one plant has expanded over 180 km in Shark Bay, making it the largest known plant on Earth,” she said via email.

Sinclair and her colleagues took samples from 10 locations across the range of the seagrass meadow in Shark Bay in 2012 and 2019. The research team also measured the environmental conditions including depth, water temperature and salinity.

“We have been studying cool water seagrasses in southern Australia for a while, to understand how much genetic diversity is in them and how connected the meadows are,” Sinclair said.

The scientists were able to sequence DNA from the seagrass samples, which revealed that it was a single plant.

“The plant has been able to continue growing through vegetative growth — extending its rhizomes (rootstalks) outwards — the way a buffalo grass would in your back garden, extending runners outwards. The only difference is that the seagrass rhizomes are under a sandy seafloor so you don’t see them, just the shoots within the water column,” she said.

“What was even more interesting was that it has double the number of chromosomes than in other populations we had been studying. It has 40, not the usual 20,” she added.

Seagrasses inhabit marine coastlines and estuaries globally.

The study suggested that reproducing via cloning helped the seagrass meadow adapt to habitat conditions that were more extreme than where seagrass is usually found — saltier water, high levels of light and wide temperature fluctuations.

The seagrass meadow covered almost 200 square kilometers (77 square miles or 49,000 acres), Sinclair said — bigger than Brooklyn. That’s a much larger area than the Pando quaking Aspen trees in Utah, which are often described as the world’s largest plant. The clone spreads over 106 acres, consisting of over 40,000 individual trees, according to the USDA Forest Service.

At about 4,500 years old, the Shark Bay seagrass is ancient, but its age isn’t record-breaking, the researchers said. A Posidonia oceanica plant discovered in the western Mediterranean that spans up to 9.3 miles (15 kilometers) may be greater than 100,000 years old.

“Individual seagrass clones may persist almost indefinitely if left undisturbed, as they rely on vegetative, horizontal rhizome expansion, rather than sexual reproduction,” Sinclair said.

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