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The Biggest, Oldest Plant on Earth Discovered off the Australian Coast

In awe at the size of this plant. An absolute unit.

A field of green seagrass stalks about a meter under the ocean surface in Western Australia
An incredibly resilient seagrass is the oldest and largest plant ever discovered.
University of Western Australia

Whenever I think about the largest things in the world, I start with Australia's Big Pineapple, but then come back to the "absolute unit" meme. I might be terminally online, sure, but there's nothing quite like the absolute unit for immediately conveying the scale and size of an object. 

And when it comes to plants, Australian scientists have found the absolute unit to end all absolute units: a seagrass that has stretched itself over 111 miles, lying a few feet underwater in a meadow in Shark Bay, Western Australia. That's a distance akin to driving across the Golden Gate Bridge... 65 times.

The researchers were studying a meadow of seagrass, sampling various clippings across a wide area to understand how genetically diverse the meadow was -- a fact that could help protect the seagrass from the looming threat of climate change. They wanted to know exactly how many different plants were growing in the meadow. But they were shocked.

"The answer blew us away -- there was just one!" said Jane Edgeloe, a student at the University of Western Australia who led the study published in the journal Proceedings of the Royal Society B on June 1.

They found that the meadow was home to a single clone of Poseidon's ribbon weed, or Posidonia australis. The plant is likely to have first sprouted around 4,500 years ago and has thrived in Shark Bay that whole time with seemingly little fuss. 

"How it's survived and thrived for so long is really puzzling," said Martin Breed, an ecologist at Flinders University in South Australia. Perhaps, Breed notes, some subtle genetic differences across its range in Shark Bay could be helpful for the seagrass to adapt to changing local conditions across its kingdom under the sea.  

Efforts will now be focused on understanding how it has evolved such resilience amid the highly variable environment it lives in. Ultimately, the insights could be useful for understanding seagrass adaptation, especially as the world's oceans begin to warm.

The discovery comes on a big day for seagrass (never thought I'd write that). Another report, published on the same day in Frontiers in Marine Science, suggests that the future of Australia's seagrass meadows is threatened by coastal development and climate change. The challenges to seagrass are extensive, especially as flooding and extreme rain affects the country's eastern coastlines and dirties waterways. Understanding the resilience of Poseidon's ribbon weed could play a key role in protecting and preserving the ecosystems.