The Ancient Giant Under the Sea: Unveiling the World's Largest Plant, a 4,500-Year-Old Seagrass Clone

It sounds like a fact from a fantasy novel: a single plant, stretching 180 kilometers (112 miles) across the seafloor, that has been alive for at least 4,500 years. But in 2022, this is exactly what scientists discovered off the coast of Western Australia. The organism, a type of seagrass called Posidonia australis, isn't just a sprawling meadow—it's one individual, making it the largest and oldest known plant on Earth.

An Accidental Discovery in Shark Bay

The discovery was, in many ways, a happy accident. Researchers from The University of Western Australia (UWA) and Flinders University were studying the genetic diversity of the seagrass meadows in the UNESCO World Heritage Area of Shark Bay. Their goal was to understand which plants were best suited for seagrass restoration projects. They took samples from various locations across the bay, expecting to find a patchwork of genetically distinct individuals, like a forest of different trees.

What they found stunned them. After analyzing 18,000 genetic markers, the data kept coming back with the same result: it was all one plant.

The existing data showed the meadows were all quite genetically diverse, so we were very surprised to find out that this was not the case. The 18,000 genetic markers we get from these seagrasses is like a fingerprint, and they all had the same one.

The team had stumbled upon a single, massive clone that had been patiently spreading itself for millennia.

The Secret to Its Immense Size and Age

So, how does a single plant achieve such a colossal scale and ancient lifespan? The answer lies in two key biological traits: cloning and polyploidy.

Like many grasses, Posidonia australis can spread via rhizomes—underground stems that send out new roots and shoots. Over thousands of years, this single plant has duplicated itself over and over again, creating a genetically identical meadow that now covers approximately 200 square kilometers (77 square miles).

But its resilience is supercharged by a rare genetic feature. This particular plant is a polyploid, meaning it carries two complete genomes from its parent species instead of one from each. It's a hybrid that, at some point in its distant past, doubled its own chromosome count.

Polyploid plants often reside in places with extreme environments, are often sterile, but can continue to grow if left undisturbed, and this giant seagrass has done just that. Even without successful flowering and seed production, it appears to be really resilient, experiencing a wide range of temperatures and salinities plus extreme high light conditions, which together would typically be highly stressful for most plants.

This genetic quirk gave the plant a robust toolkit to survive the challenging and fluctuating conditions of Shark Bay, from its high salinity to its variable temperatures, allowing it to grow undisturbed for eons.

An Ancient Survivor

To put its age into perspective, this seagrass began its life around the same time the Great Pyramids of Giza were being built. It has been steadily growing long before the rise and fall of the Roman Empire and has weathered countless environmental shifts. Its slow and steady growth, estimated at up to 35 centimeters per year, allowed it to reach its current size, which is roughly equivalent to the distance from San Diego to Los Angeles.

This discovery not only redefines the limits of what a single organism can be but also offers crucial insights into how life can adapt and persist in a changing world. This ancient giant is a living testament to resilience, a 4,500-year-old survivor thriving just beneath the waves.

Sources

• Scimex: World's oldest and largest known plant found in WA
• The University of Western Australia: World's largest plant found in WA
• Proceedings of the Royal Society B: Extensive polyploid clonality was a successful strategy for seagrass to expand into a newly submerged environment