Take a careful look at those amazing long, vivid green ribbons swaying with the water's ebb and flow the next time you go diving or snorkeling. They're seagrasses, which are marine plants that, like their land-based counterparts, produce blooms, fruit, and seedlings every year.
These underwater seagrass meadows expand in two ways: by sexual reproduction, which aids in the generation of new gene combinations and genetic variety, and by spreading their rhizomes, which are subterranean stems from which roots and shoots arise.
A DNA test is required to determine how many different individual plants are flourishing in a seagrass meadow. In the shallow sun-drenched seas of the Shark Bay World Heritage Area in Western Australia, we did this for fields of ribbon weed seagrass called Posidonia australis.
The end outcome astounded us: it was all one plant. One single plant has grown across a distance of 180 kilometers (110 miles), making it the world's biggest known plant.
We took shot samples from 10 seagrass meadows scattered around Shark Bay, in conditions ranging from typical ocean salinity to nearly twice as saline. We looked over 18,000 genetic markers in all of the samples to establish that 200 km2 (77 square miles) of ribbon weed meadows grew from a single, colonizing seedling.
We collected shoot samples from ten seagrass meadows from across Shark Bay, in waters where the salt levels range from normal ocean salinity to almost twice as salty. In all samples, we studied 18,000 genetic markers to show that 200 km² (77 square miles) of ribbon weed meadows expanded from a single, colonizing seedling.
How did it evolve?
Apart from its gigantic size, what distinguishes this seagrass plant from others is that it contains twice as many chromosomes as its cousins. This qualifies it as a "polyploid," according to experts.
The majority of the time, a seagrass seedling will inherit half of each parent's DNA. Polyploids, on the other hand, have both of their parents' genomes.
Many polyploid plant species exist, including potatoes, canola, and bananas. They frequently live in regions with harsh environmental conditions in nature.
Polyploids are generally infertile, but if left alone, they can grow indefinitely. That's exactly what this seagrass has done.
How old is this plant?
When the water level rose following the last ice age, the sand dunes of Shark Bay inundated 8,500 years ago. The increasing seagrass meadows created shallow coastal banks and sills over millennia by generating and absorbing silt, making the water saltier.
There's a lot of light in Shark Bay's waters, as well as low amounts of nutrients and big temperature swings. Despite its harsh surroundings, the plant has managed to grow and adapt.
It's difficult to say how ancient a seagrass meadow is, but based on its size and development rate, we think the Shark Bay plant is roughly 4,500 years old.
Other massive plants, such as a 6,000-tonne quaking aspen in Utah, have been documented in both marine and terrestrial environments, but this seagrass looks to be the biggest to date.
Other massive seagrass plants have been discovered, notably Posidonia oceanica, a Mediterranean seagrass that covers more than 15 km and is estimated to be 100,000 years old.
Why does this matter?
A severe heatwave struck land and sea ecosystems along the Western Australian coastline in the summer of 2010/11.
The heatwave caused extensive damage to Shark Bay's seagrass meadows. The ribbon weed meadows, on the other hand, have begun to rebound.
This is remarkable because this seagrass does not appear to reproduce sexually, which would ordinarily be the most effective way to adapt to changing circumstances.
Although we have spotted seagrass blooms in the Shark Bay meadows, which suggests that the seagrass is sexually active, we have seldom seen their fruits (the result of successful seagrass sex).
It's possible that our solitary plant isn't sterile. This makes their survival in the choppy waters of Shark Bay a puzzle: plants without sex have minimal genetic variety, which should limit their capacity to adapt to changing conditions.
Our seagrass in Shark Bay, on the other hand, may contain genes that are particularly well-suited to its local, yet fluctuating habitat, which is why it does not require sex to thrive.
The enormous plant looks to be quite robust even without successful blooming and seed production. It has a broad range of water temperatures and salt levels (from 17 to 30 degrees Celsius – 62 to 86 degrees Fahrenheit – in certain years).
Despite the fluctuating circumstances and strong light levels (which are normally stressful for seagrass), the plant is able to sustain its physiological processes and survive. So, how does it deal with it?
We believe that this plant possesses a modest number of somatic mutations (little genetic alterations that are not passed on to children) that let it survive in its 180 kilometer span.
However, this is simply a hunch, and we're testing the theory. In Shark Bay, we've set up a number of studies to learn more about how the plant lives and flourishes in such harsh surroundings.
The future of seagrass
Seagrasses safeguard our coastlines from storm damage, store a lot of carbon, and provide home for a wide range of creatures. Seagrass meadow conservation and restoration are critical for climate change mitigation and adaptation.
Seagrasses are not immune to the effects of climate change, which include rising temperatures, ocean acidity, and extreme weather events.
However, the precise image we now have of the gigantic seagrass of Shark Bay's incredible resilience gives us optimism that they will be around for many years to come, especially if real climate change action is implemented.
