Striped weeds, meadows of South Sea Posidonia, Shark Bay in Western Australia. Image credit: Rachel Austin, University of Western Australia
Scientists are using DNA analysis to study meadows of banded weed seagrass. The shocking results blew them away. The 18,000 DNA markers they analyzed in the Shark Bay World Heritage Area in Western Australia showed that one plant extended 112 miles. This makes it the largest known plant in the world.
Somehow, the seedling grew to an area of 77 square miles, crossing waters with completely different temperatures and salinities. Based on its size and growth rate, researchers estimate it to be 4,500 years old.
The next time you go diving or snorkeling, take a closer look at those fantastic long, bright green ribbons that wave as the tide rises and falls. They are seagrasses - marine plants that produce flowers, fruits and seedlings every year, just like their land relatives.
These underwater seagrass meadows grow in two ways: through sexual reproduction, which helps them produce new gene combinations and genetic diversity, and by expanding their rhizomes, roots and shoots to produce underground stems from them.
To find out how many different plants grow in seagrass meadows, you have to test their DNA. We did this work in the sun-drenched shallow waters of the Shark Bay World Heritage Area in Western Australia for the banded weed seagrass grass.
The result surprised us: it was all a plant. A single plant has been extended to an extension of 180 kilometers (112 miles), making it the largest known plant on Earth.
We collected bud samples from ten seagrass meadows in Shark Bay, where salinity ranged from normal ocean salinity to nearly twice as salinity. In all samples, we studied 18,000 genetic markers indicating that a 200 square kilometre (77 mi²) of striped weed meadows expanded from a single colonized seedling.
Sampling of Posidonia. Image credit: Rachel Austin
How did it evolve?
Apart from its enormous size, this seagrass plant differs from other plants in that it has twice as many chromosomes as its relatives. This makes it what scientists call a "polyploidy."
Most of the time, seagrass seedlings will inherit half of the genome of each of their parents. However, polyploidy carries the entire genome of each of its parents.
There are many polyploid plant species such as potatoes, rapeseed and bananas. In nature, they usually live in places with extreme environmental conditions.
Polyploidies are usually sterile, but can continue to grow indefinitely if undisturbed. This seagrass does just that.
How old is this plant?
The dunes of Shark Bay were flooded about 8,500 years ago, when sea levels rose after the last ice age. Over the next few thousand years, ever-expanding seagrass meadows formed shallow coastal shores and sills by creating and capturing sediments, which made the water more salty.
The waters of Shark Bay also have a lot of light, low nutrient levels, and large temperature fluctuations. Despite this harsh environment, the plant has been able to thrive and adapt.
The shallow, salty waters of Shark Bay. Image credit: Angela Rossen
Determining the exact age of seagrass meadows is challenging, but we estimate the age of shark bay plants to be about 4,500 years, based on their size and growth rate.
Other large plants have been reported in both marine and terrestrial systems, such as Utah's 6,000-ton seismic poplar, but this seagrass appears to be the largest to date.
Other giant seagrass plants have also been found, including a closely related Mediterranean seagrass called Posidonia oceanica, which covers more than 15 kilometers (9 miles) and may be about 100,000 years old.
Why is this important?
In the summer of 2010-11, a severe heat wave hit terrestrial and marine ecosystems along the Western Australian coastline.
The seagrass meadows of Shark Bay have suffered extensive damage during heat waves. However, banded weed meadows have begun to recover.
This is a bit surprising because this seaweed doesn't seem to reproduce sexually – which is often the best way to adapt to changing conditions.
We observed seagrass flowers in Shark Bay meadows, which indicates that seagrasses are sexually active, but their fruits (the result of successful seagrass sexual behavior) are rare.
Flowers emerge from the Seagrass of Australasia. Image credit: Angela Rossen
In fact, our single plant may be sterile. This makes its success in the volatile waters of Shark Bay a conundrum: Sexless plants also tend to have low levels of genetic diversity, which should reduce their ability to cope with changing environments.
However, we suspect that our seagrass in Shark Bay has genes that are well suited to its local but changeable environment, and perhaps that's why it doesn't need sex to succeed.
Even without successful flowering and seed production, giant plants seem to be very resilient. It has experienced a wide range of water temperatures (from 17ºC/63ºF to 30ºC/86ºF in some years) and salt content.
Despite these variable conditions and high light levels (often stressful for seagrass), plants can maintain their physiological processes and thrive. So how did it respond?
We hypothesized that this plant had a small number of somatic mutations (tiny genetic changes not passed on to offspring) over its 180 km (112 mi) range, which helped it persist under local conditions.
However, this is only a hunch, and we are working on this hypothesis through experimentation. We conducted a series of experiments at Shark Bay to really understand how this plant can survive and thrive in such a changeable condition.
The future of seagrass
Seagrass protects our coast from storm damage, stores large amounts of carbon, and provides habitat for a wide variety of wildlife. Protecting and restoring seagrass meadows plays a vital role in mitigating and adapting to climate change.
Seagrasses are also not immune to the effects of climate change: rising temperatures, ocean acidification and extreme weather events are major challenges for them.
However, the details of the enormous resilience we now have about the giant seagrasses in Shark Bay give us hope that they will be around for many years to come, especially if serious action is taken on climate change.