Next time you’re diving or snorkeling, take a look at these wonderfully long bright green ribbons, rippling with the ebb and flow of the water. They are seagrasses – marine plants that produce flowers, fruits and seedlings every year, like their terrestrial relatives.
These seagrasses grow in two ways: through sexual reproduction, which helps them generate new combinations of genes and genetic diversity, and also by spreading their rhizomes, the underground stems from which roots and shoots emerge. .
To find out how many different individual plants grow in a seagrass, you need to test their DNA. We did it for the seagrass meadows called Posidonia australis in the shallow, sunny waters of Shark Bay World Heritage Area, Western Australia.
The result blew us away: it was a single plant. A single factory grew over an area of 180 km (110 miles), making it the largest known factory on Earth.
We collected shoot samples from ten seagrass beds throughout Shark Bay, in waters where salt levels range from normal ocean salinity to nearly twice as salty. Across all samples, we studied 18,000 genetic markers to show that 200 km² (77 square miles) of ribbon weed meadows developed from a single colonizing seedling.
How has it evolved?
What makes this seagrass plant unique from others, besides its huge size, is that it has twice as many chromosomes as its parents. This makes it what scientists call a “polyploid”.
Most of the time, a herbarium seedling will inherit half of the genome from each of its parents. Polyploids, however, carry the entire genome of each of their parents.
There are many species of polyploid plants, such as potatoes, canola, and bananas. In the wild, they often reside in places with extreme environmental conditions.
Polyploids are often sterile, but can continue to grow indefinitely if left undisturbed. This herbarium did just that.
How old is this plant?
Shark Bay’s sand dunes were flooded around 8,500 years ago when sea levels rose after the last ice age. Over the following millennia, expanding seagrass beds formed shallow coastal shoals and sills by creating and capturing sediment, which made the water saltier.
There is also plenty of light in the waters of Shark Bay, as well as low nutrient levels and large temperature fluctuations. Despite this hostile environment, the plant was able to thrive and adapt.
It is difficult to determine the exact age of a seagrass bed, but we estimate the Shark Bay plant to be around 4,500 years old, based on its size and growth rate.
Other huge plants have been reported in the marine and terrestrial systems, such as a 6,000-ton trembling aspen in Utah, but this seagrass appears to be the tallest yet.
Other huge seagrasses have also been found, including a closely related Mediterranean seagrass called Ocean Posidoniawhich extends for more than 15 km and may be around 100,000 years old.
Why is this important?
During the summer of 2010/11, a severe heat wave hit terrestrial and marine ecosystems along the west coast of Australia.
Shark Bay’s seagrass suffered extensive damage during the heat wave. Still, the ribbon meadows have begun to recover.
This is somewhat surprising, as these seagrasses do not appear to reproduce sexually – which would normally be the best way to adapt to changing conditions.
We have observed seagrass flowers in the grasslands of Shark Bay, indicating that the seagrasses are sexually active, but their fruits (the result of successful sex with seagrass) are rarely seen.
Our only plant may actually be sterile. This makes its success in the variable waters of Shark Bay quite puzzling: plants that don’t have a sex also tend to have low levels of genetic diversity, which should reduce their ability to cope with changing environments.
However, we suspect that our Shark Bay seagrass has genes extremely well suited to its local, but variable, environment, and that may be why it doesn’t need to have sex to be successful.
Even without successful flowering and seed production, the giant plant seems to be very resilient. It experiences a wide range of water temperatures (from 17 to 30 degrees Celsius – 62 to 86 degrees Fahrenheit – in some years) and salt levels.
Despite these variable conditions and the high light levels (which are usually stressful for seagrasses), the plant can maintain its physiological processes and thrive. So how does he cope?
We hypothesize that this plant has a small number of somatic mutations (minor genetic changes that are not passed on to offspring) across its 180 km range that help it persist under local conditions.
However, this is only a hunch and we are addressing this hypothesis experimentally. We set up a series of experiments at Shark Bay to really understand how the plant survives and thrives in such variable conditions.
The future of seagrass
Seagrasses protect our coasts from storm damage, store large amounts of carbon, and provide habitat for a wide variety of wildlife. Conserving and restoring seagrass beds plays a vital role in mitigating and adapting to climate change.
Seagrass beds are not immune to the impacts of climate change: warming temperatures, ocean acidification and extreme weather events pose a significant challenge for them.
However, the detailed picture we now have of the great resilience of Shark Bay’s giant seagrasses gives us hope that they will be around for many years to come, especially if serious action is taken against climate change.
Elizabeth Sinclair, Senior Researcher, University of Western Australia; Gary Kendrick, Winthrop Professor, Oceans Institute, University of Western Australia; Jane Edgeloe, PhD Candidate (Marine Biology), University of Western Australia, and Martin Breed, Lecturer in Biology, Flinders University.
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