Scientists have discovered the world’s largest known plant with a length of more than 180 km

(ORDO NEWS) — The next time you’re diving or snorkeling, take a close look at these amazingly long bright green ribbons that wave in time with the ebb and flow of the water. These are sea grasses – sea plants, which, like their terrestrial relatives, annually produce flowers, fruits and seedlings.

These underwater seagrass meadows grow in two ways: by sexual reproduction, which helps them create new combinations of genes and genetic diversity, and by proliferation of rhizomes – underground stems from which roots and shoots emerge.

To find out how many different plants grow in a seagrass meadow, you need to examine their DNA. We did this for Posidonia australis banded seagrass meadows in sun-drenched shallow waters in the Shark Bay World Heritage Area in Western Australia.

The result amazed us: it was all one plant. One single plant has grown over 180 km (110 miles), making it the largest known plant on Earth.

We collected shoot samples from ten seagrass meadows across Shark Bay, in waters where salt levels range from normal ocean salinity to nearly twice as salty. In all samples, we examined 18,000 genetic markers to show that 200 km² (77 square miles) of ribbongrass meadows grew from a single colonial sprout.

How did it develop?

A distinctive feature of this marine plant from others, in addition to its huge size, is that it has twice as many chromosomes as its relatives. This makes it what scientists call a “polyploid”.

In most cases, a seagrass seedling inherits half of the genome from each of its parents. Polyploids, on the other hand, carry the entire genome of each of their parents.

There are many types of polyploid plants such as potatoes, canola and bananas. In nature, they often live in places with extreme environmental conditions.

Polyploids are often sterile, but can continue to grow indefinitely if left undisturbed. This sea grass did just that.

How old is this plant?

The sand dunes of Shark Bay were flooded about 8,500 years ago when sea levels rose after the last ice age. Over the following millennia, overgrown seagrass beds created shallow coastal shoals and rapids, creating and trapping sedimentary material that made the water saltier.

The waters of Shark Bay also have a lot of light, low nutrient levels and large temperature fluctuations. Despite such a hostile environment, the plant was able to thrive and adapt.

It is difficult to determine the exact age of the seagrass meadow, but judging by its size and growth rate, the plant from Shark Bay is about 4,500 years old.

Other huge plants have been reported in both marine and terrestrial systems, such as the 6,000-ton trembling aspen in Utah, but this seagrass appears to be the largest to date.

Other huge seagrasses have been discovered, including the closely related Mediterranean seagrass Posidonia oceanica, which covers over 15 km and may be around 100,000 years old.

Why is it important?

In the summer of 2010/11, a severe heatwave hit terrestrial and marine ecosystems along the coast of Western Australia.

Seagrass meadows in Shark Bay have been affected by extreme heat. However, the band weed meadows have begun to recover.

This is somewhat surprising since this seagrass does not seem to reproduce sexually, which is usually the best way to adapt to changing conditions.

We have observed seagrass flowers in the meadows in Shark Bay, which indicates that the seagrass is sexually active, but its fruits (the result of successful sexual reproduction of the seagrass) are rare.

Our only plant can be virtually sterile. This makes it quite a challenge to succeed in the changing waters of Shark Bay: nonsexual plants usually have low levels of genetic diversity, which should reduce their ability to cope with changing environmental conditions.

However, we suspect that our seagrass in Shark Bay has genes that are very well adapted to the local but changing environment, which may be why it doesn’t need sex to thrive.

Even without successful flowering and seed production, the giant plant proves to be very hardy. It experiences a wide range of water temperatures (17 to 30 degrees Celsius – 62 to 86 degrees Fahrenheit – in some years) and salt levels.

Despite these changeable conditions and high light levels (which are usually stressful for seagrass), the plant can maintain its physiological processes and thrive. How does it cope?

We hypothesize that this plant has a small number of somatic mutations (minor genetic changes that are not passed on to offspring) throughout its 180 km range that help it survive locally.

However, this is only an assumption, and we test this hypothesis experimentally. We set up a series of experiments in Shark Bay to understand how the plant survives and thrives in such changing conditions.

The future of sea grasses

Sea grasses protect our coasts from storm damage, store large amounts of carbon, and provide habitat for a wide variety of wildlife. Conservation as well as restoration of seagrass meadows plays a vital role in climate change mitigation and adaptation.

Seagrasses are not immune to the impacts of climate change: Warming temperatures, ocean acidification and extreme weather events pose a major challenge to them.

However, the detailed picture we now have of the enormous resilience of the giant sea grasses in Shark Bay gives us hope that they will continue to exist for many years to come, especially if serious action is taken to combat climate change.

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