BIODIVERSITY – These bright green ribbons undulate to the rhythm of the currents, and form real meadows in the sea. These are sea grasses, which are not algae, but indeed flowering plants, producing fruit every year.
To know their number and the diversity forming these “marine meadows”, a study published on June 1 in the journal Royal Society tested their DNA. Specifically, the researchers went to take samples of the plant Posidonia australis between 2012 and 2019 in 10 different seagrass beds. These are located in the shallow, sunny waters of Shark Bay on the west coast of Australia.
And the result was surprising to say the least: it was actually a single plant. More precisely, it is a single plant factory that has developed over 180 km (the distance between Paris and Auxerre), reproducing and “cloning” these sea grasses on a scale never seen before.
A 4500-year-old plant grandpa
Shark bay, previously simple sand dunes, was submerged about 8500 years ago; sea level swallowed it up at the end of the Ice Age. While it remains difficult to determine the exact age of these seagrasses, they would be approximately 4500 years old based on its size and growth rate.
As they developed, the sea grass beds shaped the reef, creating veritable sea fields. This demonstrates the resilience of these plants, as the waters of Shark Bay are bathed in strong light, low nutrient levels and wide fluctuations in temperature.
The Posidonia australis seagrass beds in Australia’s Shark Bay stretch for over 180km.
Besides its gigantic size, what makes this plant unique comes from its number of chromosomes, twice that of its parents. Researchers call this kind of plant a “polyploid”. Generally, a herbarium inherits half of the genome from each of its parents, but here it carries all of the two parents. Potatoes or bananas are also part of this family, which usually resides in extreme environmental conditions.
Resilient to climate change
If other organisms such as corals are very fragile to the warming and acidification of the oceans, sea grass beds are holding up better. How do they achieve this feat? This is all the more surprising as these plants, and more broadly the polyploids, are often sterile. Strange then that reproducing sexually is the best option to adapt. Indeed, sex and the resulting offspring help foster genetic diversity, increasing the ability to cope with changing environments.
But it’s not just sex in the life of these seagrasses that develop in a second way. To grow, they spread their rhizomes, underground stems from which new shoots emerge. Another advantage, the researchers believe that the Shark Bay herbarium has somatic mutations (minor genetic modifications) allowing it to be extremely well adapted to its local environment, despite its variability.
Indeed, the waters of this bay can vary between 17 and 30 degrees in temperature, while the level of salinity can go from simple to double. Nevertheless, the researchers point out that experiments must be carried out in order to really understand how this plant achieves this feat. This in particular with the aim of protecting it because, if it is more resistant, it is not immune to climate change.