Seagrass ‘tens of thousands of years old’

Posidonia oceanica (Image: M. San Felix) Seagrasses play a key role in marine ecosystems but are declining around the world, say researchers

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Meadows of seagrass found in the Mediterranean Sea are likely to be thousands of years old, a study shows.

Researchers found genetically identical samples of Posidonia oceanica up to 15km apart, which suggested that the species was extremely long-lived.

The team added that the organism - which provides food and shelter for many species - is under threat from climate change.

They report their findings in the open access journal Plos One.

The seagrass is "partially clonal", explained co-author Sophie Arnaud-Haond from the French Research Institute for the exploration of the Sea (Ifremer).

"This means they can reproduce sexually through flowering and recombination of male and female genomes, or clonally through the exact replication of the genome of an individual forming a new plant module through clonal growth," she told BBC Nature.

Asexual reproduction, such as cloning, means an individual organism's genetic identity is preserved and passed on from generation to generation, whereas the offspring of sexual organisms is made up of genetic material from both parents.

'Copy errors'

Dr Arnaud-Haond added that there was a theory that even asexual reproduction could not continue indefinitely because tiny "copy errors" accumulated in the genes over time.

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[There are] concerns as to the ability of this slow growing species... to face new and rapidly occurring environmental challenges”

End Quote Sophie Arnaud-Haond Ifremer

"Most of [these errors] are expected to have a negative impact; through generations [the organism] will degenerate and eventually disappear," she said.

"The age of clonal organisms should therefore be limited as well."

However, the study - which sampled seagrass across 3,500km of the Mediterranean Sea - found seagrasses with identical genomes spreading across large areas and large distances, challenging that theory.

"We were able, through modelling, to demonstrate that Posidonia oceanica has a mode of clonal spread, common to other seagrasses, that allows the plant to avoid the accumulation of deleterious mutations and explains how it escapes this theoretical rule," Dr Arnaud-Haond observed.

She said the results were one of the first times that such a long life - tens of thousands of years - had been predicted for an organism's genetic material.

The researcher added that the plants' extreme longevity also indicated that the species displayed an ability to adapt in order to survive over such a length of time.

"The estimated age of Posidonia oceanica clones imply these have been surviving under a broad range of environmental conditions, including much lower sea-water temperatures than those recorded nowadays, showing therefore extraordinary adaptive capacities."

Changing times

But she warned that past success did not mean that seagrass species were well suited to cope with future changes to the marine environment.

Green turtles swimming over seagrass Many species, including green turtles depend on the green meadows

She pointed out that the clones of Posidonia oceanica revealed in the study had "crossed the past millenia... with seawater temperatures 10-15C lower than they are nowadays, and... the drop of the sea level 100m below its present day level about 10,000 years ago".

"These findings show that Posidonia oceanica's clones have been able to survive fluctuating environmental conditions, and meadows we observe nowadays are the outcome of millenia of evolution.

"On the other hand, changes are occurring nowadays at an unprecedented rate and the sharp decline of seagrass meadows in general - and Posidonia oceanica in particular - are raising concerns as to the ability of this slow growing species and its old, patiently selected clones, to face these new and rapidly occurring environmental challenges."

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