An underwater photograph of a section of the seagrass meadow

Posidonia australis is thousands of years old, and covers hundreds of square kilometres, making it the largest known organism by area in the world. Image © Rachel Austin, UWA

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The world's largest plant is an Australian seagrass clone

The world's largest plant, and clone, may have just been discovered off the Australian coast.

Believed to be at least 4,500 years old, Shark Bay's Posidonia australis covers an area bigger than Washington DC. 

A meadow supporting thousands of species has just been revealed as the world's largest clone.

Measuring more than 180 square kilometres, the meadows of Posidonia australis in Shark Bay, Australia, form part of one genetically identical individual. Until 2010, the meadow measured 200 square kilometres, but since then cyclones and heat waves have killed off a portion of the seagrass.

PhD student Jane Edgeloe, the lead author of a paper announcing the discovery, sampled seagrass shoots from across the bay to deduce the plant's genetic fingerprint.

'The answer blew us away – there was just one!' Jane says. 'That's it, just one plant has expanded over 180 kilometres in Shark Bay, making it the largest known plant on Earth.'

Co-author Dr Elizabeth Sinclair adds, 'Shark Bay is a pretty unique environment which is largely undisturbed and has remained that way for some time, leading to its listing as a World Heritage Site. It is largely protected from the ocean which provides a fairly calm environment for the plant to keep growing.

'This could help explain why the clone is so big. It's possible there is a larger one out there, as there is a lot we don't know about our natural world.'

While the P. australis meadow has survived extreme weather events in recent years, and began recovering, there are concerns that it could perish as global temperatures continue to rise.

The study was published in Proceedings of the Royal Society B

A forest of white-barked aspen trees amongst green shrubs

Pando is thought to be the heaviest known organism, being comprised of over 40,000 trees.  Image © R Scott Jones, licensed under CC BY-NC-SA 2.0 via Flickr

The biggest and oldest clones in the world

While many organisms reproduce sexually, there are others that reproduce by cloning themselves. This occurs more often in plants than in animals, with the spider plant one species where asexual plantlets develop from a parent.

Being a clone can be problematic to an organism, as disease which affects one individual will likely affect them all. 

For instance, the Gros Michel was the world's most consumed banana for over a century and was widely grown through asexual reproduction. However, the fungal Panama disease decimated plantations during the 1950s, which caused producers to switch to the Cavendish banana instead.

Despite this, clonal organisms are among some of the largest in the world. Pando is the name given to a group of around 40,000 aspen trees in Utah, USA, which are all clones of a single organism and share the same root system.

Meanwhile, meadows of clonal Posidonia oceanica found around the island of Formentera in the Mediterranean Sea are at least 12,500 years old but may be tens of thousands of years older.

The secret of P. australis's success in Shark Bay may lie in its genetics. Researchers found that the seagrass is a polyploid organism, with double the normal number of chromosomes. 

'Diploid organisms, like you and me, only inherit half of their parents' genome, whereas this seagrass has all of it,' Elizabeth explains. 'We suspect that this gives the plant an advantage over a diploid organism as it has all of the genes that help it survive in its current environment, as well as more genes which could help it survive in others. 

'Reproducing clonally ensures the seagrass keeps the same genetics, as sexual reproduction in a diploid organism leads to new combinations of genes. While some will lead to new adaptations and an advantage, others will put it at a disadvantage.'

The genetic makeup of the seagrass is well suited to Shark Bay, which is buffered from ocean swells that could break the plant apart, and help it tolerate a wide range of temperatures and salinities. But climate change could pose a threat to the future of this enormous organism. 

An aerial photo of the shallow, salty waters of Shark Bay

The seagrass meadow provides habitat for a variety of endangered mammals, including the dugong. Image © Angela Rossen

How is climate change affecting the seagrass?

Since 1910, Australia has warmed on average by 1.4⁰C as a result of climate change. This increases the risk of extreme weather events, and from 2010 to 2011, Western Australia experienced a marine heatwave that saw temperatures rise by up to 5⁰C in some coastal waters.

In Shark Bay, around 36% of the seagrass was damaged by this heatwave, with the ecosystem still recovering today. Damage to the seagrass is likely to have released some of the carbon locked up within it into the atmosphere, as well as affecting the animals it supports.

'Shark Bay supports megafauna such as dugongs, turtles and dolphins as well as thousands of fish species,' Jane says. 'When the seagrass is affected by climate change, all of the animals at a higher trophic level are affected. 

'As this clonal seagrass survived the marine heatwave and has been recovering since then, it looks promising, and perhaps the merging of its parental genomes has made it more resilient to climate change. 

'However, there is a climate threshold. We've set up monitoring experiments within and outside Shark Bay to try and figure out this threshold, and to see how Shark Bay's organisms will react.'

'While there has been some recovery, there are other areas that need some help,' Elizabeth adds. 'Once the seagrass is gone, the seafloor keeps moving and this makes it hard for plants to establish again.

'We've been working with the indigenous rangers of Shark Bay, the Malgana, to figure out how best to restore seagrass in the areas that were most affected.'