World’s Biggest Organism Found in Australia, Is 4,500 Years Old

A seagrass clone that stretches across 110 miles will shed light on adaptation to extreme weather and climate change.
A seagrass clone that stretches across 110 miles will shed light on adaptation to extreme weather and climate change.
Ribbon weed, Posidonia australis, meadow in Shark Bay, Western Australia. Image: Rachel Austin, University of Western Australia
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Scientists have discovered a hybrid seagrass that extends for an astonishing 110 miles across Australia’s Shark Bay, making it the largest known organism on Earth, according to a new study. 

The huge plant has managed to clone the DNA of a seagrass called Poseidon’s ribbon weed (Posidonia australis) and merge it with another unknown grass around 4,500 years ago, which also makes this clone one of the oldest living things on Earth.  

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Researchers led by Jane Edgeloe, a marine biologist at the University of Western Australia, discovered the seagrass by analyzing DNA samples collected by SCUBA divers in 2012 and 2019 from Shark Bay. The results revealed that this record-breaking plant likely achieved its huge expanse by duplicating genomes, the biological instructions embedded in the cells of living things, using a process known as polyploidy. 

Polyploids are sometimes called “hopeful monsters” because they can experience “a rapid increase in genetic diversity” that outpaces their regular “diploid” progenitors, meaning species that have one set of chromosomes, according to a study published on Tuesday in the Proceedings of the Royal Society B.

“We identified that P. australis meadows sampled across Shark Bay, Western Australia, consisted of a single polyploid clone spanning more than 180 kilometers in fragmented, near-shore meadows,”  Edgeloe and her colleagues said in the study. “This makes it the most widespread known clone on Earth.” 

“Our findings suggest a significant new example of polyploidy as a successful evolutionary strategy that enabled an advantage over diploid progenitor(s) and access to new, disturbed or harsher habitats as they developed,” the researchers added.

Genetic samples revealed that this enormous seagrass hybrid contains 40 chromosomes—important structures that contain DNA—compared to the 20 chromosomes inside the genomes of regular Poseidon’s ribbon weed. It’s not clear how the hybrid seagrass is able to use these extra chromosomes to its advantage, but the study suggests the clone may be especially adept at enduring extreme weather, a phenomenon that is exacerbated by human-driven climate change.

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“Shark Bay is situated at the temperate–tropical interface meaning it is exposed to temperate and tropical extremes of climate change and extreme weather events, such as marine heatwaves and cyclones,” according to the new study. “The west coast of Australia was exposed to an unprecedented heatwave in the summer of 2010–2011, which impacted both terrestrial and marine ecosystems with sea surface temperatures greater than 3°C above long-term averages in Shark Bay.”

The researchers point to evidence that the hybrid seagrass survived and bounced back from this heatwave more quickly than its diploid relatives, and they also note that “past polyploid events appear to be associated with periods of significant changes in global climate.” As a result, the discovery of this enormous ancient clone could unravel mysteries about adaptation to climate change across time, including our own era of human-driven global warming.

“The polyploid P. australis clone showed a capacity to recover from an extreme climate event via vegetative growth,” the researchers said in the study. “The proposed superiority of this polyploid clone over diploid P. australis suggests vegetative material from the polyploid is best for the restoration of degraded meadows in Shark Bay.” 

“Exactly how the polyploid clone varies its response to local environmental conditions is unknown and the subject of further research, but its relative abundance suggests that it has evolved a resilience to variable and often extreme conditions that enable it to persist now and into the future,” they concluded.