Next time you go diving or snorkeling, look at the long, bright green ribbons that wave with the ebb and flow of the water. They are marine plants which produce flowers, fruit, and seedlings annually, like their land-based relatives.
Sexual reproduction and extending the roots of the underground stems help the underwater seagrass grow in two ways.
To find out how many different plants are growing in a meadow, you have to test their genes. We planted Posidonia australis in the Shark Bay World Heritage Area in Western Australia.
We were blown away by the result: it was all one plant. The largest known plant on Earth is one single plant that has expanded over 180 km.
Sampling Posidonia. (Rachel Austin)
In the waters of Shark Bay, the salt levels range from normal ocean salinity to almost twice as salty. We studied 18,000 genetic markers and found that 200 km of ribbon meadow weed expanded from a single, colonizing seedling.
This seagrass plant is unique in that it has twice the number of chromosomes as its relatives. Scientists call it a polyploid.
Half the genome of each of its parents will be passed on to a seagrass seedling. The whole genome of each of their parents is carried by polyploids.
Potato, canola, and bananas are some of the polyploid plant species. They live in places with extreme environmental conditions.
Polyploids can grow indefinitely if left undisturbed. That has been accomplished by this seagrass.
When the sea level rose after the last ice age, the sand dunes of Shark Bay flooded. The expanding seagrass meadow made shallow coastal banks and sills, which made the water saltier.
There is a lot of light in the waters of Shark Bay, as well as low levels of nutrients and large temperature fluctuations. The plant has been able to adapt despite the hostile environment.
The Shark Bay plant is around 4,500 years old, based on its size and growth rate, but it is difficult to determine the exact age of a seagrass meadow.
The largest plant reported to date is a 6,000-tonne quaking aspen in Utah, but this is the largest seagrass to date.
Posidonia oceanica is a Mediterranean seagrass that is around 100,000 years old and covers more than 15 km.
In the summer of 2010/11, a severe heatwave hit the Western Australian coastline.
The seagrass meadow in Shark Bay was damaged in the heatwave. The ribbon weed meadow has begun to recover.
This is somewhat surprising, as the best way to adapt to changing conditions would be to reproduce sexually.
seagrass flowers in the Shark Bay meadow are sexually active, but their fruits are rarely seen, which is a sign that they are successful in sex.
Our plant may be sterile. Plants that don't have sex tend to have low levels of genetic diversity, which should affect their ability to deal with changing environments.
We believe that the seagrass in Shark Bay has genes that are suited to its local environment, and perhaps that is why it doesn't need sex to be successful.
The giant plant is very resistant even without successful flowering and seed production. It has a wide range of water temperatures and salt levels.
The plant can thrive despite the high light levels and variable conditions. How does it cope?
We think that this plant has a small number of small genetic changes that are not passed on to offspring, that help it persist under local conditions.
This is a hunch and we are tackling it. We have set up a series of experiments in Shark Bay to understand how the plant thrives in variable conditions.
Seagrasses help protect our coast from storm damage, store large amounts of carbon, and provide habitat for a variety of wildlife. Climate change is mitigated and adapted through restoration of the seagrass meadow.
Warming temperatures, ocean acidification and extreme weather events are challenges for seagrasses.
If serious action is taken on climate change, the detailed picture we now have of the great resilience of the giant seagrass of Shark Bay provides us hope they will be around for many years to come.
The University of Western Australia has a Senior Research Fellow, Elizabeth Sinclair, and a PhD candidate, Jane Edgeloe.
This article is free to use under a Creative Commons license. The original article is worth a read.
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