Past migrations and Pleistocene ice ages still impact the size and structure of modern seagrass communities

Deep evolution casts a longer shadow than previously thought, scientists report in a new post published the week of August 1 in the Proceedings of National Academy of Sciences. Smithsonian scientists and colleagues have examined seagrass communities — the foundation of many coastal marine food webs along the northern Atlantic and Pacific coasts — and found that their ancient genetic history may play a more crucial role. than the current environment in determining their size, their composition. and who inhabits them. And that could have implications for how seagrass adapts to threats like climate change.

There are approximately half a million Years ago, when the world was warmer, some eelgrass plants made the difficult journey from their homes in the Pacific to the Atlantic. Not all plants were hardy enough to cross the Arctic. For those who succeeded, a series of ice ages during the Pleistocene epoch further affected their extent. These millennial struggles have left lasting signatures in their DNA: Even today, seagrass populations in the Atlantic are much less genetically diverse than those in the Pacific.

Yet , in the classic “nature versus nurture” debate, scientists have been amazed to discover that genetic inheritance sometimes does more to shape modern seagrass communities than the current environment.

“We already knew there was a great genetic separation between the oceans, but I don’t think any of us ever dreamed that would be more important than environmental issues,” said Emmett Duffy, marine biologist at the Smithsonian Environmental Research Center and lead author of the report. “It was a big surprise for everyone.”

Eelgrass in hot water

Eelgrass is one of the most widespread shallow water plants in the world. Its range extends from semi-tropical regions such as Baja California to Alaska and the Arctic. In addition to providing food and habitat for many underwater animals, eelgrass provides a plethora of services to humans. It protects coastlines from storms, absorbs carbon and can even reduce harmful bacteria in the water.

But in most places where it grows, eelgrass is the dominant – or only – species of seagrass present. This makes its survival essential for the people and animals that live there. And the lower genetic diversity in the Atlantic could make it difficult for some populations to adapt to sudden changes.

“Diversity is like having different tools in your tool belt,” said Jay Stachowicz, co-author and ecologist from the University of California, Davis. “And if all you have is a hammer, you can put some nails in, but that’s about it. But if you have a full set of tools, each tool can be used to do different jobs more efficiently.”

Ecologists have already seen eelgrass disappear from some areas as the waters warm. In Portugal, its southernmost point in Europe, eelgrass has begun to recede and move further north into cooler waters.

“I don’t don’t think we’re going to lose in the sense of extinction,” said co-author Jeanine Olsen, professor emeritus at the University of Groningen in the Netherlands. “It won’t be like that. He has a lot of tricks up his sleeve. But local extinctions, she pointed out, are going to happen in some places. This could leave regions dependent on their local eelgrass struggling.

Achieving a world view more ZEN

Realizing the urgent need to understand – and conserve – global seagrass, Duffy and his colleagues come together to form a global network called ZEN. The name stands for Zostera Experimental Community, a nod to the scientific name for eelgrass, Zostera marina. The idea was to bring together seagrass scientists from around the world, doing the same experiments and surveys, to get a coordinated global picture of seagrass health.

For the new study, the team studied seagrass communities at internet sites in the Atlantic and Pacific. With 20 plots sampled by web-site, the team left with data from 1 eelgrass plots.

They first collected basic data on eelgrass: size, shape, total biomass and the different animals and algae living on and around them. Next, they collected genetic data on all seagrass populations. They also measured several environmental variables at each website: temperature, water salinity, and nutrient availability, to name a few.

Ultimately, they hoped to find out what shapes seagrass communities the most: the environment or genetics?

After running a series of models, they discovered a multitude of differences between the Atlantic and Pacific seagrass ecosystems – differences closely aligned with the genetic divergence from Pleistocene migration and later ice ages.

While Pacific seagrass often grew in “forests” that regularly exceeded 3 feet tall and sometimes reached as More than twice that height, the Atlantic was home to smaller “meadows” that rarely approached that height. Genetic differences also aligned with total eelgrass biomass. In the Atlantic, evolutionary genetics and the current environment have played an equally critical role in eelgrass biomass. In the Pacific, genetics had the upper hand.

These impacts also rippled through other get-togethers in the ecosystem. When it comes to small animals that lived in eelgrass, such as invertebrates, the Pleistocene genetic signature again played a more important role than the environment in the Pacific, while the two played equally important roles. important in the Atlantic.

“The ancient legacy of this Pleistocene migration and the bottleneck of seagrass in the Atlantic had consequences for the construction of the ecosystem 20 years later,” said Duffy. “Probably more than 20 .”

Preserving the future

That ancient genetics can play such a vital role – sometimes stronger than the environment – ​​worries some ecologists about the ability of eelgrass to adapt to faster changes.

“Global warming – in and of itself – is probably not the main threat to eelgrass,” Olsen said. Air pollution from cities and farms, which can cloud water and lead to harmful algal blooms, also puts seagrass beds at risk. That said, the wide range of environments in which eelgrass can survive speaks to its hardiness.

“I am hopeful because our results illustrate long-term resilience to repeated and major changes in thermal tolerances and wide range of eelgrass habitats over approximately half of the northern hemisphere,” Olsen said. “With the genomic resources now available for eelgrass, we are beginning to analyze gene functional changes and their regulation in real time. It’s very exciting.”

To protect existing eelgrass beds, maintaining current diversity is a good first step. In places that have already lost eelgrass beds, restoration is promising. Some successes already exist, such as on the east coast of Virginia. But many restoration endeavors achieve only limited success. As Stachowicz pointed out, this raises additional issues.

“Should you restore seagrass beds using plants from local environments, or should you consider the future and try plants with genetics better suited to future environmental conditions?” He asked. “Or should you hedge your bets?” Maintaining or enhancing genetic diversity may be the best way to provide seagrass populations with the diverse toolbox needed to survive in an uncertain future.

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