The legacy of ancient ice ages determines how seagrasses respond to environmental threats today

The legacy of ancient ice ages determines how seagrasses respond to environmental threats today

Seagrass from the Finnish Archipelago Sea. Seagrasses migrated from the Pacific to the Atlantic hundreds of millennia ago, and that ancient migration left traces on their DNA that still make up them. Credit: Christoffer Boström, Åbo Akademi University

Deep evolution casts a longer shadow than previously thought, scientists report in a new paper published the week of Aug. Proceedings of the National Academy of Sciences. Smithsonian scientists and colleagues looked at seagrass communities — the basis of many marine food webs along the North Atlantic and Pacific coasts — and found that their ancient genetic histories may play a stronger role than the current environment in determining their size, structure and who lives. in them. And this could have implications for how well seagrasses adapt to threats like climate change.

About half a million years ago, when the world was warmer, some seagrass plants made the arduous journey from their homes in the Pacific to the Atlantic. Not all plants were hardy enough to make the journey across the Arctic. For those who succeeded, a series of ice ages during the Pleistocene further influenced the extent to which they were able to spread. That millennia-old struggle 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 vs. nurture” debate, scientists were stunned to find that: genetic inheritance sometimes does more to shape modern seagrass communities than the current environment.

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

Seagrasses in hot water

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

The legacy of ancient ice ages determines how seagrasses respond to environmental threats today

Smithsonian biologist Emmett Duffy on a research boat in Bocas Del Toro, Panama. Credit: Sean Mattson/Smithsonian Tropical Research Institute

But in most places where it grows, eelgrass is the dominant or only eelgrass species present. That makes its survival critical to the people and animals that live there. And the lower genetic diversity in the Atlantic can 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 at the University of California, Davis. “And if all you have is a hammer, you can put nails in it, but that’s about it. But if you have a full set of tools, each tool can be used to do different tasks more efficiently.”

Ecologists have already seen seagrass disappear in some regions as the water warms. In Portugal, the southernmost spot in Europe, seagrass is beginning to retreat and move further north to cooler waters.

“I don’t think we’re going to lose” [eelgrass] in the sense of extinction,” says co-author Jeanine Olsen, professor emeritus at the University of Groningen in the Netherlands. “It won’t be like that. It has a lot of tricks up its sleeve.” But local extinctions, she said, will happen in some places. That could put regions that rely on their local seagrass in trouble.

Achieving a more ZEN worldview

Duffy and his colleagues realized the urgent need to understand and conserve seagrass worldwide and formed a global network called ZEN. The name stands for Zostera Experimental Network, a nod to the scientific name of seagrass, Zostera marina. The idea was to bring seagrass scientists around the world together, by conducting the same experiments and studies, to get a coordinated global picture of seagrass health.

The legacy of ancient ice ages determines how seagrasses respond to environmental threats today

Emeritus professor Jeanine Olsen of the University of Groningen, on an expedition at a ZEN location near Bodø, Norway. ZEN (Zostera Experimental Network) tracks the health of seagrass at sites in the Northern Hemisphere. Credit: Wytze Stam

For the new study, the team studied seagrass communities at 50 sites in the Atlantic and Pacific Oceans. With 20 sampled plots per location, the team came up with data from 1,000 seagrass plots.

First, they collected basic seagrass data: size, shape, total biomass, and the different animals and algae that live on and around them. They then collected genetic data from all seagrass populations. They also measured different environmental variables at each location: temperature, water salinity, and water nutrient availabilityto name a few.

Ultimately, they hoped to discover what more shaped seagrass communities: the environment or the genetics?

After running a series of models, they discovered a large number of differences between the seagrass ecosystems in the Atlantic and Pacific Oceans — differences closely related to the genetic divergence of the Pleistocene migration and subsequent ice ages.

While Pacific eelgrass often grew in “forests” that were regularly more than 3 feet tall and sometimes more than twice as high, the Atlantic was home to more small “meadows” that rarely came close to that height. The genetic differences are also consistent with the total biomass of seagrass. In the Atlantic, evolutionary genetics and the current environment played an equally strong role in seagrass biomass. In the Pacific, genetics prevailed.

These effects also spread to other parts of the ecosystem. When it came to small animals living in the seagrass, such as invertebrates, the Pleistocene genetic signature again played a stronger role than the Pacific environment — while the two played an equally strong role in the Atlantic.

“The ancient legacy of this Pleistocene migration and bottleneck of seagrass in the Atlantic Ocean has had an impact on the structure of the ecosystem 10,000 years later,” Duffy said. “Probably over 10,000.”

The legacy of ancient ice ages determines how seagrasses respond to environmental threats today

UC Davis biologist Jay Stachowicz (left) and postdoc Deanna Beatty on a sampling expedition in Tomales Bay, California. Credit: Liz Allen

Preserving the future

That ancient genetics can play such a strong role — sometimes stronger than the environment — worries some ecologists about whether seagrass can adapt to faster changes.

“Climate warming — by itself — is probably not the main threat to seagrass,” Olsen said. Pollution from cities and farms, which can cloud waters and lead to harmful algal blooms, also endangers seagrasses. That said, the sheer number of environments seagrass can survive in is testament to its hardiness.

“I’m hopeful because our results illustrate the long-term resilience to repeated, large changes in thermal tolerances and the wide range of seagrass habitats in about half of the Northern Hemisphere,” Olsen said. “With the genomic resources now available for seagrass, we are starting to analyze functional changes in genes and their regulation in real time. It’s very exciting.”

To protect existing seagrass beds, maintaining the current diversity is a good first step. In places where seagrass beds have already been lost, recovery offers some promise. Some success stories already exist, such as on the eastern shore of Virginia. But many restoration efforts have had only limited success. As Stachowicz noted, this raises additional questions.

“Should you restore seagrass using plants from local environments, or should you think about the future and try plants with genetics better suited to future generations? environmental conditionshe asked. “Or should you hedge your bets?” Preserving or enhancing genetic diversity could be the best way to provide seagrass populations with the diverse toolkit needed to survive in an uncertain future.

Global warming causes northward shift in southern boundary of seagrass Zostera marina

More information:
A Pleistocene legacy structures variation in modern seagrass ecosystems, Proceedings of the National Academy of Sciences (2022). DOI: 10.1073/pnas.2121425119

Quote: Legacy of ancient ice ages determines how seagrasses respond to environmental threats today (2022, August 1), retrieved August 1, 2022 from

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