Deep evolution is casting a longer shadow than previously thought, scientists report in a new article published the week of August 1 in the Proceedings of the National Academy of Sciences. Smithsonian scientists and colleagues studied seagrass communities — the basis of many near-shore food webs along the North Atlantic and Pacific coasts — and discovered that their ancient genetic history may play a more powerful role than modern-day environments in determining their size, structure, and who dwells 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 home in the Pacific to the Atlantic. Not all plants were hardy enough to survive the journey through the Arctic. For those that were successful, a series of ice ages during the Pleistocene further affected how far they could spread. These millennia-old struggles left lasting imprints on their DNA: Even today, seagrass populations in the Atlantic are far less genetically diverse than those in the Pacific.
Still, in the classic “nature versus nurturing” debate, scientists were stunned to discover that genetic heritage sometimes shapes modern seagrass communities more than the current environment.
“We already knew there was a major genetic divide between the oceans, but I don’t think any of us ever dreamed that this was more important than environmental conditions,” said Emmett Duffy, a marine biologist at the Smithsonian Environmental Research Center and lead author of the report. “It was a big surprise for everyone.”
seaweed in hot water
Seagrass 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 aquatic animals, seagrass provides a wealth of services for humans. It protects coasts from storms, absorbs carbon and can even reduce harmful bacteria in the water.
But in most places where it grows, seagrass is the dominant — or only — species of seagrass present. That makes its survival vital to the people and animals that live there. And the lower genetic diversity in the Atlantic could make it harder 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 you’ve only got a hammer you can hammer in nails, but that’s about it. But when you have a full suite of tools, each tool allows you to do different tasks more efficiently.”
Ecologists have already observed that seagrass is disappearing from some regions as the water warms. In Portugal, its southernmost point in Europe, the seagrass has started to retreat and move farther north to cooler waters.
“I don’t think we’re going to lose [eelgrass] in terms of annihilation,” 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 pointed out, will occur in some places. That could put regions that depend on their local seagrass in trouble.
Achieving a more ZEN worldview
Recognizing the urgent need to understand and conserve seagrass worldwide, Duffy and his colleagues came together to form a global network called ZEN. The name stands for Zostera Experimental Network, a nod to the scientific name of seaweed, Marina Zostera. The idea was to bring together seagrass scientists around the world doing the same experiments and research to get a coordinated global picture of seagrass health.
For the new study, the team examined seagrass communities at 50 locations in the Atlantic and Pacific. With 20 plots sampled per site, the team received data from 1,000 seagrass plots.
First, they collected basic seagrass data: size, shape, total biomass, and the various animals and algae that live on and around them. Then they collected genetic data on all seagrass populations. They also measured several environmental variables at each site: temperature, water salinity, and nutrient availability, to name a few.
Ultimately, they hoped to find out what shaped seagrass communities more: the environment or genetics?
After running a series of models, they discovered a variety of differences between the Atlantic and Pacific seagrass ecosystems — differences that closely aligned with genetic divergence from the Pleistocene migration and subsequent ice ages.
While Pacific seagrasses often grew in “forests” that regularly exceeded 3 feet in height and sometimes more than doubled, the Atlantic hosted smaller “meadows” that rarely reached that height. The genetic differences were also consistent with the total biomass of the seagrass. In the Atlantic, evolutionary genetics and contemporary environment played equally strong roles in seagrass biomass. In the Pacific, genetics ruled.
These impacts flowed into other parts of the ecosystem as well. For small animals that lived in seagrass, such as invertebrates, the Pleistocene genetic signature played a stronger role than the Pacific environment—while both played just as strong a role in the Atlantic.
“The ancient legacy of this Pleistocene migration and bottleneck of seagrass into the Atlantic had consequences for ecosystem structure 10,000 years later,” Duffy said. “Probably more than 10,000.”
preserve the future
That ancient genetics can play such a powerful role — sometimes more so than the environment — has some ecologists concerned about whether seagrass can adapt to more rapid changes.
“Global warming – per se – is probably not the main threat to seagrass,” Olsen said. Pollution from cities and farms, which can cloud the water and lead to harmful algal blooms, also threatens seagrasses. However, the variety of environments that seagrass can survive in is a testament to its resilience.
“I’m hopeful because our results demonstrate long-term resilience to repeated, large changes in thermal tolerances and the wide range of seagrass habitats across about half of the northern hemisphere,” Olsen said. “With the seagrass genomic resources now available, we are starting to analyze functional changes in genes and their regulation in real time. That’s very exciting.”
To protect existing seagrass beds, maintaining current diversity is a good first step. In places where seagrass beds have already been lost, restoration offers some prospects. There are already some success stories, such as on the east coast of Virginia. However, many restoration efforts have met with limited success. As Stachowicz pointed out, this raises additional questions.
“Should you recreate seagrasses with plants from local environments, or should you think ahead and try plants with genetics better suited to future environmental conditions?” he asked. “Or should you be hedging your bets?” Maintaining or enhancing genetic diversity may be the best way to equip seagrass populations with the diverse toolkit they need to survive in an uncertain future.