Foundational Species: How Eelgrass Spread Around the World

Eelgrass Seagrass Illustration

Scientists traced the colonization history of eelgrass using nuclear and chloroplast genomes from 200 individuals across 16 locations. They found that eelgrass, a key species for carbon storage and biodiversity in the ocean, crossed the Pacific in two colonization events and reached the Atlantic about 243 thousand years ago.

An international research team led by GEOMAR reconstructs the worldwide colonization history of the most widespread marine plant.

Seagrasses evolved from freshwater plants and use sunlight and carbon dioxide (CO2) for photosynthesis. They are able to thrive in depths down to 50 meters (165 feet). In contrast to algae, they possess roots and rhizomes that grow in sandy to muddy sediments. The grass-like, leaf-shoots produce flowers and complete their life cycle entirely underwater. Seeds are negatively buoyant but seed-bearing shoots can raft, thus greatly enhancing dispersal distances at oceanic scale.

As foundational species, eelgrass provides critical shallow-water habitats for diverse biotas and contributes numerous ecosystem services, including carbon uptake. Seagrasses have recently been recognized as one of the crucial nature-based contributions to oceanic carbon storage. The sediment below seagrass meadows can sequester between 30 and 50 times more carbon annually than the roots of terrestrial forests. However, the ongoing loss of seagrass beds worldwide – including eelgrass – is a serious concern.

Understanding the Evolution of Eelgrass

An international group of researchers coordinated by Professor Thorsten Reusch, Head of the Research Division Marine Ecology at GEOMAR Helmholtz Centre for Ocean Research Kiel, used complete nuclear and chloroplast genomes from 200 individuals and 16 locations to reconstruct and date the colonization history of the eelgrass Zostera marina from its origin in the Northwest Pacific Ocean to the Pacific, Atlantic and the Mediterranean. The findings described in a peer-reviewed publication and a Research Briefing published on July 20 in the scientific journal Nature Plants beg the question, “How well will eelgrass adapt to our new, rapidly changing climate?”

Using a phylogenomic approach the scientists were able to determine that eelgrass plants first crossed the Pacific from west to east in at least two colonization events, probably supported by the North Pacific Current. The scientists then applied two DNA “molecular clocks” – one based on the nuclear genome and one based on the chloroplast genome – to deduce the time when eelgrass populations diverged into new ones. The DNA mutation rate was calculated and calibrated against an ancient, whole genome duplication that occurred in eelgrass.

Tracing Eelgrass Migration Patterns

Both nuclear and chloroplast genomes revealed that eelgrass dispersed to the Atlantic through the Canadian Arctic about 243 thousand years ago. This arrival is far more recent than expected – thousands of years versus millions of years, as is the case with most Atlantic immigrant species during the Great Arctic Exchange some 3.5 million years ago.

Reusch explains: “We thus have to assume that there were no eelgrass-based ecosystems – hotspots of biodiversity and carbon storage – in the Atlantic before that time. Recency was also mirrored in an analysis of the associated faunal community, which features many fewer specialized animals in the Atlantic as compared to the Pacific eelgrass meadows. This suggests that there was less time for animal-plant co-evolution to occur,” said Reusch.

Mediterranean populations were founded from the Atlantic about 44 thousand years ago and survived the Last Glacial Maximum. By contrast, today’s populations found along the western and eastern Atlantic shores only (re)expanded from refugia after the Last Glacial Maximum, about 19 thousand years ago – and mainly from the American east coast with help from the Gulf Stream.

Genomic Diversity and Future Concerns

Furthermore, the researchers confirmed significant differences in genomic diversity between the Pacific and Atlantic eelgrass populations, including latitudinal gradients of reduced genetic diversity in northern populations.

“Both Atlantic compared to Pacific populations, and northern versus southern ones are less diverse on a genetic level than their ancestors by a factor of 35 among the most and least diverse one,” summarized postdoctoral scientist Dr. Lei Yu, first author of the publication which was a chapter in his doctoral thesis. “This is due to bottlenecks arising from past ice ages, which raises concerns as to how well Atlantic eelgrass, will be able to adapt to climate change and other environmental stressors based on its genetic capacity.”

“Warming oceans have already caused losses of seagrass meadows at the southern range limits, in particular North Carolina and southern Portugal. In addition, heat waves have also caused losses in shallow waters in some the northern parts of the distribution,” noted Reusch. “This is not good news because seagrass meadows form diverse and productive ecosystems, and no other species is able to take on the role of eelgrass if meadows cannot persist under future conditions.”

Potential Solutions and Future Research

“One possibility for restoration might be to borrow some genetic diversity from Pacific eelgrass to fortify diversity in the Atlantic. Our next step is to interrogate the eelgrass pangenome. A new reference genome from Pacific eelgrass is currently under development and should tell us more about the adaptive ecotypic capacity across its global range of habitats,” said Prof. Jeanine Olsen, emeritus professor from the University of Groningen who initiated the study and coordinated the work between the Joint Genome Institute (JGI) and the research team. Thus, the verdict on rapid adaptation is out but there’s reason for optimism.

Reference: “Ocean current patterns drive the worldwide colonization of eelgrass (Zostera marina)” by Lei Yu, Marina Khachaturyan, Michael Matschiner, Adam Healey, Diane Bauer, Brenda Cameron, Mathieu Cusson, J. Emmett Duffy, F. Joel Fodrie, Diana Gill, Jane Grimwood, Masakazu Hori, Kevin Hovel, A. Randall Hughes, Marlene Jahnke, Jerry Jenkins, Keykhosrow Keymanesh, Claudia Kruschel, Sujan Mamidi, Damian M. Menning, Per-Olav Moksnes, Masahiro Nakaoka, Christa Pennacchio, Katrin Reiss, Francesca Rossi, Jennifer L. Ruesink, Stewart T. Schultz, Sandra Talbot, Richard Unsworth, David H. Ward, Tal Dagan, Jeremy Schmutz, Jonathan A. Eisen, John J. Stachowicz, Yves Van De Peer, Jeanine L. Olsen and Thorsten B. H. Reusch, 20 July 2023, Nature Plants.
DOI: 10.1038/s41477-023-01464-3

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