Eel grass is not a seaweed but a flowering plant that migrated to the sea, say scientists who have now mapped the eel grass genome. The study also shows that eel grass ( Zostera marina) is crucial in absorbing carbon dioxide in the soft sediments of the coasts.
Eel grasses form a carbon dioxide sink: “they store more carbon than tropical forests,” says Jeanine Olsen of the University of Groningen in the Netherlands who led the study.
Coastal sea grass ecosystems cover some 200,000 square kilometers, the study says. Those ecosystems account for an estimated 15 per cent of carbon fixed in global ocean, and also impact sulphur and nitrogen cycles.
The scientists argue that since sea grasses are the only flowering plants to have returned to the sea that is the most extreme adaptation a terrestrial (or even freshwater) species can undergo.
The science team says the Zostera marina genome is “an exceptional resource that supports a wide range of research themes, from the adaptation of marine ecosystems under climate warming and its role in carbon burial to unraveling the mechanisms of salinity tolerance that may further inform the assisted breeding of crop plants.”
Sea grasses form the backbone of one of the most productive and biodiverse ecosystems on Earth, rivaling coral reefs and rain forests in terms of the ecosystem services they provide to humans.
Sea grass meadows are part of the soft-sediment coastal ecosystems found in all continents, with the exception of Antarctica. They not only form a nursery for young fish and other organisms, but also protect the coastline from erosion and maintain water clarity. ‘
The study, which sequenced the genome of the eel grass taken from the Archipelago Sea off Finland. published today, in the journal Nature, is the work of an international consortium of 35 labs, most of them in Europe, working with researchers from the U.S. Department of Energy Joint Genome Institute.
The study showed that eel grasses are completely submerged marine flowering plants, called by science angiosperms. It shows that eel grass is a member of the ancient monocot family.
The monocots include about 60,000 species, flowering plants that first appear above the soil as a single leaf. They include orchids, “true grasses,” as well as rice, wheat, maize and “forage grasses” such as sugar cane, and the bamboos. According to Wikipedia, other economically important monocot crops include palms bananas , gingers, onions, garlic, lilies, daffodils, irises, amaryllis, bluebells and tulips.
Zostera marina is the first marine flowering plant have its genome fully sequenced. As well as finding the eel grass’s genetic ancestors the researchers were interested in understanding how the plant–and by extension other plants in the ecosystem–adapt to climate change.
As it adapted to an underwater, coastal lifestyle, eel grass gained genes that allowed it to live in saltwater but lost genes involved in traits associated with land-based plants.
Olsen called this “arguably the most extreme adaptation a terrestrial (and even a freshwater) species can undergo.”
What she describes as the “use it, lose it, or change it” scenario, eelgrass modified its cell walls. The eel grass cell wall is very different from normal plant cell walls and more like that of sea algae, similar to the cell in seaweeds. The eel grass has lost genes associated with light-sensing, pollination and regulation of internal water balance.
Eel grass lost its stomata (which are used by land plants to ‘breathe’) but also all of the genes involved in stomatal differentiation. “The genes have just gone, so there’s no way back to land for sea grass,” Olsen says. Sex is entirely underwater involving long naked sperm filaments especially adapted for underwater fertilization of the tiny flowers.
The team compared the eel grass genome to duck weed, one of the simplest flowering plants and Zostera marina’s closest sequenced relative. They noted differences in genes related to cell wall structure due to adaptations to freshwater or terrestrial conditions. For example, plants such as duckweed have seemingly lost genes that help plants retain water in the cell wall, while eel grass has regained these genes to better deal with osmotic stress at low tide.
“They have re-engineered themselves,” said Olsen of the changes affecting the eelgrass cell walls. “Crop breeders may benefit from lessons on how salt tolerance has evolved in these plants.”
With Zostera marina meadows stretching from Alaska to Baja California, and from the White Sea to southern Portugal, Olsen noted that these ecosystems afford researchers “a natural experiment to investigate rapid adaptation to warmer or colder waters, as well as to salinity tolerance, ocean acidification and light.”
Eel grass endangered
Jeremy Schmutz, head of the US Department of Energy’s genetic plant program, emphasized that while eel grasses are key players in coastal marine ecosystem functions and considered the “lungs of the sea,” they are also endangered. “There are estimates that nearly a third of the eel grass meadows worldwide have been destroyed by runoff into the ocean,” he said, “reducing their potential capabilities as carbon sinks. Thus, studying the adaptive capacity of eel grass is urgent to assist conservation efforts.”
An overarching question for Olsen’s team is how quickly eel grass can adapt to rapid climate change. The fact that Zostera marina grows along the coastline from Portugal to Scandinavia is being used as a natural experiment to investigate adaptation to warmer or colder water, as well as to salinity, ocean acidification and light.