Newly Discovered Symbiosis Could Power Carbon Removal in The Sea

The lungs of the planet are tropical rainforests, which absorb large amounts of carbon dioxide from our atmosphere and exhale oxygen. However, our oceans have even larger lungs.
Seagrass meadows, which are large areas of aquatic plants that can capture carbon 35 times faster than tropical forests, are huge. These coastal ecosystems, along with tundra, are the largest carbon sinks in the world, but we don't know much about their high productivity.

New research suggests that these marine meadows wouldn't be as effective at capturing carbon if there wasn't a hidden hero within the grass.

Researchers in the Mediterranean discovered a new bacteria that can convert nitrogen gas into a nutrient marine plant can use.

This is strikingly similar to the way that plants on land absorb nitrogen, but this type of symbiotic relationship was never found in marine plants.

Although the study was limited to a particular species of Mediterranean Sea seagrass, C. neptuna relatives are found all over the world, so the authors believe similar relationships may be occurring elsewhere.

"It was assumed the so-called fixed nitrogen for seagrasses is from bacteria that lives around their roots in seafloor," says Wiebke Mohr, a marine microbiologist from the Max Planck Institute Bremen, Germany.

"We now demonstrate that the relationship between bacteria and seagrasses is closer. The bacteria lives inside the seagrass roots. This is the first time such intimate symbiosis was demonstrated in seagrasses.

The novel bacteria was identified by microscopic methods. Different bacterial species are highlighted with different color labels to highlight their presence.

The plant was able store eight times more carbon dioxide than the sandy sediment that is scattered around seagrass. This was true even though there were no nitrogen nutrients detected in the water column.

These findings indicate that something else is happening. The marine plant was able to fix nitrogen gas and convert it. The team discovered that the microbiome in the roots of the plant was different from the one found in the sediment.

This difference was attributed to a single bacterium that the authors discovered in seagrass roots. It is found in large quantities in summertime when nitrogen is scarcest. These roots seem to quickly spread the nitrogen once they are obtained.

The authors state that the transfer was quick, with as much as 20 percent of newly fixed nitrogen being assimilated in leaf biomass within 24 hours.

The team believes that the seagrass provides sugars to its bacteria, a relationship that we may have overlooked previously and which could be ancient.

The seagrasses were thought to have originated from flowers that had been reintroduced into the sea around 100 million years ago.

The authors suspect that the marine version of the nitrogen-fixing root microbiomes they had previously established on land may have been lost during this transition.

Mohr speculates that they "practically copied the system that was very successful on land, and then, to survive in the nutrient poor seawater, acquired marine symbionts."

It is not clear where C. neptuna came from. Its genetic characteristics suggest that its ancestors were from the coast environment. This is where it was most likely to have been associated with seaweed which doesn't need roots.

However, the closest relatives are found in the roots of tropical seagrasses and saltmarshgrasses. There, the bacteria forms a similar symbiotic relationship to seaweed, although this is still not confirmed.

The authors say that, just like microorganisms [nitrogen]-fixing might have helped the colonization soils by early land plant colonies, the ancestors and relatives of C. neptuna probably allowed flowering plants to invade marine habitats with low nitrogen, where they created extremely efficient blue carbon ecosystems."

Now, the authors plan to study the new root bacteria in a lab to understand nitrogen-fixing in greater detail. Mohr is eager to find out if other seagrass systems in the world depend on similar relationships.

It's time to learn how seagrass ecosystems work, given how important they are in the current climate crisis.

This is especially true when we are destroying seagrass meadows faster than tropical rainforests.

Nature published the study.