Seagrass meadows store much more carbon than previously realised

The research, published in Nature Ecology & Evolution, reveals that seagrass ecosystems store a surprising amount of carbon in the form of sucrose.

Seagrasses form lush underwater "meadows" and act as efficient carbon sinks: they can store nearly twice as much carbon, and 35 times quicker, as the same area of land forest.

A team of marine microbiologists has now investigated how that carbon is stored and cycled through the seagrass ecosystem, with some surprising results.

Firstly, the researchers found that seagrass ecosystems store much more carbon in the form of the sugar sucrose than previously suspected. The sucrose is mostly found in the rhizosphere - the environment around the seagrass roots.

The study found that sugar concentrations in the seagrass rhizosphere were about 80 times higher than previous marine records.

"To put this into perspective, we estimate that worldwide there are between 0.6 and 1.3 million tons of sugar, mainly in the form of sucrose, in the seagrass rhizosphere," says senior author Manuel Liebeke, head of the metabolic interactions research group at the Max Planck Institute for Marine Microbiology in Germany, and senior author on the paper.

"That is roughly comparable to the amount of sugar in 32 billion cans of Coke!"

The researchers believe that these high sucrose levels are the "overflow" of seagrass photosynthesis - the process by which plants convert sunlight energy and carbon into sugars and oxygen.

"Under average light conditions, these plants use most of the sugars they produce for their own metabolism and growth," explains Nicole Dubilier, director of the Max Planck Institute for Marine Microbiology.

"But under high light conditions, for example at midday or during the summer, the plants produce more sugar than they can use or store. Then they release the excess sucrose into their rhizosphere."

The rhizosphere is also full of marine microbes - but unexpectedly, most of them don't feast on the easy-to-digest sugars.

"What we realised is that seagrass, like many other plants, release phenolic compounds to their sediments," explains first author Maggie Sogin.

The phenolic compounds have antimicrobial properties, which likely keep most microbes at bay around the seagrass roots.

The researchers tested this idea in a lab.

"In our experiments we added phenolics isolated from seagrass to the microorganisms in the seagrass rhizosphere - and indeed, much less sucrose was consumed compared to when no phenolics were present," says Sogin.

However, there were a few "sucrose specialist" microbes that could thrive in the seagrass rhizosphere - possibly by degrading the phenolics. Sogin thinks the microbes may exist in a mutually beneficial relationship with the seagrass, perhaps by producing nutrients that the plants can use.

"Such beneficial relationships between plants and rhizosphere microorganisms are well known in land plants, but we are only just beginning to understand the intimate and intricate interactions of seagrasses with microorganisms in the marine rhizosphere," she says.

It's actually good news for the planet that microbes don't consume most of the sucrose produced by seagrasses.

The team estimated that if the amount of sucrose present in the seagrass rhizosphere were metabolised by microbes, at least 1.54 million tons of carbon dioxide would be released into the atmosphere.

"That's roughly equivalent to the amount of carbon dioxide emitted by 330,000 cars in a year," says Liebeke.

While "blue carbon" - carbon stored in the ocean - is a hot topic, researchers had not previously appreciated the role of carbon stored in the seagrass rhizosphere as well as in the plants themselves. Unfortunately, just like coral reefs and rainforests, seagrass ecosystems are under threat: it's been estimated that up to a third of the world's seagrass population already may have been lost.

Sogin concludes: "Our study contributes to our understanding of one of the most critical coastal habitats on our planet, and highlights how important it is to preserve these blue carbon ecosystems."