Fold mountains

Mountains of sugar under the seagrass

Lush meadows of the Posidonia oceanica seagrass in the Mediterranean. Scientists at the Max Planck Institute for Marine Microbiology predict that their findings are relevant to many marine environments with plants, including other seagrass species, mangroves and salt marshes. Credit: HYDRA Marine Sciences GmbH

Seagrasses play an important role in the climate. They are one of the most efficient carbon dioxide sinks on Earth. A team of scientists from the Max Planck Institute for Marine Microbiology now reports that seagrass beds release large amounts of sugar, mostly in the form of sucrose, into their soils – worldwide more than a million tons of sucrose, enough for 32 billion cans of coke. Such high concentrations of sugar are surprising. Normally, microorganisms quickly consume all the free sugars in their environment. Scientists have found that seagrasses secrete phenolic compounds, and these deter most microorganisms from degrading sucrose. This ensures that the sucrose stays buried under the grasslands and cannot be converted into CO2 and returned to the ocean and atmosphere. They now describe their discovery in the journal Nature ecology and evolution.

Seagrass beds form verdant grasslands in many coastal areas around the world. These seagrasses are one of the most efficient global sinks of carbon dioxide on Earth: a square kilometer of seagrass stores almost twice as much carbon as terrestrial forests, and can do so 35 times faster. Today, scientists from the Max Planck Institute for Marine Microbiology in Bremen, Germany, discovered that seagrasses release huge amounts of sugar into their soils, the so-called rhizosphere. Sugar concentrations under the seagrass beds were at least 80 times higher than those previously measured in marine environments.

“To put this into perspective: we estimate that there are between 0.6 and 1.3 million tonnes of sugar worldwide, mainly in the form of sucrose, in the rhizosphere of seagrass beds”, explains Manuel Liebeke, head of the research group on metabolic interactions at Max Planck. Institute of Marine Microbiology. “That’s roughly comparable to the amount of sugar in 32 billion cans of Coke.”

Polyphenols prevent microbes from eating sugar

Microbes love sugar. It is easy to digest and full of energy. So why is sucrose not consumed by the large community of micro-organisms in the rhizosphere of seagrass beds? “We spent a lot of time trying to figure this out,” says first author Maggie Sogin, who led the research off the Italian island of Elba and at the Max Planck Institute for Marine Microbiology. “What we’ve realized is that seagrasses, like many other plants, release phenolic compounds into their sediments. Red wine, coffee, and fruit are full of phenolic compounds, and many people take them. as dietary supplements.What is less well known is that phenolic compounds are antimicrobials and inhibit the metabolism of most microorganisms.

“In our experiments, we added phenolic compounds isolated from seagrass beds to microorganisms in the seagrass rhizosphere – and indeed, significantly less sucrose was consumed compared to no phenolic compounds.”

Beautiful to look at, hard to sample: It is difficult to measure metabolites like sucrose and polyphenols in seawater. Scientists at the Max Planck Institute for Marine Microbiology in Bremen had to develop a special method to deal with large amounts of salt in seawater that make measurements of metabolites so difficult. Credit: HYDRA Marine Sciences GmbH

Some specialists feed on sugars in the rhizosphere of seagrasses

Why do seagrasses produce such large quantities of sugars, then dump them into their rhizosphere? Nicole Dubilier, Director of the Max Planck Institute for Marine Microbiology explains: “Seagrasses produce sugar during photosynthesis. Under average light conditions, these plants use most of the sugars they produce for their own metabolism and growth. But in bright light conditions, such as midday or summer, plants produce more sugar than they can use or store. Then they release the excess sucrose into their rhizosphere. Think of it as an overflow valve.

Interestingly, a small group of microbial specialists are able to thrive on sucrose despite the harsh conditions. Sogin speculates that these sucrose specialists are not only able to digest sucrose and degrade phenolic compounds, but could also benefit seagrasses by producing the nutrients they need to thrive, such as nitrogen. “Such beneficial relationships between plants and rhizosphere microorganisms are well known in land plants, but we are only beginning to understand the intimate and complex interactions of seagrass beds with marine rhizosphere microorganisms,” she adds.

Sweet spots at sea: Sugar mountains under the seagrass beds

Manuel Liebeke and Nicole Dubilier in the lab. Credit: Achim Multhaupt

Threatened and critical habitats

Seagrasses are among the most threatened habitats on our planet. “By examining how much blue carbon, that is, the carbon captured by the world’s ocean and coastal ecosystems, that is lost when seagrass communities are decimated, our research clearly shows: it is not not just the seagrass itself, but also the large amounts of sucrose underlying the seagrass beds which would result in a loss of stored carbon.Our calculations show that if the sucrose in the rhizosphere of the seagrass beds were degraded by microbes, at least 1.54 million tonnes of carbon dioxide would be released into the global atmosphere,” says Liebeke. “This is roughly equivalent to the amount of carbon dioxide emitted by 330,000 cars in one year.”

Seagrass beds are rapidly declining in all oceans and annual losses are estimated at 7% at some sites, which is comparable to the loss of coral reefs and tropical rainforests. Up to a third of the world’s seagrass beds may already have been lost. “We don’t know as much about seagrasses as we do about terrestrial habitats,” Sogin points out. “Our study contributes to our understanding of one of our planet’s most critical coastal habitats and highlights how important it is to preserve these blue carbon ecosystems.”

Dead or Alive: Seagrasses Continue to Release Methane After They Die

More information:

Maggie Sogin, Sugars Dominate the Herbarium Rhizosphere, Nature ecology and evolution (2022). DOI: 10.1038/s41559-022-01740-z.

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Sweet Spots in the Sea: Sugar Mountains Under Seagrass (2022, May 2)
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