Sweet spots in the sea

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Lush meadows of the seagrass  Posidonia oceanica  in the Mediterranean. Scientis

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

Scientists from the Max Planck Institute for Marine Microbiology now report that seagrasses release large amounts of sugar, largely in the form of sucrose, into their soils - worldwide more than one million tons of sucrose, enough for 32 billion cans of coke. Such high concentrations of sugar are surprising. Normally, microorganisms quickly consume any free sugars in their environment. The scientists found that seagrasses excrete phenolic compounds, and these deter most microorganisms from degrading the sucrose. This ensures that the sucrose remains buried underneath the meadows and cannot be converted into CO2 and returned to the ocean and atmosphere.
 

Seagrasses form lush green meadows in many coastal areas around the world. These marine plants are one of the most efficient global sinks of carbon dioxide on Earth: One square kilometer of seagrass stores almost twice as much carbon as forests on land, and 35 times as fast.

Now scientists from the Max Planck Institute for Marine Microbiology in Bremen, Germany, have discovered that seagrasses release massive amounts of sugar into their soils, the so-called rhizosphere. Sugar concentrations underneath the seagrass were at least 80 times higher than previously measured in marine environments. "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", explains Manuel Liebeke, head of the Research Group Metabolic Interactions at the Max Planck Institute for Marine Microbiology. "That is roughly comparable to the amount of sugar in 32 billion cans of coke!"

Polyphenols keep microbes from eating the sugar

Microbes love sugar: It is easy to digest and full of energy. So why isn't the sucrose consumed by the large community of microorganisms in the seagrass rhizosphere? "We spent a long 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 realized is that seagrass, like many other plants, release phenolic compounds to their sediments."

Red wine, coffee and fruits are full of phenolics, and many people take them as health supplements. What is less well known is that phenolics are antimicrobials and inhibit the metabolism of most microorganisms. "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."

E. Maggie Sogin, Dolma Michellod, Harald Gruber-Vodicka, Patric Bourceau, Benedikt Geier, Dimitri V. Meier, Michael Seidel, Soeren Ahmerkamp, Sina Schorn, Grace D’Angelo, Gabriele Procaccini, Nicole Dubilier, Manuel Liebeke

  • Max Planck Institute for Marine Microbiology, Bremen, Germany 
  • University of California at Merced, CA, USA
  • University of Vienna, Vienna, Austria
  • University of Oldenburg, Oldenburg, Germany 
  • Stazione Zoologica Anton Dohrn Napoli, Naples, Italy 


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