Madrid. Seagrasses release large amounts of sugar into their soils, mostly in the form of sucrose: more than a million tons, enough for 32 billion cans of soda.
Such high concentrations are surprising. Microorganisms generally quickly consume any free sugar in their environment. Scientists discovered that seagrasses excrete phenolic compounds, and these deter most of these beings from breaking down sucrose, ensuring that it stays buried under the seagrass beds and is not converted to carbon dioxide and returned to the ocean and the atmosphere.
Seagrasses form lush green meadows in many coastal areas of the world. They are one of the most efficient global sinks of carbon dioxide on Earth: a square kilometer of them stores almost twice as much carbon as forests on land, and 35 times faster.
Now scientists at the Max Planck Institute for Marine Microbiology have found that these grasses release massive amounts of sugar into their soils, the so-called rhizosphere. The sugar concentrations below them were at least 80 times higher than those measured in other marine environments. “To put this in perspective: we estimate that in the world there are between 0.6 and 1.3 million tons of sugar, mainly in the form of sucrose, in the rhizosphere of seagrasses,” explained Manuel Liebeke, head of the Metabolic Interactions Research Group of the Max Planck Institute for Marine Microbiology. “That’s roughly comparable to the amount of sugar in 32 billion cans of Coca-Cola.” Microbes love sugar: it’s easy to digest and full of energy. So why is sucrose not consumed by the large community of microorganisms in the rhizosphere? “We spent a lot of time trying to figure this out,” said Maggie Sogin, who led the research on the Italian island of Elba and at the Max Planck Institute for Marine Microbiology. “We found that seagrass, like many other plants, releases phenolic compounds into its sediments.”
Red wine, coffee, and fruit are full of phenolic compounds, and many people take them as health supplements.
What is less known is that phenols are antimicrobial and inhibit the metabolism of most microorganisms. “In our experiments we added phenols isolated from seagrasses to microorganisms in the rhizosphere and much less sucrose was consumed than when no phenols were present.”
Why do seagrasses produce such large amounts of sugars, only to discharge them into the rhizosphere? Nicole Dubilier, director of the Max Planck Institute for Marine Microbiology, noted in a statement: “They produce sugar during photosynthesis. Under medium light conditions, these plants use most of these sugars for metabolism and growth. However, in high light conditions, for example at noon or during the summer, they make more than they can use or store and release the excess in the rhizosphere”.
Interestingly, a small group of microbial specialists can thrive on sucrose despite challenging conditions.
Sogin speculates that these specialists are not only capable of digesting sucrose and degrading phenolic compounds, but may also provide benefits to the grass by producing nutrients it needs to grow, such as nitrogen. “Such beneficial relationships between plants and microorganisms in the rhizosphere are well known in terrestrial plants, but we are only beginning to understand the intimate and intricate interactions of seagrasses with microorganisms in the rhizosphere,” he added.
Seagrass beds are among the most threatened habitats. “Our calculations show that if sucrose in the rhizosphere were degraded by microbes, at least 1.54 million tons of carbon dioxide would be released into the atmosphere worldwide,” Liebeke said. “That’s roughly equivalent to the amount of carbon dioxide emitted by 330,000 cars in a year.”
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