Sweet spots in the sea: Mountains of sugar under seagrass meadows

Date:
May 2, 2022
Source:
Max Planck Institute for Marine Microbiology
Summary:
Seagrasses play an important role in our climate. They are one
of the most efficient sinks of carbon dioxide on Earth. A team
of scientists now reports that seagrasses release large amounts
of sugar, largely in the form of sucrose, into their soils --
worldwide more than 1 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.



FULL STORY ========================================================================== Seagrasses play an important role in our climate. They are one of the
most efficient sinks of carbon dioxide on Earth. A team of scientists
from the Max Planck Institute for Marine Microbiology now reports
that seagrasses release large amounts of sugar, largely in the form
of sucrose, into their soils - - worldwide more than 1 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. They now describe their discovery in the
journal Nature Ecology & Evolution.


========================================================================== 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." Some specialists thrive on
sugars in the seagrass rhizosphere Why do seagrasses produce such large
amounts of sugars, to then only dump them into their rhizosphere? Nicole Dubilier, Director at 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 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. Think of it as an overflow valve." Intriguingly,
a small set of microbial specialists are able to thrive on the sucrose
despite the challenging conditions. Sogin speculates that these sucrose specialists are not only able to digest sucrose and degrade phenolics, but might provide benefits for the seagrass by producing nutrients it needs
to grow, such as nitrogen. "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 adds.

Endangered and critical habitats Seagrass meadows are among the most
threatened habitats on our planet. "Looking at how much blue carbon --
that is carbon captured by the world's ocean and coastal ecosystems --
is lost when seagrass communities are decimated, our research clearly
shows: It is not only the seagrass itself, but also the large amounts
of sucrose underneath live seagrasses that would result in a loss
of stored carbon. Our calculations show that if the sucrose in the
seagrass rhizosphere was degraded by microbes, at least 1,54 million
tons of carbon dioxide would be released into the atmosphere worldwide,"
says Liebeke. "That's roughly equivalent to the amount of carbon dioxide emitted by 330,000 cars in a year." Seagrasses are rapidly declining in
all oceans, and annual losses are estimated to be as high as 7% at some
sites, comparable to the loss of coral reefs and tropical rainforests. Up
to a third of the world's seagrass might have been already lost. "We do
not know as much about seagrass as we do about land-based habitats,"
Sogin emphasizes. "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."

========================================================================== Story Source: Materials provided by Max_Planck_Institute_for_Marine_Microbiology. Note: Content may be edited
for style and length.


========================================================================== Journal Reference:
1. Sogin, E.M., Michellod, D., Gruber-Vodicka, H.R. et al. Sugars
dominate
the seagrass rhizosphere. Nat Ecol Evol, 2022 DOI:
10.1038/s41559-022- 01740-z ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/05/220502120422.htm

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