A natural CO2-sink thanks to symbiotic bacteria
Like many land plants, seagrasses live in symbiosis with nitrogen-fixing bacteria

Date:
November 3, 2021
Source:
Max Planck Institute for Marine Microbiology
Summary:
Seagrasses cover large swathes of shallow coastal seas, where
they provide a vital habitat. They also remove large amounts
of carbon dioxide (CO2) from the atmosphere and store it in
the ecosystem. However, seagrasses need nutrients to thrive,
particularly nitrogen. Up to now, researchers have assumed that the
plants take up the nitrogen primarily from the surrounding seawater
and sediment. However, in many of the regions where seagrasses are
most successful there is little nitrogen to be found. Researchers
now show that seagrass in the Mediterranean Sea lives in symbiosis
with bacteria that reside in their roots and provide the nitrogen
necessary for growth. Such symbioses were previously only known
from land plants.



FULL STORY ========================================================================== Seagrasses are widespread in shallow coastal regions of both temperate
and tropical seas, covering up to 600,000 square kilometers, which is
roughly the area of France. They form the basis of the entire ecosystem,
which is home to numerous organisms, some of them endangered species
such as turtles, seahorses and manatees, and nursery ground for many economically important fish species.

Moreover, seagrasses protect coastlines from erosion by storm surges and sequester millions of tons of carbon dioxide every year, which is stored
in the ecosystem as so-called "blue carbon" for long periods of time.


==========================================================================
Lush life despite a lack of nutrients The habitat of many seagrasses
is poor in nutrients, such as nitrogen, for much of the year. Although
nitrogen is abundant in the sea in its elemental form (N2), seagrasses
cannot use it in this form. How can the plants still thrive? It is thanks
to their now discovered microscopic partners: Bacterial symbionts living
within the plants roots that convert N2 gas into a form that the plants
can use. Wiebke Mohr and her colleagues from the Max Planck Institute for Marine Microbiology in Bremen, Germany, Hydra Marine Sciences in Bu"hl, Germany, and the Swiss Water Research Institute Eawag now describe how
this intimate relationship between seagrass and bacteria works.

Harmony in the roots "It was assumed that the so-called fixed-nitrogen
for the seagrasses comes from bacteria that live aroundtheir roots in
the seafloor," Mohr explains. "We now show that the relationship is
much closer: The bacteria live inside the roots of the seagrass. This
is the first time that such an intimate symbiosis has been shown in
seagrasses. It was previously only known from land plants, especially agriculturally important species such as legumes, wheat and sugar
cane." These, too, have symbiotic bacteria, to which they supply
carbohydrates and other nutrients in return for fixed nitrogen. A very
similar exchange of metabolic products also occurs between the seagrass
and its symbiont.

The bacteria that live in the seagrass roots are a new discovery. Mohr
and her team named them Celerinatantimonas neptuna, after their host,
the neptune grass (Posidonia). Relatives of C. neptunahave previously
been found in association with seaweeds. "When the seagrasses moved
from land to sea about 100 million years ago, they probably adopted the bacteria from the seaweeds," Mohr speculates. "They virtually copied the
system that was highly successful on land and then, in order to survive
in the nutrient-poor seawater, acquired a marine symbiont." The current
study looked at seagrasses of the genus Posidoniain the Mediterranean
Sea. However, such symbioses may also occur elsewhere. "Genetic analyses suggest that similar symbioses also exist on tropical seagrasses and
in salt marshes," says Mohr. "This way, these flowering plants manage
to colonize a wide variety of seemingly nutrient-poor habitats, both in
the water and on land." Going with the seasons As the seasons change,
the amount of nutrients present in coastal water varies.

In winter and spring, the nutrients present in the water and sediment
seem sufficient for the seagrasses. "At that time, we do find scattered symbionts in the roots of the plants, but they are probably not very
active," says Mohr. In summer, when sunlight increases and more and more
algae grow and consume the few available nutrients, nitrogen quickly
becomes scarce. Then the symbionts take over. They directly supply the seagrasses with the nitrogen they need.

This is how seagrasses can reach their largest growth in summer, when
nutrients are most scarce in the environment.

Many different methods for a clear picture The present study bridges
the entire ecosystem, from seagrass productivity to the symbionts that
live in their roots and ultimately fuel the system. To accomplish
this, the researchers used a variety of different methods to
understand the symbiosis as fully as possible: Oxygen measurements
carried out in the waters of the Mediterranean Sea revealed the
productivity of the seagrass meadow. Microscopy techniques, in which
individual bacterial species are color-labeled (known as FISH),
helped to visualize the bacteria in and between the root cells of
the seagrass. In the NanoSIMS, a state-of-the-art mass spectrometer,
they showed the activity of the individual bacteria. Genomic and
transcriptomic analyses revealed which genes are probably particularly important for the interaction and that these pathways are heavily
used. As a result, the researchers succeeded in providing a sound
and detailed description of this amazing collaboration. "Our next
step is to study these new bacteria in more detail," says Mohr. "We
want to isolate them in the laboratory to further investigate how
the symbiosis works and how it developed. It will certainly also be
exciting to search for comparable systems in other regions and habitats." ========================================================================== Story Source: Materials provided by Max_Planck_Institute_for_Marine_Microbiology. Note: Content may be edited
for style and length.


========================================================================== Journal Reference:
1. Wiebke Mohr, Nadine Lehnen, Soeren Ahmerkamp, Hannah K. Marchant,
Jon S.

Graf, Bernhard Tschitschko, Pelin Yilmaz, Sten Littmann, Harald
Gruber- Vodicka, Nikolaus Leisch, Miriam Weber, Christian Lott,
Carsten J.

Schubert, Jana Milucka, Marcel M. M. Kuypers. Terrestrial-type
nitrogen- fixing symbiosis between seagrass and a marine
bacterium. Nature, 2021; DOI: 10.1038/s41586-021-04063-4 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/11/211103140110.htm

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