Many coastal areas around the world are home to lush green grasslands, all thanks to seagrass beds.
As the only flowering plants growing in marine environments, these grasslands are magical: a square kilometer of seagrass stores almost twice as much carbon as terrestrial forests, and it does so 35 times faster. This makes seagrasses one of the most efficient global sinks of carbon dioxide on Earth.
And that’s not the only remarkable thing about them, a new study has revealed. Submerged beneath the waves, seagrass ecosystems hold colossal reserves of sugar we didn’t know existed before, with an estimated 32 billion cans of candy worth Coca-Cola hidden in the seabed.
Naturally, this has major implications for climate change mitigation and carbon storage.
Sweet, sweet seagrass
Scientists from the Max Planck Institute for Marine Microbiology in Bremen, Germany, reported in a study published in the journal Nature ecology and evolution that seagrasses release colossal amounts of sugar into their soils, also known as the rhizosphere. Beneath the seagrass, sugar concentrations were unexpectedly 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. Marine Microbiology Institute, in a press release. “That’s roughly comparable to the amount of sugar in 32 billion cans of Coke!”
This happens because sea grasses produce sugar during photosynthesis. Most of the sugar produced by these plants is used for their metabolism and growth in medium light conditions. However, under high light conditions, such as midday or summer, plants produce more sugar than they can store or use, and the excess sucrose is then released into the rhizosphere.
You might be wondering why sucrose is stored on the seabed rather than being consumed by the billions upon millions of microorganisms in the rhizosphere. After all, microbes love sugar because it’s easy to digest and full of energy. The researchers behind the study were also intrigued by this question.
“We spent a lot of time trying to figure this out,” says first author Maggie Sogin. “What we realized is that seagrasses, like many other plants, release phenolic compounds into their sediments.”
In case you didn’t know, red wine, coffee, and fruit are full of phenolic compounds, which 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.”
The study highlights the very importance of seagrasses: while they are carbon storage powerhouses that can help solve our climate problems, they are also some of the most threatened habitats on earth.
“By examining how much blue carbon – that is, the carbon captured by the world’s ocean and coastal ecosystems – 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 under living seagrass that would lead to a loss of stored carbon,” says Liebeke.
“Our calculations show that if sucrose in the rhizosphere of seagrass were degraded by microbes, at least 1.54 million tonnes of carbon dioxide would be released into the global atmosphere. This is roughly equivalent to the amount of carbon dioxide carbon emitted by 330,000 cars in one year.”
Unaltered, seagrass can store carbon for millennia, while tropical forests do it for decades. However, as the tech industry races to take advantage of climate change and engineered solutions to suck carbon of the atmosphere, seagrasses are disappearing at an alarming rate, with annual losses of up to 7% in some areas. Tragically, up to a third of the world’s seagrass beds may have already disappeared.
“We don’t know as much about seagrasses as we do about terrestrial habitats,” notes Sogin. “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.”