Over a million tons of sugar could be stored in the root zone of the world’s seagrass beds, according to a discovery: Researchers have found that the sea plants release surprisingly large amounts of sucrose through their roots into the sea floor. They can also explain why the seaweed releases the sugar and why the “sweets” are not subsequently eaten by the bacteria in the root zone. The results shed new light on the role of marine plants in the climatically important storage of carbon in the ocean, say the scientists.
Meadows that sway in the water instead of in the wind: The coastal areas of many marine areas of the world are characterized by lush seagrass fields. In total, these ecosystems cover up to 600,000 square kilometers, which is roughly the size of France. The green underwater landscapes are therefore of far-reaching importance: they provide a home for many organisms, protect coastal areas from erosion – and they play a role in the carbon cycle and thus for the global climate. Previous studies have already shown that sea gas meadows absorb carbon dioxide much more effectively than forests on land. As so-called “blue carbon”, it can then remain bound in the biomass for a long time and thus does not contribute to climate change.
In their current study, however, the researchers led by Manuel Liebeke from the Max Planck Institute for Marine Microbiology (MPIMM) did not deal with the carbon bound in the biomass. Her focus was on the compounds that the seaweed makes. Because it is well known that under certain circumstances plants excrete carbohydrates through their roots. In order to record what seagrass releases into its so-called rhizosphere, the scientists examined samples from the sediment under meadows of different seagrass species in the Mediterranean, the Caribbean and the Baltic Sea.
Heaps of sugar
Their analyzes revealed that the plants apparently give off large amounts of sucrose, also known as table sugar, and surprisingly this substance accumulates in the soil. The researchers report that the sugar concentration under the seagrass meadows is about 80 times higher than anything that has been measured in the sea to date. “For classification: We estimate that between 0.6 and 1.3 million tons of sugar, mainly in the form of sucrose, are stored in the seagrass rhizosphere worldwide,” says Liebeke. “That corresponds roughly to the amount of sugar in 32 billion cans of cola!” the scientist compares.
A fundamental question arises: why do the plants produce such amounts of this energy-rich substance? Apparently there is sometimes a need for disposal when there is a large excess, explains co-author Nicole Dubilier from the MPIMM: “The seaweed produces the sugar during photosynthesis. Under average light conditions, plants use most of these sugars for their own metabolism and growth. But when the light is very strong, for example at noon or in summer, they produce more sugar than they can use or store. Then they release the excess sucrose into their rhizosphere – it’s like an overflow valve,” says the scientist.
However, the question remained unanswered as to why the energy-rich and easily digestible sugar can accumulate so strongly. Specifically: Why don’t the microorganisms in the seaweed rhizosphere attack the sweet treasure? “We’ve been trying to figure that out for a long time,” says lead author Maggie Sogin from the MPIMM. To do this, the researchers used genetic methods to study the metabolism of the bacteria that live in the root area of the seaweed. It turned out that they usually have the prerequisites for the breakdown of sucrose – but the corresponding genes are often not activated. Apparently something inhibits the microbial conversion of the sugar with the release of carbon dioxide.
Phenols inhibit sugar breakdown
As further analyzes revealed, it is the combination of low-oxygen conditions with certain substances that the plant roots also release in addition to the sugar: “We found that seaweed releases phenols into the sediment,” says Sogin. These are substances that are known to be able to inhibit the metabolism of microorganisms. “We conducted experiments in which we exposed the microorganisms in the seaweed rhizosphere to phenols isolated from the seaweed – and in fact, much less sucrose was consumed there than if we had not added phenols,” reports Sogin.
Interestingly, the investigations also provided evidence that a small group of microbial specialists are adapted to the conditions of the seagrass rhizosphere: They can digest sucrose and break down the phenols. The researchers suspect that these microbes could be partners with seagrass: they may give the plants access to the nutrients they need to grow. “We are familiar with such advantageous relationships between plants and microorganisms in the rhizosphere from land plants. But we are only just beginning to understand the intimate and complicated interactions of seagrasses with microorganisms in the marine rhizosphere,” says Sogin.
As the team concludes, their study underscores once again the importance of protecting the world’s threatened seagrass beds. Up to a third of the world’s seagrass population may already have been lost, and annual losses are estimated at up to seven percent in some locations. “Large amounts of stored carbon would be released as seagrass beds continue to decline. Our research shows very clearly: not only the seagrass itself, but also the large amounts of sucrose under the meadows must be considered,” Liebeke sums up.
Source: Max Planck Institute for Marine Microbiology; Specialist article: Nature Ecology & Evolution, doi:
10.1038/s41559-022-01740-z