Seagrasses are some of the most efficient carbon sinks on Earth: one square kilometer can store nearly twice as much carbon as forests on land, and 35 times as fast. Now, a research team led by the Max Planck Institute for Marine Microbiology in Germany has found that seagrasses also release massive amounts of sugars into their soils (the so-called “rhizosphere”), during the process of photosynthesis.
While under average light conditions, the plants use most of the sugars for their own metabolism and growth, under high light conditions, such as at midday or during the summer, they produce much more than they can use, releasing the surplus into their rhizosphere.
“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,” said study senior author Manuel Liebeke, the head of the Metabolic Interactions group at the Max Planck Institute. “That is roughly comparable to the amount of sugar in 32 billion cans of coke!”
Knowing that almost all microbes thrive on sugars, Dr. Liebeke and his colleagues have pondered why is the sucrose not consumed by the vast population of microorganisms from the seagrass rhizosphere.
“We spent a long time trying to figure this out,” said study first author Maggie Sogin, who conducted the study during a postdoctoral fellowship at Max Planck.
“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.”
However, a small set of microbes seem to be able to thrive on sucrose. According to Dr. Sogin, these creatures are not only able to digest sucrose and degrade phenolics, but might also produce essential nutrients, such as nitrogen, which help seagrass grow. “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 said.
Since seagrass meadows are among the most threatened habitats on the planet and are rapidly declining in all oceans, scientists worldwide fear that their disappearance will release massive amounts of carbon into the atmosphere, thus exacerbating global warming.
“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,” concluded Professor Liebeke.