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| Photo Credit: Walter Frehner |
A new study reveals that methane can form in the upper layers of sandy seabeds — something that has taken scientists by surprise. Special microorganisms are at work, and the phenomenon may be happening along coastlines all over the world.
Methane is a powerful greenhouse gas, produced in many natural environments by microorganisms.
Until recently, scientists believed these microbes were intolerant of oxygen and could only survive in oxygen-free zones. But new research shows they can, in fact, persist in oxygenated environments — lying dormant until the oxygen disappears. That means an entirely new source of methane emissions has just been discovered.
“We do not yet know how much methane these microbes are producing. That is the next big question. But we suspect the contribution is significant and widespread in sandy coastal zones. This is not something confined to a few isolated spots on the globe,” says Ronnie N. Glud, professor at the Department of Biology and an expert in biogeochemistry.
May contribute negatively to the climate balance
Glud, together with colleagues Amelia-Elena Rotaru and Satoshi Kawaichi from the Department of Biology are co-authors of the new study published in Nature Geoscience. Lead authors Ning Hall and Perran Cook are from Monash University in Australia. The findings are based on fieldwork in both Denmark and Australia.
According to Glud, the discovery is important: "This tells us there is a methane-producing process in coastal zones that we have been overlooking. It is a process that could contribute negatively to the climate balance."
Methane producing organisms in nature When a microorganism produces methane, it is called a methanogen. Methanogenic archaea thrive in oxygen-free environments such as a cow’s stomach, swamps, marshes, and sediments. According to the Danish Meteorological Institute, the concentration of methane in the atmosphere has increased by 150% since the mid-1700s.
The clean sand
To understand what the scientists found, picture yourself walking barefoot along the shore. You roll up your pants and wade into the water. As your toes sink into the seabed, it feels like nothing but clean sand and water—surely no biological activity here, right?
Wrong. Sand teems with microorganisms, busily breaking down organic matter carried in with the water. Because sand is permeable, water flows through it, flushing away waste products and keeping the sediment relatively oxygen-rich. In fact, it works a bit like the biofilters in many water treatment plants.
"Contrary to what intuition tells us, sandy seabeds host a vibrant microbial community," says Glud.
Samples from Australia and Avernakø
Until now, researchers thought methanogenic archaea could not possibly survive here because oxygen from waves and currents penetrates the upper layers of sand. But analyses of sandy seabeds in Australia and Denmark (near Avernakø) prove otherwise.
The microbes remain inactive in oxygen-rich conditions. Once oxygen levels drop—say, in still periods—they awaken and start producing methane.
"Coastal waters are highly dynamic, with oxygen levels rising and falling. When the wind stirs, oxygen enters the water. But on calm days, oxygen is quickly consumed and this is when the archaea switch on methane production. Such low-oxygen episodes occur frequently especially when seaweed and seagrass material settles on sandy sediments, making methane emissions potentially substantial," explains professor Amelia-Elena Rotaru.
What is the microbes’ food source?
For archaea to produce methane, they need a food source—and the team investigated that too.
The microbes feast on metabolites of marine plants: dissolved organic compounds released by marine plants growing in or near sandy sediments. Typically, these are seagrasses or seaweeds, with older plants releasing the highest supply of metabolites.
"We did not know archaea were capable of this flexible lifestyle. That is the beauty of biology—it is so diverse and complex that it keeps surprising us with new mechanisms," says Rotaru.
Nature as a tool?
The findings also serve as a reminder that we should be careful using nature as a tool for alleviating human-made problems.
"We need to be cautious and not rush to implement apparent solutions without fully understanding how an ecosystem functions, like greenhouse gas production" warns Glud, "restoring seagrass meadows and coastal vegetation is important and provide healthier ecosystems, greater biodiversity and better water quality. But as this study shows, it might also mean more methane escaping from the seafloor. Discoveries like this highlight just how much we still have to learn about nature. And that is the wonderful part—there is always more to discover," says Glud.
Denmark has about 8,500 km of coastline, most of it sandy. Globally, there are around one million kilometers of ice-free coastline — about 30% of which is sandy or gravelly.
Published in journal: Nature Geoscience
Title: Coastal methane emissions driven by aerotolerant methanogens using seaweed and seagrass metabolites
Authors: N. Hall, W. W. Wong, R. Lappan, F. Ricci, K. J. Jeppe, R. N. Glud, S. Kawaichi, A-E. Rotaru, C. Greening, and P. L. M. Cook
Source/Credit: University of Southern Denmark | Birgitte Svennevig
Reference Number: env091652_01