We usually see the coast as a natural line of defense. Seagrass beds and mangroves are often praised for pulling carbon from the air. But there’s another story unfolding in the sand.
New research from Monash University shows that sandy coastlines are not silent climate helpers. They also release methane, a greenhouse gas that warms the planet much faster than carbon dioxide.
The study points to half of the world’s continental margins. These sandy stretches cover vast areas and have gone mostly unnoticed in climate models. They now appear to be an important, and underestimated, source of emissions.
Sandy coastlines release methane
Professor Perran Cook, a biogeochemist in the Department of Chemistry at Monash, is the principal investigator of the study.
“This new finding not only challenges a fundamental assumption in marine science, but calls into question what we thought we knew about the role of sandy coastline ecosystems in greenhouse gas production,” said Professor Cook.
In simple terms, the sand isn’t just storing carbon. It is leaking methane, and much of it comes from seaweed and seagrass breaking down. This means the gains we credit to coastal ecosystems may be smaller than once thought.
Sand microbes producing methane
The researchers worked in Australia’s Port Phillip Bay and Westernport Bay, and in Denmark’s Avernakø region.
The team found microbes living in sandy sediments that can survive oxygen and keep producing methane. Until now, scientists believed these methanogens could only work in oxygen-free zones.
Tests revealed two new strains. These microbes feast on the leftovers of decaying seaweed and seagrass. Instead of dying when oxygen floods their habitat, they bounce back quickly and restart methane production within hours. That recovery is much faster than anything seen in soils or rice fields.
How the microbes make methane
The microbes use a pathway called methylotrophic methanogenesis. They prefer compounds like trimethylamine, choline, and dimethyl sulfoniopropionate.
All of these compounds are released when marine plants break down. Other potential food sources, such as acetate or hydrogen, did little to stimulate methane.
This discovery shows why sandy sediments matter. They sit under mats of seaweed, soak up plant byproducts, and then feed methane straight into the atmosphere. Waves and currents push oxygen into the sand, but the microbes endure.
Algal blooms make it worse
Warmer water leads to more algae. The algae wash onto beaches and rot. As they decay, methane escapes into the air. This cycle keeps feeding itself, creating a feedback loop.
Professor Cook warned that large blooms add fuel to the problem. “With rising sea temperatures, species invasions and increasing nutrient pollution, we’re seeing more frequent algal blooms and biomass accumulation on beaches.”
“This could lead to larger and more frequent pulses of methane to the atmosphere, which in turn contributes to rising sea temperatures.”
Sandy beaches match major methane sources
The researchers calculated methane flux from sandy sediments. In some cases, emissions were as strong as wetlands, which are already known as major methane sources.
The average methane flux matched mangroves and salt marshes but beat out seagrass meadows.
Because these sands lie under shallow, turbulent waters, methane doesn’t linger. It escapes quickly instead of breaking down in deeper layers. This rapid release means sandy coasts may punch above their weight in the global methane budget.
Persistent methane production
The team isolated methanogen strains from both Australia and Denmark. Belonging to the genus Methanococcoides, they showed surprising resilience. When hit with oxygen, they paused. Once conditions shifted back, they resumed methane production within an hour.
Genomic analysis revealed why. The microbes carry antioxidant defenses, including enzymes normally linked to oxygen-using organisms. These tools allow them to handle sudden oxygen bursts and keep working in shifting coastal sands.
Local and seasonal drivers
Not every coastline behaves the same. Seasonal growth and decay of different seaweed and seagrass species matter. Warm waters, nutrient levels, and grazing pressure also play roles. Tropical and temperate coasts may follow very different emission patterns.
Human activity adds another layer. Fertilizer runoff and nutrient pollution drive eutrophication, leading to dense blooms.
As these blooms wash ashore, they deliver more fuel for methane-producing microbes. Climate change then sharpens the cycle further.
Sandy coastlines as methane sources
Study first author and Monash Ph.D. candidate Ning Hall emphasized the path forward. “From here, we need to understand this process in more detail,” Hall said.
Future work will examine how different species of seaweed and local ocean conditions affect emissions. These insights will help refine climate models and give a clearer picture of how much methane coastal zones release.
The study changes how we see sandy shores. They store carbon but also release methane. Microbes, decaying plants, and shifting ocean conditions decide how much they give or take.
By pulling this hidden source into focus, researchers show that coasts can’t be counted only as climate allies. They are active, dynamic players in the greenhouse gas puzzle, and they demand a place in future climate strategies.
The study is published in the journal Nature Geoscience.
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