Behind the "CO₂ Absorption Source" - Could Seaweed Washed Ashore Contribute to Global Warming? Invisible Greenhouse Gases Rising from the Beach

Behind the "CO₂ Absorption Source" - Could Seaweed Washed Ashore Contribute to Global Warming? Invisible Greenhouse Gases Rising from the Beach

2025年09月03日 00:51

Introduction: The Beach Was a "Quiet Emission Source"

On September 1 (local time), Phys.org reported news that shook the conventional wisdom of coastal ecosystems. It was revealed that on sandy beaches, macroalgae (macrophytes) washed ashore decompose, and the metabolites produced are used by archaea to produce methane, which is then released into the atmosphere. The paper was published in Nature Geoscience on August 7. Until now, it was believed that sandy beaches "suppress methane production due to exposure to oxygen," but this assumption has been overturned.Phys.orgDOI


What Was Discovered: Methane Producers That "Tolerate" Oxygen

The research was conducted on the sandy beaches of Port Phillip Bay and Western Port Bay (Australia), and Avernakø (Denmark). The surface waters of shallow coastal areas showed extreme methane supersaturation, ranging from **380% to 189,000%** relative to the atmosphere. Methane levels surged in microtopographies where mats of seaweed and seagrass tend to accumulate, indicating that the source was not groundwater (radon indicator).DOI


The Key Is Methanotrophic Methanogenic Archaea, Centered on Methanosarcinaceae. They utilize methyl compounds abundant in seaweed and seagrass, such as trimethylamine (TMA), methylamine, and dimethyl sulfide (DMS), to produce methane. In culture and inhibition experiments, the addition of BES, which stops the terminal enzyme of archaea, almost completely suppressed methane production. Methane production did not increase with the more competitive acetate (acetoclastic) or hydrogen, but increased significantly only with methyl substrates. Furthermore, "oxygen tolerance," where production quickly recovers even after repeated oxygen exposure, was also confirmed.DOI


Scale Estimation: A Global "Overlook"

It is estimated that shallow coastal seas account for about 75% of oceanic methane emissions, but the breakdown was uncertain. The new findings highlight the reality that sandy beaches (about 50% of the world's continental margins) are supplying more methane than previously assumed. This strongly demands an **offset** perspective on the narrative that blue carbon has claimed—"coastal ecosystems are powerful CO₂ sinks."DOI


Why It Is Becoming an Issue Now: Algal Blooms and Warming Feedback

The massive stranding of seaweed tends to increase with the progression of high water temperatures, invasive species intrusion, and eutrophication. The algal blooms that troubled various parts of South Australia in 2025 are part of this phenomenon. When stranded algae accumulate, methyl compounds are released during decomposition, and archaea in the sand emit a **"methane pulse"**—the warmer the sea, the more algae increase, potentially further driving warming through methane.Phys.org


Research Methods: Field Observations × Metagenomics × Cultivation

This research employed a multi-layered approach: (1) local methane concentration and radon measurements, (2) slurry experiments combining sand, seawater, and seaweed, (3) more field-like conditions using a flow-through reactor (FTR), (4) metagenomic analysis, and (5) isolation of two new strains of the Methanococcoides genus, to demonstrate the involvement of archaea and their substrate specificity. The study also revealed that the decomposition of choline, glycine betaine, DMSP, and other osmolytes contained in seaweed extracts drives methane production.DOI


Reactions on Social Media: Voices of Surprise and Reevaluation

 


  • Nature Geoscience Official introduced the paper on August 7, emphasizing the unexpected methane source from coastal sandy beaches. The Altmetric indicator, showing the academic community's attention, also rose.X (formerly Twitter)DOI

  • Marine Microbiologist Cameron Thrash shared the paper link on X, spreading reactions to the perspective that "seaweed and seagrass metabolites are key."X (formerly Twitter)

  • Monash Science's official post and various news sites also reported that sandy beaches are an "overlooked emission source." Researchers and general users commented, calling for a "reassessment of blue carbon evaluation."X (formerly Twitter)FacebookEarth.com


In summary, the simple narrative of **"coasts as carbon sinks = good" has collapsed, and we have entered a phase of reevaluation as a **"double-edged sword of absorption and emission."


Three Points to Avoid Misunderstanding

  1. Seaweed ≠ BadThe fact that seaweed forests and seagrass beds absorb CO₂ remains unshaken. However, it is necessary to simultaneously account for the "methane return." Previous studies have also reported examples where methane emissions from seaweed areas could reach 20-40% of CO₂ absorption.news.mongabay.com

  2. Location and Condition Are Everything
    On sandy beaches where seaweed is stranded and accumulated and decomposition progresses, methane production is stronger in areas rich in methyl compounds. The data also showed that it is not groundwater-derived.DOI

  3. Oxygen Doesn't Always Act as a "Stopper"
    Contrary to conventional wisdom (methane production occurs only in anoxic conditions), the main player this time is oxygen-tolerant. It can maintain activity even in the highly disturbed surface sands of coastal areas.DOI


Impact: How Policy and On-Site Decision-Making Will Change

  • Updating Climate Models
    It is necessary to re-estimate the spatial distribution and seasonal pulses of oceanic methane by overlaying maps of sandy beaches and accumulated seaweed. The contribution in eutrophic coastal areas cannot be ignored.DOI

  • Redesigning Coastal Management
    (a) Shorten the accumulation time of stranded algae within a range that does not impair ecosystem services (nursery grounds for juvenile fish, coastal nutrient cycling), and (b) reduce anaerobic and semi-anaerobic hotspots through measures such as covering with soil, composting, and dispersing accumulation sites. This **"methane-smart cleaning"** becomes a subject of consideration.

  • Increasing the Priority of Eutrophication Measures
    Reducing the nitrogen and phosphorus loads in river basins also leads to suppressing algal blooms, which in turn suppresses the supply of methyl compounds.

  • Observation and Monitoring
    Methane supersaturation in coastal areas varies on a four-digit scale depending on the location. It is necessary to design continuous monitoring by combining remote sensing and on-site sensors.DOI


Counterarguments and Cautions

  • The Global Contribution Is Still RoughThis time, the focus is on mechanism confirmation. To directly connect to global flux, long-term and wide-area observations

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