Methanogenesis: Part 1: Discovery of New Oxygen-Tolerant Methanogens May Partially Explain Recent Increase in Biogenic Atmospheric Methane: Coastal Methanogenesis is More Abundant Than Thought and Implications for Methane Budgeting

      Researchers have discovered the culprit behind the increase in atmospheric methane over the past decade or so. It is likely more than one culprit. This post looks at research in intertidal coastal areas with permeable sand bottoms. Part 1 will follow this study. In another post, Part 2 will examine the recent discovery of the growing proliferation of deep-sea methanogens as a possible source of the methane. According to Part 1, the methane is thought to be from two previously unknown methanogen strains. The researchers from Monash University in Australia studied sandy coastal regions in Australia and Denmark. The new methanogen species can tolerate some oxygen, once thought to be impossible with methanogens. Lead author of their paper in Nature Geoscience, Professor Perran Cook, noted;

“Methanogenesis was thought to occur only in oxygen-free environments. We’ve now shown that these microbes survive oxygen exposure with no ill effects.”

     The microbes generate methane by metabolizing compounds released from decaying seaweed and seagrass, even in the presence of oxygen. The implications of this new knowledge include the new likelihood that sandy coastal areas with permeable sediments contribute much more methane than previously thought. In addition, the fact that they metabolize chemicals from decaying seaweed and sea grass roots means that so-called “blue carbon” carbon offset schemes may not be providing as much greenhouse gas sequestration as previously thought, possibly much less. The new understanding suggests that such schemes may not be as useful as thought. This certainly needs further research. There are other reasons, however, than sequestering greenhouse gases, for coastal restoration via seagrasses and seaweed.

     The paper notes the changing implications of the study for climate change analysis:

“The evidence presented here shows the activity of the most oxygen-tolerant methanogens described so far, both in whole-community and isolate settings. Combined with high-rate measurements and evidence of macrophyte biomass being the primary driving factor, this redefines the range of environments that can be described as highly methanogenic and suggests climate consequences to changes in coastal permeable environments, which have not been previously considered.”

“The shallow and turbulent nature of waters overlying coastal permeable sediments, combined with advective transport in the sediments, gives the study additional importance. In deeper waters and cohesive sediments, the balance of methanogenesis and methanotrophy is such that in the bulk of the ocean, the volume is undersaturated in methane with respect to the atmosphere. However, in rippled permeable sediments, the redox seal is broken, with flow from reduced reaction zones exported directly through ripple peaks or lee sides depending on bedform and flow interactions. This allows methane produced in shallow anoxic regions to reach the shallow overlying water where low residence times and high turbulence causes high rates of export to the atmosphere. Therefore, the contribution of methane production in shallow permeable sediments to total marine methane emissions is probably disproportionately large.”

     It seems likely that much of the methane emissions are seasonal, related to the growth and decay of plants.

     Other implications involve two increasingly common phenomena: 1) the presence of eutrophication and large algae blooms along coastal zones receiving excess nutrients, phosphorus in particular, from rivers draining agricultural areas, and 2) rising sea temperatures – these higher temperatures support algal blooms and biomass collecting on beaches that will later decay.

     The study utilized a combination of in situ monitoring, laboratory experiments, and genomic analysis. Lab experiments were conducted with slurries. There are implications for climate modeling and carbon budgeting, especially since these permeable sandy coasts make up half of the world’s continental margins.

“Here, we have shown that deposition of this excess algal biomass on sandy coasts may result in increasingly large and frequent pulses of methane to the atmosphere and should be accounted for in future marine methane budgets and modelling. In particular, we note that many studies that quantify the net carbon sink/source dynamics of vegetated ecosystems focus on the sites where these macrophytes grow, and we suggest that future work should focus on the mobility of degrading biomass and its potential greenhouse gas emissions when deposited in different ecosystems. As well as unintentional excess macrophyte growth caused by eutrophication, the results of this study further complicate CO2 removal by macrophytes, seagrasses or ‘blue carbon’ as a climate change mitigation strategy, as enhanced methane emissions may offset much of the CO2 removal by these ecosystems.”

     The researchers were able to rule out groundwater as a source of the extra methane by comparing methane concentrations to radon concentrations in the groundwater. While methane increased, radon did not, indicating a non-groundwater source. They used liquid chromatography and mass spectrometry (LC-MS) to evaluate chemical components in the study. They also isolated and sequenced the genomes of the methanogens. They found that methylotrophic archaea dominate the intertidal coastal emissions. This was unexpected since it was thought that the frequent presence of oxygen in the environment would inhibit archaeal methanogenesis.    

“We investigated acetoclastic, hydrogenotrophic and methylotrophic methanogenesis pathways using targeted substrate addition (Fig. 2d) and found that methylotrophic methanogenesis predominated…”   

     The metagenomic analysis and the isolation of novel microbes confirm that aerotolerant methanogens were the culprit in the enhanced methane generation. They also found that in both locations, Australia and Denmark, with much different climates, the methanogenesis pathways were remarkably similar:

“Analyses of the genome sequences of both isolates revealed remarkable similarities in their methanogenesis pathways and antioxidant systems, despite being isolated from geographically and climatically distinct locations, suggesting that these traits are important for adaptation in sandy sediments.”

 

     

 

References:

 

New Data Says Earth’s Dangerous Warming Traced To A Hidden Methane Culprit. Julie Majid. Petsnpals. September 1, 2025. New Data Says Earth’s Dangerous Warming Traced To A Hidden Methane Culprit

Coastal methane emissions driven by aerotolerant methanogens using seaweed and seagrass metabolites. N. Hall, W. W. Wong, R. Lappan, F. Ricci, K. J. Jeppe, R. N. Glud, S. Kawaichi, A-E. Rotaru, C. Greening & P. L. M. Cook. Nature Geoscience. August 7, 2025. Coastal methane emissions driven by aerotolerant methanogens using seaweed and seagrass metabolites | Nature Geoscience

Monash University scientists unlock seaweed secrets that could transform climate models.Monash University.  August 11, 2025. Monash University scientists unlock seaweed secrets that could transform climate models - Science