Methanogenesis: Part 2: Deep-Sea Methanogens and Chemosynthesis: Quantifying Its Contribution

     New research is showing that there is much more to the global methane cycle than previously assumed, especially regarding methane seeping from cracks in deep-sea trenches. These deep methane seeps support strange forms of deep-sea life in those trenches. There is a clear need for a better understanding of deep-sea methane and the possibility that it is contributing to increases in atmospheric methane. The new discovery, published in July 2025 in Nature, was written about recently in Forbes by Ingmar Rentzhog.  She explains:

“This Deep-Sea discovery is so new it’s rewriting the map of life on Earth and it could reshape our understanding of the climate system. More than 9,000 meters below the Pacific Ocean, scientists have uncovered a 2,500-kilometer stretch of extraordinary life that doesn’t depend on sunlight at all — it runs on methane.”

“Between Russia and Alaska, in the deep-sea of the Kuril–Kamchatka and Aleutian trenches, clams, red-tipped tube worms, and invisible microbes thrive on gases seeping from cracks in the seafloor. These are the deepest methane-fueled ecosystems ever recorded — and they may be doing far more than surviving. They might be helping regulate our climate.”

     This methane-based deep-sea biological system is known as chemosynthesis. The scientists involved in the recent paper relied on an expedition to the Kuril–Kamchatka Trench and the western Aleutian Trench using the manned submersible Fendouzhe. The source of the methane is organic matter that is processed by deep-ocean microbes. As noted in the abstract, this chemosynthesis-based life may be more widespread than previously thought. There are also potentially huge implications for these hadal trenches for deep ocean carbon cycling. The new research underscores the fact that we need a better understanding of ocean methane and carbon cycling. It is difficult to explore these hadal trenches due to water depths reaching 9000 meters or more. Deep trenches occur at passive and active plate margins. The authors note that “probable chemosynthetic mats have been observed at a depth of 10,677 m at the bottom of the Mariana Trench.” The Mariana Trench is a Mid-Ocean Ridge where new basaltic sea floor is being created and spreading out from the mid-ocean toward the next tectonic boundary, which is likely a subduction zone associated with continental plates. The process of the emergence of sea floor spreading to re-submergence takes about 180 million years. The Kuril–Kamchatka Trench occurs at a subduction zone where one plate subducts under another plate. These cold seeps are very high-pressure environments.

“The detection of anomalously high methane concentrations and the potential for gas hydrate formation in the hadal zone provide new insights into deep carbon cycling. The widespread methane-rich environments in two hadal trenches, where microbial reduction of CO2 from sedimentary organic matter presumably results in methane production, suggest a vibrant and active microbial community in the hadal sediments. This indicates that the deep-subsurface biosphere may exert a more important influence on biogeochemical processes in subduction zones, representing a previously unrecognized energy supply. The accumulation of methane in sedimentary layers generated by this deep-subsurface biosphere could potentially sequester considerable amounts of sedimentary organic carbon, suggesting a portion of subducting organic carbon can be stored in the trench sediments in a form of methane for prolonged geological time, rather than being subducted to the deep lithosphere. It remains unknown whether the current findings can be extrapolated to other trench systems, but given the geological similarities hadal methane reservoirs may be more widespread, irrespective of the presence of fault zones that could serve as conduits for the release of methane-rich fluids. This hypothesis is supported by the recovery of gas hydrates from drilling sediments in the Middle America Trench and the Peru–Chile Trench at depths surpassing 5,000 m (ref. 48), and the presence of similar seep communities in the Japan Trench. These findings underscore the complex nature of carbon cycling in the deep sea and highlight the critical need to integrate hadal processes into global carbon models to improve the accuracy of predictions about carbon dynamics and climate change responses on geological timescale. Furthermore, the potential presence of methane hydrates at great depths in hadal trenches may enhance global inventory of methane gas hydrate resources.”

     Microbes in the bodies of the deep-sea life of the trenches consume methane. They do not rely on sunlight at all.

     Below is the paper's abstract, pictures of some of the fauna, and an exploration of deep-sea methane origins.

     Below is a section from the Forbes article on how methane is measured in these deep-sea environments. It should be noted that under the high-pressure conditions of deep water, methane occurs in liquid form in seafloor sediments, usually as methane hydrates. It should also perhaps be noted that the amount of methane hydrate stored in seafloor sediments is much higher than the amount of methane generated thermogenically in the subsurface. This suggests that unknown fluxes releasing methane from methane hydrates could result in significantly higher biogenic emissions, since the source is so big. Those fluxes could be initiated by ocean currents, deep seafloor mining or disturbance, and the availability of organic matter and hydrogen.

     Difficult as it is, more research is needed about deep-sea methane and methanogenesis. Part 3 will explore how methane is fingerprinted by chemical isotopes.

     Below is an awesome video of "life in the trenches!'

    

References:

 

Flourishing chemosynthetic life at the greatest depths of hadal trenches. Xiaotong Peng, Mengran Du, Andrey Gebruk, Shuangquan Liu, Zhaoming Gao, Ronnie N. Glud, Peng Zhou, Ruoheng Wang, Ashley A. Rowden, Gennady M. Kamenev, Anastassya S. Maiorova, Dominic Papineau, Shun Chen, Jinwei Gao, Helu Liu, Yuan He, Inna L. Alalykina, Igor Yu. Dolmatov, Hanyu Zhang, Xuegong Li, Marina V. Malyutina, Shamik Dasgupta, Anastasiia A. Saulenko, Vladimir A. Shilov, …Andrey V. Adrianov. Nature (July 2025). Flourishing chemosynthetic life at the greatest depths of hadal trenches | Nature

Origin of natural gas within the deep-sea uncompacted sediments of the Shenhu area, northern South China Sea: Geochemical and methanogenic cultivation results. Hongfei Lai, Yinan Deng, Lu Yang, Jinqiang Liang, Lirong Dai, Ling Li, Yunxin Fang, Laiyan Liu, and Zenggui Kuang. Marine and Petroleum Geology. Volume 147, January 2023. Origin of natural gas within the deep-sea uncompacted sediments of the Shenhu area, northern South China Sea: Geochemical and methanogenic cultivation results - ScienceDirect

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

Deep-Sea Discovery Reveals Hidden Methane Cycle. Ingmar Rentzhog. Forbes. August 14, 2025. Deep-Sea Discovery Reveals Hidden Methane Cycle