In the past, signs suggestive of the presence of biogenic methane found during drilling have been considered to indicate the absence of natural gas beneath them, whereas the presence of thermogenic methane has been considered to indicate the presence of methane deposits beneath it and received as a cue to proceed with extraction. A research group led by Senior Researcher Daisuke Mayumi and Invited Senior Researcher Susumu Sakata at the Institute for Geo-Resources and Environment, Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (AIST), in collaboration with the Massachusetts Institute of Technology, has discovered that methanogenic archaea in subsurface environments convert signatures of natural gas of thermogenic origin to those of biogenic origin. The findings overturn previous theories and are expected to lead to the discovery of new natural gas deposits that have been overlooked. The work was published in Science.
Understanding where and how methane, which is the main component of natural gas, is produced is important for assessing the available global natural gas resources. To date, stable isotope signals have been used as important signatures to distinguish the origins of methane formation. However, this identification method has a major problem, which has remained unsolved for nearly half a century: the stable isotope signals of biogenic methane produced by methanogens in subsurface environments cannot be reproduced in laboratory methanogenic cultures.
The joint research group developed a methanogen culture system that faithfully simulates the subsurface environments. This system comprises stainless steel pressure-resistant vessels and pressure pumps for use in methane hydrate synthesis experiments that are modified for microbial culture experiments and allows for culturing methanogens under hydrostatic pressure conditions equivalent to those in subsurface environments (equivalent to ∼150 atmospheres). Moreover, instead of adding hydrogen at the beginning of the culture, methanogens were cocultured with bacteria that continue to decompose organic matter and slowly produce hydrogen, to reproduce the low hydrogen concentrations characteristic of the subsurface environments.
The stable isotope signals of methane produced under these conditions noticeably differed from those produced under atmospheric pressure and high hydrogen concentrations and were clearly in agreement with the stable isotope signals of biogenic methane in the subsurface environments, which could not be reproduced previously. In cultures under hydrostatic conditions, the concentration of dissolved methane in the culture medium increased, the backward reaction of methanogenesis was accelerated, and eventually, the forward and backward reaction rates became almost the same, indicating the establishment of equilibrium. The aforementioned reproduction has been found to be achieved due to this equilibrium.
Additionally, the research team observed changes over time of the stable isotope signals with increases in dissolved methane concentration. They found that methanogens produced methane with nonequilibrium stable isotope signals while the dissolved methane concentration was low, whereas the signals reached equilibrium as the dissolved methane concentration increased.
Next, they added thermogenic methane to the culture vessel before initiating methanogen culture and found that the stable isotope signals of the added thermogenic methane and those of biogenic methane approached equilibrium as methanogenesis occurred.
This result indicates that methanogens overwrite the stable isotope signals of thermogenic methane with those of biogenic methane. By reviewing data on stable isotope signals of natural gas from different countries worldwide, it seems likely that methanogens are actually overwriting stable isotope signals in natural gas deposits around the world. Moving forward, the origins and formation processes of natural gas may need to be reexamined on a global scale.
Journal Information
Publication: Science
Title: Hydrogenotrophic methanogens overwrite isotope signals of subsurface methane
DOI: 10.1126/science.ado0126
This article has been translated by JST with permission from The Science News Ltd. (https://sci-news.co.jp/). Unauthorized reproduction of the article and photographs is prohibited.