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. 2018 Oct 12;18(10):1221–1242. doi: 10.1089/ast.2018.1917

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© Yuk L. Yung et al., 2018; Published by Mary Ann Liebert, Inc.

This Open Access article is distributed under the terms of the Creative Commons License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited.

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FIG. 4. — Free-energy profile of a hydrothermal pathway (in purple) to methane (Seewald et al., 2006) is contrasted with the reduction profiles of the acetogenic bacteria (triangles) and methanogenic archaea (squares); both biological mechanisms use the acetyl coenzyme-A pathway. We can think of the geochemical pathway as a chemical siphon while the much more rapid biochemical pathways are driven by chemiosmosis over the intermediates formate and formaldehyde (or the formyl group). Adapted from Maden (2000).