Figure 1. (a) The “burning air” sketch included in a letter by Alessandro Volta to Carlo Guiseppe Campi. In this letter, Volta describes an experiment he had carried out on November 3, 1776, at the Lago Maggiore. The letter is dated November 14, 1776. From: Carlo Paolini: Storia de metano, Milano, 1976. (b) biogas, liberated in the Eel Creek, Woods Hole, USA. (Photograph: Gerhard Gottschalk, Goettingen, Germany.)

Figure 2. Structure of the nickel-tetrapyrrole coenzyme F ₄₃₀. This compound is part of the machinery that catalyzes the methane-forming step in methanoarchaea. It is structurally related to heme (iron-tetrapyrrole), chlorophyll (magnesium-tetrapyrrole) and vitamin B ₁₂ (cobalt-tetrapyrrole). (Drawing: Rolf Thauer, Marburg, Germany.)