NOVELTY - An isolated Methanothermobacter microorganism (A1) is new.    USE - As isolated Methanothermobacter microorganism useful for producing methane from carbon dioxide; for converting electric power or electricity into methane (claimed).    ADVANTAGE - The isolated Methanothermobacter microorganism or its variant or progeny produces methane from carbon dioxide via a process called methanogenesis; exhibits high methane production efficiency i.e. at least 25 (preferably 40, especially 50, particularly 60) CO2 molecules converted to methane per CO2 molecule converted to cellular material (where the methane production efficiency from CO2 is maintained for at least 30 consecutive days); produces at least 96 (preferably 97, especially 98) molecules of methane per 100 molecules of carbon dioxide supplied to the microorganism; produces at least 17 g (preferably 20 g, especially 30 g, particularly 40 g) of methane from CO2 per gram of Methanothermobacter cellular material (i.e. biomass) produced from CO2; and exhibits a doubling time of at least 72 hours (preferably 80 hours, especially 200 hours, particularly 1 month) in a stationary phase (where the doubling time is maintained for at least 7 (preferably 30) consecutive days). By virtue of the slow doubling time, the Methanothermobacter microorganism or its variant or progeny exhibits a significant reduction in required nutrients for maintenance and growth of the microorganisms. The microorganism exhibits the methane production efficiency when not greater than 25 molecules of hydrogen are supplied to the microorganism for every 6 molecules of methane produced, or not greater than 200 molecules of hydrogen are supplied for every 49 molecules of methane produced. The isolated microorganism produces at least 20 g (preferably 30 g, especially 40) of methane from CO2 per gram of cellular material produced from CO2. The microorganism exhibits a doubling time of at least 72 hours in a stationary phase when provided with CO2 gas at a rate of at least 34 volumes of methane at standard temperature and pressure per volume of culture per day (VVD) and with reducing power sufficient to reduce at least 90% of the CO2 (where the reducing power is H2 gas supplied at a rate of at least 122 VVD or the reducing power is electrical current). The microorganism returns to at least 80% of the methane productivity level in the operating state within 20 minutes of exposure to at least 3 minutes of either oxygen or carbon monoxide; or returns to at least 80% of the methane productivity level in the operating state within 10 minutes of exposure to at least 3 minutes of oxygen; exhibits a cell culture density of at least 6 (preferably 6-8) mg dry mass of cells/ml culture in a stationary phase for at least 15 consecutive days; and returns to at least 80% of the methane productivity in the operating state within 20 minutes of re-supplying hydrogen (H2) gas or electricity, after being in a dormant state for at least 2 hours as induced by interrupting or ceasing hydrogen supply or electricity. The progeny retains the CO2 conversion phenotypic characteristics of the strain.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) a substantially pure culture or monoculture comprising the microorganism (A1);    (2) a system (A2) for converting electric power into methane, comprising a biological reactor having at least a cathode, an anode, the microorganism (A1), water, and carbon dioxide;    (3) converting electricity into methane involving supplying electricity and carbon dioxide to the system (A2), where the biological reactor having an operating state, and the microorganism is maintained at greater than or equal to 60 degrees C, and collecting methane from the first chamber;    (4) a porous cathode comprising the isolated microorganism (A1);    (5) a kit comprising the microorganism (A1), substantially pure culture or monoculture, system and/or porous cathode, and instructions for care or for use;    (6) a cell culture inoculum comprising the isolated Methanothermobacter microorganism (at least 1.6 kg dry weight); and    (7) a large-scale culture which is ready for use in methane production comprising at least 1000 l of a culture at a density of at least 6 g dry weight cell/l culture of the isolated microorganism.