Proteins are shown by ellipses: pMMO, particulate methane monooxygenase; Mxa, PQQ-linked methanol dehydrogenases; H4MTP, methanopterin-linked one-carbon metabolic pathway; Fdh, formate dehydrogenases; C1, NADH:quinone dehydrogenase; C3, ubiquinol-cytochrome c reductase or bc1 complex; C4, cytochrome c oxidase; F1F0, F1F0-ATPsynthase. cytC, cytochrome c; UQ, ubiquinone; UQ H2, ubiquinol; ICM, intracytoplasmic membrane. The physiological electron donor to the pMMO is still not known. So we proposed three possible mechanisms (shown below): redox arm mode in which electrons driving methane oxidation come from NADH produced by formate and/or formaldehyde oxidation, while electrons produced from methanol oxidation are linked to redox arm and used for ATP production; direct coupling that is a direct transfer of reducing power from MDH (Mxa) to pMMO and uphill electron transfer model, in which methanol oxidation partially supports methane oxidation.