The endosymbiosis theory of the origin of eukaryotic cell was first proposed more than a hundred years ago. In the second half of the 20th century, Lynn Margulis suggested a new interpretation of the origin of the nucleus in modern eukaryotes. The background was the study of the consortium "Thiodendron", a symbiotic bacterial community, which includes anaerobic aerotolerant motile spirochaetes and sulfidogenic bacteria (sulfidogens) of vibrioid form with a fermentation type of metabolism. Spirochaetes supply sulfidogens with metabolites (pyruvate and, probably, organic nitrogenous products of cell lysis) and get hydrogen sulfide from sulfidogens that helps to maintain a low redox potential. At low oxygen concentrations, spirochaetes are able to assimilate glucose more efficiently. Margulis hypothesized about the symbiotic origin of the nucleus by adding the bacterium Spirochaeta to the Thermoplasma-like archaea. She considered the "Thiodendron"-like consortium to be an intermediate stage in evolution. According to Margulis, the conversion of carbohydrates and the oxidation of Н₂S to S⁰ by the bacterium provided the archaea with electron acceptors for anaerobic respiration, as shown for modern thermoplasmas and products saturated with carbon. The use of carbon sources increased by attaching the floating bacterium to the archaea. More efficient microaerobic oxidation of glucose pre-adapted the spirochaetes for association with Thermoplasma. However, modern "Thiodendron"-like consortia are not in stable symbiosis and a sulfidogenic component of the consortium is capable for fermentation, rather than anaerobic respiration, which makes the theory by Margulis disputable.