The aim of this work was to evaluate the bioethanol productivity in an unconventional bioreactor assisted by extremely low frequency (ELF) - electromagnetic field and elucidate the biophysical mechanism of action by which ELF magnetic fields improve the bioethanol production by S. cerevisiae. Fermentations were carried out under axial field lines at 10 mT magnetic flux density (B), using three different recycling arrangements (spiral-shape tube, u-shape tube and whole bioreactor) in a closed loop. Fermentation kinetics were monitored by cell growth, substrate consumption, ethanol and by-product formation. In addition, electrophysiological measurements of the H+ ion fluxes were carried out in yeast cells sampled at different fermentation stages. ELF magnetic fields increased the glucose uptake, bioethanol production and H+ efflux, shortening in 2 h the fermentation time. The greatest effects of the ELF magnetic fields were obtained in the whole bioreactor arrangement, reaching an average increase of 33% in the bioethanol production. The results are consistent with a stimulatory effect of ELF magnetic fields on the plasma membrane H+-ATPase activity, as indicated by the specific increase of the vanadate-sensitive component of the yeast cells H+ efflux, providing a new biophysical mechanism of action for the biological effect of magnetic fields. (C) 2021 Elsevier Ltd. All rights reserved.