Growing hyphae of Achlya bisexualis generate a longitudinal pH gradient in the surrounding medium

Growing hyphae of Achlya bisexualis were found to generate a longitudinal pH gradient in the surrounding medium; the medium adjacent to the tip was slightly more alkaline than the bulk phase, while that near distal parts was acidic. The profile of external pH paralleled that of electric current, as measured with a vibrating probe; the apical alkaline zone corresponded to the region of current inflow. In organisms grown in complete medium, both current flow and apical alkalinization were inhibited when amino acid uptake was blocked, either by removing amino acids from the medium or by raising the external pH to 8.5. Achlya could, however, adapt to a medium deficient in organic nutrients; elongating hyphae again generated both the pH profile and the transcellular electric current. It is proposed that both the pH profile and the electric current are manifestations of a transcellular proton current, which arises from the segregation of proton pumps from proton leaks. Symport of protons with amino acids may be one mechanism by which protons enter the hyphal apex.