Bioenergetics and the Role of Soluble Cytochromescfor Alkaline Adaptation in Gram-Negative AlkaliphilicPseudomonas

Abstract

Very few studies have been conducted on alkaline adaptation of Gram-negative alkaliphiles. The reversed difference of H+concentration across the membrane will make energy production considerably difficult for Gram-negative as well as Gram-positive bacteria. Cells of the alkaliphilic Gram-negative bacteriumPseudomonas alcaliphilaAL15-21Tgrown at pH 10 under low-aeration intensity have a soluble cytochromeccontent that is 3.6-fold higher than that of the cells grown at pH 7 under high-aeration intensity. Cytochromec-552 content was higher (64% in all soluble cytochromesc) than those of cytochromec-554 and cytochromec-551. In the cytochromec-552-dificient mutant grown at pH 10 under low-aeration intensity showed a marked decrease inμmax⁡[h−1] (40%) and maximum cell turbidity (25%) relative to those of the wild type. Considering the high electron-retaining abilities of the three soluble cytochromesc, the deteriorations in the growth of the cytochromec-552-deficient mutant could be caused by the soluble cytochromescacting as electron storages in the periplasmic space of the bacterium. These electron-retaining cytochromescmay play a role as electron and H+condenser, which facilitate terminal oxidation at high pH under air-limited conditions, which is difficult to respire owing to less oxygen and less H+.