Abstract
Microorganisms have potential capacities to adapt to environmental changes to a certain extent. As extreme cases, a variety of microorganisms such as thermophiles, psychrophiles, acidophiles, alkalophiles and halophiles have been isolated from the natural environments. The structure and function of cell membranes play essential roles to cope with an environmental stress. We studied the modes of adaptations of a slightly halophilic marine bacterium, Vibrio alginolyticus, to salt concentrations and pH changes. As a response to osmotic stress, the cells accumulate K+, glutamate and proline as osmoprotectants. Except for extreme halophiles (archaebacteria), the mode of osmotic adaptation of halophilic bacteria was very similar to those of nonhalophilic bacteria. The adaptation to the changes in external pH is performed by maintaining the intracellular pH nearly constant. In V. alginolyticus, K+/H+ antiporter functioned as a “pH regulator” over the pH range from 6.0 to 9.0. Marine bacteria are unique in having the respiratory chain-driven Na+ pump. The site of energy coupling was assigned to the Na+_dependent NADH-quinone reductase segment, which was composed of three subunits (α, β, γ) and contained FAD (β) and FMN (α) as cofactors. Since the respiratory chain of marine bacteria also generates proton-motive force, both Na+ and H+ are utilized as energy-coupling ions. Energy coupling by the Na+ circulation is of great advantage to the energetics of marine bacteria. These examples represent the importance of membrane functions in the microbial adaptations to environmental changes.
