Energetic problems of extremely alkaliphilic aerobes

Abstract

Over a decade of work on extremely alkaliphilic Bacillus species has clarified the extraordinary capacity that these bacteria have for regulating their cytoplasmic pH during growth at pH values well over 10. However, a variety of interesting energetic problems related to their Na +-dependent pH homeostatic mechanism are yet to be solved. They include: (1) the clarification of how cell surface layers play a role in a category of alkaliphiles for which this is the case; (2) identification of the putative, electrogenic Na +/H + antiporter(s) that, in at least some alkaliphiles, may completely account for a cytoplasmic pH that is over 2 pH units lower than the external pH; (3) the determination of whether specific modules or accessory proteins are essential for the efficacy of such antiporters; (4) the mechanistic basis for the increase in the transmembrane electrical potential at the high external pH values at which the potential-consuming antiporter(s) must be most active; and (5) an explanation for the Na +-specificity of pH homeostasis in the extremely alkaliphilic bacilli as opposed to the almost equivalent efficacy of K + for pH homeostasis in at least some non-alkaliphilic aerobes. The current status of such studies and future strategies will be outlined for this central area of alkaliphile energetics. Also considered, will be strategies to elucidate the basis for robust H +-coupled oxidative phosphorylation by alkaliphiles at pH values over 10. The maintenance of a cytoplasmic pH over 2 units below the high external pH results in a low bulk electrochemical proton gradient ( Δp). To bypass this low Δp, Na +-coupling is used for solute uptake even by alkaliphiles that are mesophiles from environments that are not especially Na +-rich. This indicates that these bacteria indeed experience a low Δp, to which such coupling is an adaptation. Possible reasons and mechanisms for using a H +-coupled rather than a Na +-coupled ATP synthase under such circumstances will be discussed.