Arrhenius parameters of phototransients in Halobacterium halobium in physiological conditions.

Abstract

Halobacterium halobium is indigenous to warm saline pools exposed to bright sunlight. Strains of this bacterium synthesise a purple pigment in the cell membrane (the ‘purple membrane’). A single protein, bacteriorhodopsin, is responsible for this characteristic colour, possessing a broad band in the visible spectrum with a maximum at 570 nm (refs 1–3). Pigmented cells utilise solar radiation absorbed by bacteriorhodopsin to create a transmembrane proton gradient which serves as an energy source for ATP synthesis4,5. Most information about the mechanism of operation of this light-driven proton pump has been obtained from spectrophotometric studies of suspensions of purple membrane fragments6–10. We report here a continuation of studies initiated in this laboratory using modulation excitation11 and conventional flash photometry12,13, in which we use the latter technique to observe transient spectral changes in aqueous suspensions of intact cells at physiological temperatures and salinity (basal salt solution).