SODIUM DEPRIVATION UNDER ALKALINE CONDITIONS CAUSES RAPID DEATH OF THE FILAMENTOUS CYANOBACTERIUM SPIRULINA-PLATENSIS

Like other alkaliphiles, the cyanobacterium Spirulina platensis (Norst .) Geitler requires sodium to function properly at elevated pH values. At pH 10.0, 150-250 nM Na+ were required for optimal growth, whereas 2.5 mM were sufficient for short-term photosynthetic oxygen evolution. The complete absence of sodium, however, caused S. platensis to deter iorate. O-2 evolution stopped, the absorbance at 620 nm corresponding to phycocyanin decreased, and the cells lysed within 1 h, a process ac celerated by light. The activity of photosystem II, but not that of ph otosystem I, was affected in the process, which was irreversible unles s sodium was readded within 15 min from the onset of the deprivation. The effect was mimicked, even in the presence of sodium, by the ionoph ore nigericin. We suggest that the cascade of events leading to cell l ysis is primarily due to the inability of S. platensis to maintain a p roton gradient (acid inside), possibly due to inactivity of a sodium/p roton antiporter, as demonstrated for other alkaliphiles.