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 mM 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 deteriorate. O₂ evolution stopped, the absorbance at 620 nm corresponding to phycocyanin decreased, and the cells lysed within 1 h, a process accelerated by light. The activity of photosystem II, but not that of photosystem I, was affected in the process, which was irreversible unless sodium was readded within 15 min from the onset of the deprivation. The effect was mimicked, even in the presence of sodium, by the ionophore nigericin. We suggest that the cascade of events leading to cell lysis is primarily due to the inability of S. platensis to maintain a proton gradient (acid inside), possibly due to inactivity of a sodium/proton antiporter, as demonstrated for other alkaliphiles.