Ammonia uptake in the alkalophilic cyanobacterium Spirulina platensis

Ammonia uptake was studied in the alkalophilic cyanobacterium Spirulina platensis. In continuous cultures under optimal growth conditions ammonia supported optimal growth (doubling time of 9.3 h), causing a reduction of glutamine synthetase activity to 25% of that found in cultures grown on NO₃. Long term (20 min) ammonia uptake assays were performed to study the dependency on metabolism: 1) Ammonia uptake proceeded at the same rates in the light and in the dark, the pH dependency pattern correlating with light-dependent O₂ evolution and dark O₂ consumption. 2) The uptake of ammonia was pH dependent with an optimum at pH 9.3. 3) The uptake was totally dependent upon the activity of glutamine synthetase and was completely inhibited by methoinine sulfoximine. To study the mechanism by which NH₄⁺/NH₃ enters the cells, short term experiments (up to 1 min) were performed at pH 7.0 and pH 10.0: At pH 7.0 the uptake was slow and at a constant rate. At pH 10.0, the uptake did not saturate even at 1 mM ammonia and the kinetics were biphasic, consisting of a fast component lasting less than 5 seconds and of a subsequent slower component. The fast phase was insensitive to methionine sulfoximine, whereas the slower phase was completely inhibited by this compound. We suggest that under optimal (alkaline) pH the entry of ammonia into Spirulina cells is likely to be a δpH driven diffusion process, continuously supported by its intracellular assimilation.