Ammonia translocation in cyanobacteria

Ammonia is the inorganic nitrogen source preferentially used by most cyanobacteria. Moreover, even in nitrate utilizers or N₂-fixers, ammonia is an obligate intracellular intermediate in nitrogen assimilation. It also affects the synthesis and the activation of several key enzymes in nitrogen metabolism such as nitrogenase, glutamine synthetase and nitrate reductase. The mechanism by which NH₃/NH₄⁺ enters different cyanobacteria (N₂-fixers, non-fixers, neutrophilic) was thus studied. Using ¹⁴CH₃NH₃⁺, the convenient radioactive analog of ammonium, we have shown that the neutrophilic A. nidulans R-2 possibly possesses an active transport system for this cation. The conditions leading to repression and depression of this transport system have been studied; it appears that de novo protein synthesis is required for the acquisition of the transport ability. We have also provided evidence that methionine sulfoximine affects ammonium uptake only through its inhibition on glutamine synthetase, and found no support for the possible interaction of this inhibitor with the ammonium transporter. In the alkalophilic cyanobacterium Spirulina platensis, an active mechanism to translocate ammonium is probably not needed. Our data suggest that net uptake of ammonia to support optimal growth could be explained by a pH driven diffusion process. In all the different strains tested net ammonia uptake was observed only when conditions permit continuous amidation through the activity glutamine synthetase and inhibition of this enzyme by methionine sulfoximine caused the excretion of ammonia to the external medium. The weight of experimentation suggests that ammonia leaks from cells because of the inherent permeability of NH₃, and that no specific carrier is involved in its release.