Ammonia uptake and retention in some cyanobacteria

The internal pool of ammonia in strains of unicellular and filamentous cyanobacteria was found to be 6-12 nmol·mg^-1 protein. In nitrate grown Anacystis nidulans R-2 the pool size averaged 12 nmol·mg^-1 protein, which corresponds to 2.3 mM, and was little affected by N-source or medium pH during growth. Cells from NH₄⁺-limited continuous culture contained comparable pools, and cell yield was independent of medium pH (7.2-8.5). The internal pool was not bound to macromolecules. The pool fell transiently to about one-third within 2 h on shifting cells to N-free medium, but was slowly regenerated over 24 h. Added ammonia was removed from solution by illuminated cell suspensions at a linear rate, adequate to supply biosynthetic needs, to residual concentrations less than 5 μM. An apparent K_m of less than 1 μM can be inferred. Uptake rates were independent of N-source during growth, and of assay pH over the range 6.2-8.7. Bicarbonate was needed for uptake, but the rate of uptake was not influenced by the simultaneous presence of NaNO₃ (10 mM) or CH₃NH₃Cl (0.15 mM). Uptake was energydependent, and was eliminated in dark, anaerobic conditions or by the addition of protonophores. Uptake was also strongly inhibited by dicyclohexylearbodiimide, an ATPase inhibitor, by - SH reagents and methionine sulfoximine, suggesting that interference with energy supply or with ammonia metabolism prevented further entry into the cells.