Ammonia uptake and retention in some cyanobacteria

Abstract

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 NH 4 + -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.