Light Dependent Ammonium Inhibition of Nitrate Assimilation in Rhodopseudomonas capsulata AD2

Abstract

Abstract Nitrate assimilation by Rhodopseudomonas capsulata AD2 was completely inhibited by ammonium only in young well illuminated cultures. At higher densities (A660 about 0.5) the addition of ammonium had no inhibitory effect, but the nitrate was only reduced to the level of nitrite, which appeared in the medium. Under both conditions the cellular level of nitrate reductase activity remained unaffected. In marked contrast to other R. capsulata strains both ammonium sensitive and insensitive cells could reduce nitrate in the light and in the darkness. In the light up to 90% of the reduced nitrate was assimilated, but in the dark the reduced nitrate was stoichiometrically excreted as nitrite. This behaviour was only shown by R. capsulata AD2 and BK5, while in other strains the nitrate assimilation was always completely inhibited by ammonium. The role of photosynthesis and respiration by the regulation of nitrate reduction is discussed.