The influence of salinity fluctuation on the ammonium metabolism of the marine diatom Skeletonema costatum grown in continuous culture

Ammonium-limited cultures of Skeletonema costatum were grownat dilution rates from 0.019 to 0.038 h^–1 at an averagesalinity of 22.4 For a few days cultures were exposed to a freshwaterpulse. When salinity was decreased to 8.6 (average minimum)photosynthesis and cell division were inhibited. Both in vivoand DCMU-enhanced fluorescence per cell were statistically constant:photosystems I and II were not inhibited by a gradual salinitydecrease. Ammonium assimilation was affected via an inhibitionof carbon fixation. Ammonium concentrations increased in thecontinuous cultures, whereas the overcapacity of ammonium uptakedeclined: the nitrogen limitation was relieved. When salinitywas increased again, photosynthesis and cell division were stimulated.Salinity fluctuations were accompanied by a fluctuation in thepools of aspartic acid (0.5–1.0 mM), glutamine acid (0.9–4.1mM) and glutamine (0.5–2.0 mM). The pool of glutamic acidfollowed the salinity pattern (r=0.67, P<0.05). The correlationbetween the amino acid pool and the osmotic value of the mediumwas significant (r=0.72, P<0.05). Cellular glutamic acidand glutamine levels increased until the nitrogen limitationwas restored.