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JOURNAL OF PLANKTON RESEARCH | VOLUME 11 | NUMBER 2 | PAGES 297-315 | 1989
© Oxford University Press

research-article

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

J. W. Rijstenbil and J.J. Sinke

Delta Institute for Hydrobiological Research Vierstraat 28, 4401 EA Yerseke, The Netherlands

Received on March 21, 1988; accepted on December 6, 1988 Ammonium-limited cultures of Skeletonema costatum were grown at dilution rates from 0.019 to 0.038 h–1 at an average salinity of 22.4{per thousand} For a few days cultures were exposed to a freshwater pulse. When salinity was decreased to 8.6{per thousand} (average minimum) photosynthesis and cell division were inhibited. Both in vivo and DCMU-enhanced fluorescence per cell were statistically constant: photosystems I and II were not inhibited by a gradual salinity decrease. Ammonium assimilation was affected via an inhibition of carbon fixation. Ammonium concentrations increased in the continuous cultures, whereas the overcapacity of ammonium uptake declined: the nitrogen limitation was relieved. When salinity was increased again, photosynthesis and cell division were stimulated. Salinity fluctuations were accompanied by a fluctuation in the pools of aspartic acid (0.5–1.0 mM), glutamine acid (0.9–4.1 mM) and glutamine (0.5–2.0 mM). The pool of glutamic acid followed the salinity pattern (r=0.67, P<0.05). The correlation between the amino acid pool and the osmotic value of the medium was significant (r=0.72, P<0.05). Cellular glutamic acid and glutamine levels increased until the nitrogen limitation was restored.


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