| Abstrakt: |
The rhythmic changes in NH4+ concentration previously observed in an eutrophic shallow estuary under conditions of growth limitation, NH4+ addition and dominance of cyanobacteria were studied in laboratory cultures of the cyanobacterium Microcystis firma (Breb et Lenorm.) Schmidle, strain Gromov/Len. 398. The rhythm was induced in M. firma by means of carbon limitation and light/dark alternation, and was characterized by a decrease in NH4+ in the growth medium during the first hours of the light phase and an increase in NH4+ in the late light phase lasting until the end of the dark phase. Inhibition of the glutamine synthetase/glutamate synthase pathway [darkness, lack of C skeletons, inhibitors such as L-methionine-D,L-sulfoximine (MSX)] brought about NH4+ release by the cells. From the various processes generating NH4+ intracellularly (active NH4+ uptake, NO3- reduction, protein and amino acid degradation, photorespiration), NO3- reduction appears the most important one. This has been concluded from experiments with and without nitrate and/or ammonium in the growth medium and from experiments with 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) and MSX. NO3- was also taken up and reduced in the presence of external ammonium, probably because of the C limitation under otherwise optimal conditions for photosynthesis. It appears, moreover, that glutamine restricts nitrate uptake during the first hours of the tight phase hut less in subsequent light periods, when carbon limitation might he more severe. The present results may help explain previous observations under natural conditions, provided that nitrate is available in the water samples. Furthermore, this system enables the occurrence of a new, cheaper and more efficient method of photoproduction of ammonium from nitrate. [ABSTRACT FROM AUTHOR] |
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