Nitrogen uptake and fixation in the cyanobacterium Cylindrospermopsis raciborskii under different nitrogen conditions.

Ammonium and nitrate uptake and N2-fixation of the heterocystous cyanoprokaryote Cylindrospermopsis raciborskii was examined in continuous cultures under different nitrogen concentrations and dilution rates using the 15N technique. It was found that at luxury phosphorus supply (5 mg PO4-P l-1) the biomass was similar in all cultures irrespective of the amount and portioning (continuous or pulsed) of available nitrogen forms. The added ammonium and nitrate was fully taken up by C. raciborskii and the remaining nitrogen demand was met by N2-fixation. Different ammonium concentrations (300, 750, 1500 and 3000 μg 15N l-1) added at the same dilution rate did not affect the growth of C. raciborskii. In the culture supplied with pulsed ammonium, N2-fixation was detected prior to ammonium addition only. After the ammonium pulse, the N2-fixation continued for a while then decreased and stopped. In addition, the inflowing ammonium was fully taken up by the organism. The rate of nitrogen fixation reached its original level after 8–24 hours, depending on the dilution rate. It can be suggested that the nitrogen fixation system stopped and was then activated again depending on the nitrogen content of the cells. [ABSTRACT FROM AUTHOR]