Hesham M. Shafik

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.

Some special morphological features of Cylindrospermopsis raciborskii in batch and continuous cultures

Morphological characteristics of Cylindrospermopsis raciborskii were investigated in batch culture under P-starvation and in continuous cultures at two growth rates with different N forms. The species displayed an enormous morphological variability under controlled condition. In the P-starved batch culture very short and extreme morphology trichomes with conical or drop-shape heterocytes were formed. In continuous culture extremely long, twisted filaments were found near the maximum growth rate. Rarely, C. raciborskii formed intercalary heterocytes.

Relative nitrogen deficiency without occurrence of nitrogen fixing blue-green algae in a hypertrophic reservoir

The N:P ratios (TN:TP, NO3-N:TP and DIN:SRP) in theshallow,hypertrophic Marcali reservoir varied about 1, far below thephysiological optimum for the planktonic algae. Moreover,N2-fixing cyanobacteria were absent in the vegetationperiod.One possible explanation for this phenomenon could be thefiltration of the stocked silver carp (Hypophthalmichthysmolitrix Val.), a phytoplankton feeder, and bighead carp(Aristichthys nobilis Rich.), an omnivorous plankton feederpopulation (600 kg ha−1) which strongly reduces the largesized algae.Enclosure and uptake experiments were carried out to confirmtherelative N deficiency in the reservoir and, by excluding thefish,to check whether the reservoir water is suitable for thedevelopment of N2-fixing cyanobacteria, or theplanktivorousfish are responsible for the lack of blue-greens. Theenclosureswere free of fish. In one of them the NO3-N enrichmentresulted in a rapid growth of diatoms and cryptophytes,confirmingthe relative N limitation of the phytoplankton biomass. Theresultsof 15N uptake experiments showed a high preference ofalgaefor ammonium. Its actual uptake velocity (19.5 µg l−1h-1) was much higher than that for nitrate (1.0 µgl-1 h−1). But the relatively high value of thehalf-saturation constant (32 µg l−1) in the ammoniumuptake experiment indicated only a moderate affinity of algaeevento the preferred nitrogen source. The other enclosure wasinoculated with N2-fixing cyanobacteria. During the 29daysexperiment they did not grow in the enclosure and weregraduallyeliminated from the water column. The results of enclosureexperiments suggested that factors such as the relatively highabundance of inorganic N (30–50 µg l−1 ammoniumand90–140 µg l−1 nitrate, respectively) could beresponsiblefor the absence of N2-fixing cyanobacteria in thereservoir,rather than the size selective filtration of planktivorousfish.