Nitrogen fixation was investigated by means of the acetylene reduction method during the development of a water bloom of Nodularia in coastal waters of the Baltic Sea west of the island of Hiddensee and in backwaters showing different degrees of eutrophication. Depending on plankton density, the values found varied greatly. The maximum of nitrogen fixation values found in extremely dense water blooms under special conditions (Baltic Sea, 2250 pg N2/l * h ; Kleiner Jasmunder Bodden, 374 pg Nz/l * h) are up to 10' times higher than from other parts of the Baltic Sea or from inland waters. The average nitrogenase activity determined for coastal water populations of the Baltic Sea is 2.15 pg N2/heterocyst . h and that of the inmost backwaters 0.77 pg N2/ heterocyst . h. The relationship between N2-fixation and nutrient content in water is discussed.
A 400-900 nm thick slime sheath showing 2-3 layers of different thickness and fibril orientation as well as large amounts of densely packed gas vesicles are characteristic for vegetative cells of Nodulariu spumigena, a cyanobacterium, forming extended water blooms in the Baltic Sea. Comparative SEM and TEM investigations revealed typical screw-like shaped epiphytic bacterial cells anchored in the slime sheath of Nodularia trichomes. Their unique helically sculptured surface is characteristic for the genus Seliberia. Two types of Seliberia cells can be distinguished, regarding cell morphology, formation of apical.spherica1 bodies and their occurrence on host organisms. The main ultrastructural peculiarities concern a helically sculptured membraneous cell wall, a 100-250 nm large, heavily stained inclusion granulum in the cytoplasm of the anchored polar region, and a pad-structure with associated hold-fast material surrounding the polar cap, which is attached to the slime sheath. A permanent subcultivation of pure cultures of Seliberia epiphytes could not be obtained. In samples from natural water blooms, Nodularia cells were found to contain single or numerous gram-negative rod-shaped bacteria, and to show a more or less advanced destruction of the intracellular organization.
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