Ultrastructural studies on the cyanobacteriumNodularia spumigenaand its epiphytic bacteria of the genusSeliberia

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Abstract
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 ofNodularia 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 ofNodulariatrichomes. Their unique helically sculptured surface is characteristic for the genusSeliberia.Two types ofSeliberiacells can be distinguished, regarding cell morphology, formation of apical spherical 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 ofSeliberiaepiphytes could not be obtained. In samples from natural water blooms,Nodulariacells were found to contain single or numerous gram‐negative rod‐shaped bacteria, and to show a more or less advanced destructio of the intracellular organization.