INTRACELLULAR CYANOBIONT RICHELIA-INTRACELLULARIS - ULTRASTRUCTURE AND IMMUNO-LOCALIZATION OF PHYCOERYTHRIN, NITROGENASE, RUBISCO AND GLUTAMINE-SYNTHETASE

In marine tropical or subtropical plankton the filamentous, heterocyst -forming cyanobacterium Richelia intracellularis forms a symbiosis wit h the diatom Rhizosolenia clevei. An ultrastructural analysis of the a pex of Rhizosolenia clevei showed that the cytoplasm in that particula r part of the cell was present only where the cyanobiont was located. The cyanobiont was, however, always outside the host cytoplasm. Vegeta tive cells as well as the heterocysts of the cyanobiont were devoid of gas vesicles and cyanophycin granules, while carboxysomes and large g lycogen granules were common. The cyanobacterial cell wall apparently remained intact in both vegetative and heterocyst cells. In green exci tation light the heterocysts and vegetative cells emitted a bright yel low fluorescence, indicating that both cell types possessed high conce ntrations of the pigment phycoerythrin (PE) commonly associated with p hotosystem (PS) II. The presence of this pigment in both cell types wa s verified by immunogold localisation. Using the same technique, the n itrogenase (dinitrogenase reductase) enzyme was shown to be exclusivel y present in the heterocysts, while Rubisco was localised primarily to the carboxysomes, which were only detected in vegetative cells. Using an antiserum against the ammonia assimilating enzyme glutamine synthe tase (GS), we could demonstrate very low levels of this enzyme, indica ting repression of GS in the cyanobiont.