Etude ultrastructurale de la différenciation des hétérocystes chez la Cyanobactérie, Anabaena cylindrica Lemm

Abstract

The differentiation of heterocystes from vegetative cells of Anabaena cylindrica, induced after transfer from a NH4+ medium to a medium without NH4+ under aerobic conditions, was studied by electron microscopy, the material being fixed with osmium tetroxide. The narrow septum separating the heterocyst and the vegetative cell originates from the median region of the normal septum which separates two adjacent vegetative cells. Its peripheral region is transformed into a thickened ring on which rests the complex envelope of the heterocyst. This structure presumably helps to create an anaerobic condition for nitrogenase. At the final stage of heterocyst maturation, the thylakoid membranes are significantly modified. They become flattened while at the previous stages they were expanded and vesiculated. In addition, some of them seem to be closely associated either with the remaining vesicular structures or with the plasmalemma. Polyphosphates bodies, cyanophycin granules, DNA fibres, and polyhedral bodies disappear. The presence of ribosomes, few polyglucosides, and numerous lipid droplets are observed at the end of the differentiation. A possible relationship between these ultrastructural changes and N2 fixation in the heterocysts of Anabaena cylindrica is discussed.