Akinetes of the cyanobacteriumNostoc PCC 7524: morphological changes during synchronous germination

Following dilution into fresh medium in the light, akinetes ofNostoc PCC 7524 germinated synchronously. Synchrony was maintained at a high level during the first 24 h, at which time the young filaments were composed either of three cells (with N₂ as nitrogen source) or four cells (with NO ₃ ^- or NH ₄ ⁺ ), and at a slightly lower level during the next 24 h of growth. The pattern of cell division was similar in media containing the different nitrogen sources although the timing of the major events varied. In the presence of N₂ or NO ₃ ^- , heterocysts differentiated synchronously; the first developed invariably from a terminal cell of the young filament at approximately 19 h, the second from the other terminal cell after further vegetative cell division. Heterocyst differentiation did not occur in the presence of NH ₄ ⁺ . In the absence of nitrogen (gas phase argon: CO₂) akinete germination initially followed the same pattern as that observed in N₂, this early stage probably occurring at the expense of intracellular reserve materials.During germination, a new laminated layer, similar in structure and position to that found in the heterocyst envelope, appeared in the akinete envelope. This layer was not present in the germinating akinetes of a mutant which was incapable of forming heterocysts.