Thylakoid morphology of the cyanobacteria Anabaena variabilis and Nostoc MAC grown under light and dark conditions.

Anabaena variabilis (ATCC 29413) and Nostoc sp. strain MAC were grown photoautotrophically and chemoheterotrophically. Thylakoid morphology of cells from both types of culture was investigated by freeze-etching electron microscopy. Peripheral and concentric arrays of thylakoid membranes seen in autotrophic cells were replaced, in heterotrophic cells, by numerous vesicular or tubular structures; the changes were reversible. Heterotrophic cells showed a reduced overall content of chlorophyll but started photosynthetic oxygen evolution immediately upon illumination. The results apparently show that in Anabaena and Nostoc the gross morphology of the intracytoplasmic membranes (thylakoid membranes) may depend on growth conditions similarly as is known for the photosynthetic bacteria sensu stricto (Rhodospirillales).