The ultrastructure of Anabaena azollae in Azolla pinnata

Abstract

The water fern Azolla pinnata R. Br. was collected in Northwestern India and its structure was examined using scanning and transmission electron microscopy. Emphasis was given to the symbiotic cyanobacterium. Anabaena azollae, and to its relationship to the hairs of the leaf cavities. The cyanobacterial filaments are loosely arranged and often adhere to the protruding hairs and the folded cell walls of the cavities. The vegetative cells show a typical bilayered cell wall. Thylakoids are few and evenly dispersed in mature vegetative cells and appear to lack phycobilisomes. Clusters of polyglucoside granules are distinguished between the thylakoids. Thylakoid membranes are often seen forming whirls and lattices. Polyhedral bodies (carboxysomes) appear particularly frequently in younger cells and in the proximity of polyphosphate bodies. Presence of structured granules, often positioned at both sides of the cross‐wall of neighbouring vegetative cells, suggest a positive nitrogen balance. A high frequency of heterocysts is noted, while spores are not observed.The outer cell wall of the unbranched, mostly two‐celled, hairs shows frequent invaginations. The cytoplasma of the mature hair contains numerous organelles, and is penetrated by an electron transparent network with blebs and vesieles appearing. The exchange of metabolites between the symbiotic partners is discussed in relation to the structures noted.