The heterocysts of blue-green algae II. Details of ultrastructure

Abstract

Heterocysts of Anabaena cylindrica in intact filaments or separated from vegetative cells were examined using improved methods of fixation with glutaraldehyde followed by potassium permanganate or osmium tetroxide. The heterocyst envelope, formed external to the outer layer (L IV ) of the pre-existing cell wall, is composed of three distinct layers, i.e. the inner laminated, the middle homogeneous and the outermost fibrous layers. These layers are absent at the narrow point of contact between heterocyst and vegetative cell. Here the septum appears to be traversed by microplasmodesmata interconnecting the plasmalemmas of the heterocyst and adjacent vegetative cell. In addition to the characteristic reorganization of thylakoids an extensive membrane elaboration was observed in the heterocysts. This takes the form of dense layering and, frequently, coiling and leads to a marked increase of the membrane surface area within the cell. The heavily contorted membranes beneath the pore channel undergo some change in structure resulting in an appearance of membrane disintegration and in the gradual accumulation of an electron opaque material within the pore channel. The possible significance of the observed structural changes is discussed in relation to the suggested role of heterocysts in nitrogen fixation and to the interrelations between heterocysts and vegetative cells.