Ultrastructure of a cave-wall cyanophyte-Gloeocapsa NS4

Abstract

Gloeocapsa strain NS4, a cyanophyte (cyanobacterium) which grows in low light levels inside cave entrances, was studied in the electron microscope by thin sectioning and freeze-etching. The cells are surrounded by a microfibrillar sheath divided by ‘dense lamellae’, which are probably an acidic mucopolysaccharide. Inside this is a typical Gramnegative cell wall. Double-replica freeze-fracture showed that the outer envelope of the wall fractures to give two faces each consisting of densely-packed particles; the particles of the outer leaflet seem to consist of subunits arranged in a hollow cylinder. A structural model of the outer envelope is proposed. The plasma membrane fractures to give a PF face with 3000 ∼ 9 nm particles μm^-1 and an EF face with 150–700 11–12 nm particles μm^-1. The thylakoids are arranged in a pattern not previously found in a unicellular cyanophyte, parallel arrays which intersect, and may fuse with, the plasma membrane. The thylakoid membranes have 2,850 particles μm^-1, mean size 10.9 nm, on the PF face and 560 particles μm^-1, mean size 12.3 nm, on the EF face. Phycobilisomes are difficult to see, but may be unusually large. These ultrastructural features may be adaptations to a very low light habitat.