Studies on the gas-vacuoles of blue-green algae

Abstract

Gas-vacuoles were examined with optical and electron microscopes in species of blue-green algae cultured in the laboratory and collected from the field. Gas-vacuoles were characterised with the normal transmission optical microscope as reddish, pressure-sensitive structures, which possess a lower refractive index than the surrounding cellular contents. They were resolved with the electron microscope into closely packed bundles of gas-cylinders (70.0 nm wide) with a-granules lying in ordered rows between the individual gas-cylinders. The groups of gas-cylinders were found in association with lamellae at all stages of growth of Anabaena flos-aquae D124. Changes in the arrangement of lamellae were reflected in the appearance of the groups of gas-cylinders. Analyses of vacuole gases with the mass-spectrometer indicated the presence of nitrogen. However, the finding that gas-cylinder membranes are freely permeable to gases indicates that the composition of vacuole gases will reflect that of the surroundings. Gas-cylinder membranes and α-granules were isolated from A. flos-aquae D124. The membranes were striated with a periodicity of 5.0 nm and appeared globular in section. The 4.0 nm globules were proteinaceous with molecular weights of 22,000 ± 2,000. Possible homologies of these membranes with viral coat protein are discussed. α-granules were shown by a variety of techniques to be polysaccharide. A. flos-aquae D124 was grown under a variety of different environmental conditions. Two generalisations were apparent from these studies. Gas-vacuoles occupy a greater proportion of cell volume in the stationary phase of growth compared with exponential growth. Gas- vacuoles in cells grown at inhibitory light intensities are restricted in their development to the cell periphery adjacent to cell walls. Some physiological studies were conducted on the effects of the absence of gas-vacuoles on growth of A. flos-aquae D124, and the changes in sedimentation rate of cells associated with gas-vacuole redevelopment. After consideration of a variety of different evidence it was concluded that the main function of gas-vacuoles is to lower the specific gravity of blue-green algal cells.