20] Gas vesicles: Chemical and physical properties

Abstract

This chapter focuses on the chemical and physical properties of gas vesicles. Gas vesicles occur in a wide variety of cyanobacteria. In planktonic species they are often constitutive cell components; elsewhere their production may be restricted to differentiating hormogonia. Gas vesicles provide an organism with hollow spaces which bring about a reduction in cell density, and if enough are accumulated within a cell it will float. Planktonic cyanobacteria regulate their buoyancy with gas vesicles. Gas vesicles are hollow, cylindrical structures closed at either end with a conical cap. The cylinder diameter of gas vesicles varies within rather narrow limits in a given species, but marked differences in mean diameter are found when comparisons are made between some species. Length varies markedly in all species because the gas vesicle grows by elongation of the central cylinder from a biconical initial. Gas vesicles are highly permeable to gases. The outer gas vesicle surface is hydrophilic and the inner surface hydrophobic. The volume of gas vesicle gas space in a suspension of vesicles can be determined by measuring the reduction in the volume of that suspension when the vesicles are collapsed by applying pressure exceeding the maximum Pc. Gas vesicles from different species have different sizes and geometries. This means that the amount of gas space enclosed, and hence light scattered by a given amount of wall material, will vary among species.