Water relations of gas-vacuolate prokaryotes

Abstract

Gas vesicles can be used to provide an instantaneous measurement of the hydrostatic pressure that exists inside the cells of prokaryotic organisms that possess them. Such measurements, carried out on the blue-green alga Anabaena flos-aquae , have been used to determine its cell-water relations. The cell turgor pressure is determined as the difference between the mean critical pressure required to collapse gas vesicles in cells' suspended in hypertonic sucrose solution and that for the cells suspended in water. The turgor pressure falls slightly as the gas vesicles are collapsed because the volume of the cell contents decreases. The relation between the cell turgor pressure and volume is described by the volumetric elastic modulus, e, which can be determined by measuring the turgor pressure of cells suspended in sucrose solutions of increasing concentration. As the external osmotic pressure is increased, the turgor pressure, which stretches the cell wall, falls and the cell shrinks by losing water. The cell water volume can be calculated from the internal osmotic pressure, which is equal to the sum of the external osmotic pressure and the turgor pressure. The elastic modulus of the A. flos-aquae . cell is quite low, about 11 bar, and constant over a range of turgor pressures down to 1.3 bar. W hen gas vesicles are collapsed inside cells, the turgor pressure suddenly drops, but it partially recovers as water enters to restore the osmotic balance. The rate of entry can be computed from the time course of gas vesicle collapse in cells held under moderate hydrostatic pressures, and from this the hydraulic conductivity, L p , of the cell wall and membrane can be calculated; it is estimated to be about 0.14 μm s -1 bar -1 in A . flos-aquae , with the assumption that there are no errors owing to delay in the elastic contraction of the cell wall. The permeability, k , of the cell to various solutes can be determined by following the rate of turgor pressure recovery as the solutes penetrate the cells from external solutions of 0.15 osm concentration. Two methods are described in which turgor pressure rise is monitored by gas vesicle collapse, one for slowly penetrating solutes (e.g., mannitol) and one for rapidly penetrating solutes (e.g., glycerol). Passive uptake is indicated by exponential kinetics of turgor pressure recovery to the initial value in water, as for sugar alcohols; active uptake results in turgor pressures exceeding the initial value, as observed for K + and glycine—betaine. The biological significance of cell—water relations in gas-vacuolate organisms is discussed.