Water relations of immobilized giant algal cells

Cells of Halicystis parvula, Acetabularia mediterranea, and Valonia utricularis were immobilized in a cross-linked alginate matrix (4-6% w/w) in order to simulate water-relation experiments in individual cells of higher plant tissues. The immobilization of these cells did not lead to an increase in the mechanical stability of the cell walls. This was demonstrated by measuring the volumetric elastic modulus of the cell wall and its dependence on turgor pressure with the aid of the non-miniaturized pressure probe. In immobilized cells, no changes in the absolute value of the elastic modulus of the cell wall could be detected for any given pressure. At the maximum turgor pressure at which non-immobilized cells normally burst (about 3-7 bar for V. utricularis; depending on cell size, 3 bar for A. mediterranea and 0.9 bar for H. parvula) reversible decreases in the pressure are observed which are succeeded by corresponding pressure increases. This obvervation indicates that coating the cells with the cross-linked matrix protects them from rapid water and turgor pressure loss. Turgor pressure relaxation processes in immobilized cells, which could be induced hydrostatically by means of the pressure probe, yielded accurate values for the half-times of water exchange and for the hydraulic conductivity of the cell membrane. The results demonstrate that the water transport equations derived for single cells in a large surrouding medium are valid for immobilized cells, so that any influence exerted by the unstirred layer which is caused by the presence of the cross-linked matrix can be ignored in the calculations. On the other hand, the evaluation of the half-times of water exchange and the hydraulic conductivity from turgor pressure relaxation processes, which have been induced osmotically, only yields correct values under certain circumstances. The model experiments presented here show, therefore, that the correct Lp-value for an individual cell in a higher plant tissue can probably only be obtained presently by using the pressure probe technique rather than the osmotic method. The results are also discussed in relation to the possible applications of immobilized cells and particularly of immobilized micro-organisms in catalytic reaction runs on an industrial scale.