Selective permeability of the extracellular envelope of the microalga Spondylosium panduriforme (Chlorophyceae) as revealed by electron paramagnetic resonance

The aim of this work was to investigate the rob of the polysaccharide sheath of the microalga Spondylosium panduriforme (Chlorophyceae, Desmidiaceae) in the selective permeability and transport of molecules into the interior of the cell. We have used the electron paramagnetic resonance (EPR) technique applied to a variety of spin labels of a hydrophobic nature with different substitutents on the ring (-OH, =O, -N=C=S, -NH3+, and others). The spin label EPR signals were destroyed as a consequence of metabolic processes once the spin probes had entered the cells. The decay time of the EPR signal was regulated by the diffusion mechanism across the polysaccharide sheath, cell wall, and membrane. To discriminate the effect of the polysaccharide sheath from that of the cell wall and membrane, the polysaccharide sheath was removed by ultrasonic treatment. The decay times for the cells without capsule were faster than those for intact cells, and a possible mechanism of interaction involving hydrogen bonds between the spin labels and the -OH groups of the polysaccharide sheath is presented. These were expressed by their diffusion and function coefficients as derived from Ficks' Second Law and the Einstein-Stokes equation and were summarized in terms of diffusion coefficients (D-1) for the capsule medium in the order: =O < -OH < -phe < -H < -N=C=S; and for cell wall and membrane (D-2): -OH < -H < =O < -NH3+ congruent to -phe < -N=C=S. For the friction coefficients (f(1) and f(2)), the order was inverted. These results suggest the capsule plays a role in selectivity as a result of polar interactions with the spin labels.