SELECTIVE PERMEABILITY OF THE EXTRACELLULAR ENVELOPE OF THE MICROALGASPONDYLOSIUM 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, Desm idiaceae) in the selective permeability and transport of molecules int o the interior of the cell. We have used the electron paramagnetic res onance (EPR) technique applied to a variety of spin labels of a hydrop hobic nature with different substitutents on the ring (-OH, =O, -N=C=S , -NH3+, and others). The spin label EPR signals were destroyed as a c onsequence of metabolic processes once the spin probes had entered the cells. The decay time of the EPR signal was regulated by the diffusio n 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 u ltrasonic treatment. The decay times for the cells without capsule wer e faster than those for intact cells, and a possible mechanism of inte raction involving hydrogen bonds between the spin labels and the -OH g roups of the polysaccharide sheath is presented. These were expressed by their diffusion and function coefficients as derived from Ficks' Se cond 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 coef ficients (f(1) and f(2)), the order was inverted. These results sugges t the capsule plays a role in selectivity as a result of polar interac tions with the spin labels.