Transport of spin-labelled molecules through the capsule of Nephrocytium lunatum (Chlorococcales) studied by electron paramagnetic resonance

Abstract

The role of the polysaccharide capsule of the microalga Nephrocytium lunatum was investigated in the selective permeability and transport of molecules into the cell. We used hydrophobic spin-labelled molecules and electron paramagnetic resonance (EPR) to investigate the permeability of the capsule. The natural intracellular metabolism converts the spin labels into EPR-silent molecules, providing methodology for the study of molecular transport across the capsule, cell wall and membrane. The diffusion of the spin-labelled molecules across the polysaccharide capsule, cell wall and membrane regulates the decay time of the EPR signal. To investigate the effect of the polysaccharide capsule, the decay time of the EPR signal was monitored in algae with a complete capsule or with the capsule removed by sonication. The spin labels Tempo, Tempo1, Tempone, Tempamine, Maleimide and Phenyl-Imidazoline were used. The decay times, obtained from the cells without a capsule, were faster than those measured for intact cells. A mechanism responsible for the interaction between spin labels and the polysaccharide capsule is proposed. Results suggest that the capsule plays an important role in cell selectivity, probably as a consequence of its polar interactions with the spin labels.