A study of dielectric membrane breakdown in the Fucus egg

Unfertilized eggs and zygotes of the marine brown alga, Fucus serratus, have been subjected to single external electric field pulses of 1 to 1760 musec duration (tau p) and 50 to 400 V field strength (Upcm-1). During exposure, the difference in electric potential across the plasmalemma (Vm) was recorded intracellularly from single eggs, and the efflux of 86Rb+(K+) from the cytoplasm was measured on egg populations. A given single pulse instantaneously depolarizes the plasmalemma by a few (i.e., 6) millivolts and releases a certain fraction (i.e., 5%) of the cytoplasmic 86Rb+(K+). The dependence of these responses upon Up and tau p is fully consistent with the assumption that the membrane undergoes a localized reversible dielectric breakdown and reseals within less than 3 sec. The data are treated in terms of the electro-mechanical model for a compressible membrane by H.G.L. Coster and U. Zimmermann (1975, J. Membrane Biol. 22:73) and verify this model on a nonvacuolated plant cell. A threshold Vm for membrane breakdown (Vc) of 0.58 and 0.51 V is estimated for the turgorless unfertilized eggs and the turgescent (4.8 bar) zygotes, respectively. Using these values for Vc, and a reasonable value of the membrane's elastic modulus (i.e., Ym approximately 10(6) Nm-2), possible sites of membrane breakdown are discussed in terms of membrane thickness and relative permittivity.