A new organelle related to osmoregulation in ultrarapidly frozen Pelvetia embryos

Freeze-fracture electron microscopy of the cortical cytoplasm of unfixed, uncryoprotected, ultrarapidly frozen embryos of the marine brown alga Pelvetia fastigiata has demonstrated the presence of numerous 0.5-μm diameter, disc-shaped vesicles lying adjacent and nearly parallel to the plasma membrane. Some vesicles are fused with the plasma membrane through a narrow connection; this however appears to be a reversible attachment rather than an intermediate stage in the incorporation of the vesicle into the plasma membrane. The distribution of these connections in the plane of the membrane is not uniform; they tend to occur in patches. The fraction of vesicles that is fused with the plasma membrane at any one time appears to be related to a cell's perception of a stressful hypotonic imbalance between the internal and external concentrations of osmotically active compounds. Thus, a sudden 5% decrease in osmolarity of the artificial seawater medium just before freezing leads to a 38% increase in connections per unit membrane area, while a 20% decrease in osmolarity leads to a 75% increase in connections per unit area. Based on these findings and the corresponding ion-transport studies of R. Nuccitelli and L.F. Jaffe (1976, Planta 131, 315—320), we postulate that the disc-shaped vesicles mediate short-term osmoregulation in Pelvetia embryos by reversibly inserting chloride channels into the plasma membrane.