CELL-WALL STRUCTURE AND SUPRAMOLECULAR ORGANIZATION OF THE PLASMA-MEMBRANE OF MARINE RED ALGAE VISUALIZED BY FREEZE-FRACTURE

The cell-wall structure and the supramolecular organization of the pla sma membrane in 29 species of red algae were studied both in replicas of rapidly frozen cells and in ultrathin sections. Most of the marine red algae investigated have a random distribution of the microfibrils of the cell walls; in a few cases there is a tendency to parallel alig nment. Laurencia obtusa is an exception in which apart from a random d istribution, microfibrils are arranged parallelly in a certain wall la yer. The microfibrils have a cylindric or ribbon-like morphology. In a number of species, microfibrils consist of two, three or four linear subcomponents (sub-fibrils). In certain species two or three microfibr ils can be bundled. In Erythrocladia subintegra, Radicilingua reptans and Laurencia obtusa the plasma membrane exhibits randomly distributed linear microfibril terminal complexes. All results favour the suggest ion that the linear terminal complexes in the plasma membrane of the c ells of the above mentioned species are involved in the biosynthesis, assembly and orientation of microfibrils. In the plasma membrane a num ber of other intramembranous particles are aggregated in various compl exes (tetrads, complexes of six subunits, crystalline complexes, parti cle strings). Intramembranous particle complexes composed of four subu nits 'membrane tetrads' have been observed in the plasma membrane and in the membranes of mucilage sacs of all red algae investigated. The ' membrane tetrads' are thought to be membrane-bound multi-enzyme comple xes participating in the synthesis of the matrix polysaccharides. Obse rvations of ultrathin sections suggest that the Golgi system and the i nflated Golgi-derived vesicles with fibrillar contents contribute to t he formation of the wall. Our results support the view that the biosyn thesis of cell-wall skeletal and matrix polysaccharides in red algae a re spatially separated.