Cellulose microfibril assembly and orientation in some bangiophyte red algae: relationship between synthesizing terminal complexes and microfibril structure, shape, and dimensions

Abstract

As shown by freeze-fracture electron microscopy, the linear terminal complexes (TCs) in Erythrocladia subintegra Rosenvinge, Erythrotrichia carnea (Dillwyn) J. Agardh, Porphyra leucosticta Thuret, and Porphyra yezoensis Ueda typically have regularly spaced rows of particles aligned transverse to the longitudinal axis of the TCs. The spacing between neighboring transverse particle rows is almost constant and ranges from 10.5 to 13.5 nm among the four species. The TC subunits appear to be interconnected by filament-like structures in transverse as well as longitudinal directions along the long axis of the TC. In the investigated species, both linear TCs and microfibrils are randomly distributed and the microfibrils show a flat, ribbon-like morphology. Isolated cellulose microfibrils in negatively stained preparations from Erythrocladia and Porphyra appear as thin, ribbon-shaped structures, 1 to 1.5 nm thick (constant) and 5 to 70 nm wide (variable). It is proposed that each protein particle subunit in an Erythrocladia TC synthesizes three glucan chains that are then assembled into a minisheet. Each transverse row of four TC subunits contributes to a fibrillar (minicrystal) component containing 12 glucan chains, with dimensions of 1.20 × 1.59 nm. The number of fibrillar (minicrystal) components assembled by a given TC and then laterally associated equals the number of transverse particle rows. The laterally associated minicrystal components contribute to the variation in microfibrillar width. The arrangement and consolidation of polymerases (cellulose synthase) and variation in their number in a single linear TC (Erythrocladia 32 to 140 subunits, Erythrotrichia 30 to 110, Porphyra yezoensis 11 to 25, Porphyra leucosticta 6 to 24) are responsible for the structure and flat, ribbon-like morphology of the microfibrils and for the variation in the width/diameter of the cellulose microfibrils. In red algae, cellulose occurs as highly oriented crystalline microfibrils.