Non-cellulosic structural polysaccharides in algal cell walls - III. Mannan in siphoneous green algae

Abstract

The cell walls of a number of green seaweeds, all members of the Codiaceae and the Dasy-cladaceae and including Codium and Acetabularia , are shown to contain β -1.4-linked mannan as the sole crystalline polysaccharide in the complete absence of cellulose. The X-ray diagram of the native mannan (almost identical with that of the mannan of ivory nut and of other palm-seed endosperms) has been indexed to an orthorhombic unit cell a = 7.21 Å, b (fibre axis) = 10.27 Å, c = 8.82 Å. After treatment with alkali solutions the mannan recrystallizes in a different lattice; by analogy with cellulose we propose to name this form mannan II and the native mannan, mannan I. The lamellated walls of the central siphon of some of these algae (including Dasycladus , Batophora and Cymopolia ) may be separated into two layers. X-ray diffraction analysis and polarization microscopy show that the mannan crystallites of the outer layer tend to lie transversely to the siphon axis, with some dispersion, while those in the inner layer lie longitudinally. The inner layers therefore yield good X-ray fibre diagrams from which a provisional structure of mannan I has been derived. It has proved impossible to reveal in the electron microscope, by the techniques used, the presence of true microfibrils in these plants even when the mannan is well oriented. Electron microscope images of carbon replicas reveal at most the appearance of short rodlets some 100 Å wide. The outer and inner layers resemble respectively the primary and secondary wall layers of higher plants. Some peculiar growth habits of members of the Dasycladaceae are discussed in terms of wall architecture.