Non-cellulosic structural polysaccharides in algal cell walls - II. Association of xylan and mannan in Porphyra umbilicalis

Abstract

The structure of the walls of the red alga Porphyra umbilicalis , a member of the Bangiaceae, has been examined by chemical and physical methods, and observations have been made on Bangia fusco-purpurea sufficient to establish that there is no essential difference in wall structure between these two algae. Mannan and xylan, which are the two major skeletal polysaccharides of these algae, are found to be spatially segregated within an individual plant. The cell walls proper contain microfibrils of β -1,3-linked xylan identical with the microfibrils found in certain siphoneous green algae and constituted therefore of parallel arrays of double-stranded helices. The incrusting substances probably include mannan and xylan. The cuticle has been shown to consist predominantly of mannan (with no or little xylan) which becomes crystalline only after treatment such as extraction with hot water. The crystalline mannan is β -1,4-linked and appears identical with the mannan which forms the walls of still another group of siphoneous green algae. No evidence has been found for any structure such as microfibrils in the cuticle; it shows at best only a granular appearance. The cell walls are clearly lamellated; they merge gradually into the intercellular material (mainly mannan) and this in turn into the dense outer sheath of the plant—the cuticle. The microfibrils of individual lamellaelie at random in all the walls with the exception of the rhizoids. The rhizoids are as a rule narrow, with thick, compact walls in which the microfibrils lie through out parallel to rhizoid length with the short cross-connexions typical of the green algae with xylan walls. The apex of the rhizoids are thin-walled and sometimes dilated. The undilated tips are characterized by a small patch of randomly arranged microfibrils at the extreme apex with a progressive tendency towards longitudinal orientation below. The implications of this structure for the growth of rhizoids is discussed.