Biosynthesis of the Cell Walls of the Algae

Abstract

Algae constitute a large and diverse array of photosynthetic eukaryotes that are common to most of modern earth’s photic zones. Algae are critical to global primary production, CO₂ sequestering and biomineralization as well as being of economic significance in the food, pharmaceutical and biofuel industries. The extracellular matrix of algal cells, most notably the cell wall, is of fundamental significance to survival and often serves as the major product of photosynthetic carbon fixation. The cell walls of most algae consist of a framework of fibrillar polysaccharides that are embedded in a matrix composed of neutral and charged polysaccharides along with various proteins, phenolics and complexed cations. The fibrillar components include mannans, xylans and most notably, cellulose, whose synthesis occurs in membrane-bound enzyme complexes. These load-bearing fibrillar components are inserted in complex networks of polysaccharides that include hemicelluloses and polyanionic polymers such as pectins, alginates, fucoidans and the sulfated galactans of red algae, agar and carageenan. These polymers are synthesized in the Golgi Apparatus, transported to cell surface sites via actin- and tubulin-based motors and deposited in the wall complex. Often, post-secretory crosslinking with cations alters the structural architecture of these polymers that, in turn, influences the strength and function of the cell wall. Proteins are also found in algal cell walls including highly glycosylated and hydroxyproline-rich forms, some of which have structural semblance to extensins and arabinogalactan proteins of land plants. Modern molecular and immunobinding studies are now probing the specific mechanisms in wall development including modulations that occur during morphogenesis and in response to environmental triggers.