Schematic representation of algal cell walls.

<p>The cell wall composition differs between various algal groups [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0127499#pone.0127499.ref043" target="_blank">43</a>]. Within the Charophyta (A), the wall is formed of an inner fibrillar layer made of cellulose microfibrils. The fibrillar layer is enmeshed in and surrounded by a middle amorphous matrix of pectin (homogalacturonan, HG, and rhamnogalacturonan I, RGI) that anchors the inner fibrillar cellulose layer to an outer lattice of homogalacturonan. Extracellular polymeric substances or mucilages are also present outside the outer lattice [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0127499#pone.0127499.ref038" target="_blank">38</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0127499#pone.0127499.ref043" target="_blank">43</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0127499#pone.0127499.ref101" target="_blank">101</a>]. Similarly, cell walls of Chlorophyta (B) contain skeletal polysaccharides enmeshed in a matrix. However, the skeletal polysaccharides in Chlorophyta cell walls form double fibrillar layers (inner layer and outer layer) with an amorphous matrix in between. The fibrillar layers vary in composition between cellulose, β-1,3-xylans or β-1,4-mannans or complex heteropolymers, and are rich in hydroproline-rich glycoprotein such as extensins and AGPs. The amorphous matrix polysaccharides are generally in the form of ulvans (e.g. in Ulva species). Brown algal cell walls (C) consist of a fibrillar framework of cellulose microfibrils present in layers parallel to the cell surface but with no clear orientation within each layer. Two such layers are depicted in the figure. All cellulose layers are enmeshed in acidic polysaccharides, e.g. alginates. The interfibrillar matrices are composed of alginates and fucans [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0127499#pone.0127499.ref041" target="_blank">41</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0127499#pone.0127499.ref043" target="_blank">43</a>]<b>.</b>