Algal biopolymers: Diversity of charophytic and chlorophytic cell walls

Charophytic green algae are an under-explored, widely diverse division of the streptophytes. Exploring their polysaccharides’ properties could shine light on new under-exploited biomaterial sources. Initial experiments were performed on both early and late-diverging charophytic species; plus a bryophyte (Anthoceros caucasicus) and a chlorophyte (Ulva linza). They aimed at extracting and analysing, via classical ‘land-plant methods’, the different polysaccharide fractions (conventionally described as pectin, hemicellulose, and cellulose) present in algal cell walls. The bryophyte and the late-diverging charophytes (Chara vulgaris, Charales and Coleochaete scutata, Coleochaetales) showed similar features, both in terms of extractability and sugar residue compositions, to land plants. All the species were screened for the presence of the land-plant-specific polymers xyloglucan and rhamnogalacturonan-II, alongside species from the late-diverging charophytic order Zygnematales, via in vivo 14CO2 labelling followed by enzymatic hydrolysis of their cell walls. Xyloglucan-like oligomers were visible in zygnemataleans, but the key dimer isoprimeverose could not be conclusively detected. An RG-II-like polymer was present in the cell walls of axenic Chara (Charales). During the initial experiment, early-diverging charophytes presented distinctive characteristics. Upon further characterisation, the extractability of the uronic acid-free ‘pectic’ polysaccharide in Klebsormidium was described. Some sections of the polymers were characterised, namely rhamnoxylan and galactoxylan. The basal species Chlorokybus showed the presence of the previously unknown dimer β-D-GlcpA-(1→4)-L-Gal. Its ‘pectic’ fraction was found to be sulphated, contained L-Gal but not D-Gal, and was made up of two distinct polymers, different by their ionisation degree, sulphation degree, and susceptibility to hydrolysis. Finally, an investigation on the seaweed Ulva was conducted. In particular, the ‘hemicellulosic’ fraction was characterised using biochemical and spectroscopic methods: phyco-xyloglucan was found to be a linear β-(1,4)-polymer of glucose and xylose, featuring xylose stretches up to four residues long, and with a much greater affinity for cellulose than that of many well-known land-plant-specific polymers. Overall, this work indicates the presence of previously unidentified polysaccharides in algal cell walls, making the harnessing of such widely available biomass possible and desirable.