Cell wall configuration and ultrastructure of cellulose crystals in green seaweeds

Green seaweeds (Chlorophyta) are often considered a nuisance, but may in fact constitute a potential climate-friendly renewable resource in the new bioeconomy. Utilization of green seaweed polysaccharides could in particular enable a new type of green growth in coastal regions in developing countries e.g. in West Africa. The carbohydrate constituents, including cellulose, are lignin-free, but only limited knowledge is available on the cellulose ultrastructure and cellulose crystallinity in commonly occurring green seaweeds. Abundantly available green seaweeds along the coast in West Africa include Chaetomorpha linum, Caulerpa taxifolia, Ulva fasciata and Ulva flexuosa. Here, using X-ray diffraction (XRD), on cellulose gently purified from the green seaweeds, we determined that mainly cellulose I_α was present in both C. linum and U. flexuosa. We also found that the crystallite diameter was particularly large, 16 nm, in both the Ghanaian and the Danish C. linum samples, especially compared to the crystallite diameter of 2.5–5 nm in most terrestrial vascular plants (Tracheophytes), which mainly contain cellulose I_β. For U. flexuosa, increasing crystallinity of the cellulose was achieved during purification. Cellulose purification was not achievable for C. taxifolia, indicating that in this species the glucose is mainly a constituent of other carbohydrates, presumably notably of starch. Transmission electron microscopy (TEM) confirmed the large size of the crystallites of C. linum showing the cellulose forming a multi-lamellar structure with alternating perpendicular and parallel layers along the fibre axis. The cell wall nano-structure appeared amorphous in C. taxifolia and as a network in U. fasciata and U. flexuosa. As glucose was most abundant in C. linum and its cellulose was crystalline, this green seaweed species appears particularly promising as a source for manufacturing of novel cellulose-based materials, e.g. specialized paper or cellulose-based composite materials.