Unravelling the multi-scale structure of cell walls from edible seaweed species and the impact of processing treatments

Abstract

A sequential protocol combining solvent and mechanical treatments, originally designed for land plants, was adapted to concentrate the cell walls of two edible seaweed species, Ulva lacinulata and Porphyra dioica (Nori). However, complete isolation of the cell walls without altering their natural structure was challenging due to strong interactions between polysaccharides and other components, such as proteins. Instead, the process concentrated the cell walls, resulting in polysaccharide contents of 61–63 %. In Ulva, cellulose was identified as the main structural component, while in Nori, porphyrans constituted the semi-crystalline, microfibrillar backbone, closely resembling cellulose in arrangement. These structural variations affected how the cell walls responded to mechanical (ultrasounds, US) and thermal (steam cooking) treatments, as evidenced by the advanced structural characterisation of the samples, combining Small Angle X-ray Scattering (SAXS) and X-ray diffraction (XRD) with microscopy and monosaccharide analyses. In Ulva, steam cooking preserved cell wall integrity while releasing intracellular components, whereas US disrupted the crystalline cellulose structure, causing significant cell wall breakdown and the release of proteins and polysaccharides. For Nori, steam cooking promoted protein release and the migration of some polysaccharides into the extracellular matrix, while US altered the crystalline structure of porphyrans but preserved cellular integrity due to the dense, gel-like extracellular matrix. These results highlight the importance of the composition and structure of the cell wall in seaweeds in determining their mechanical integrity and transformation responses.