Exploration of the extracellular matrix of the red alga Chondrus crispus reveals unprecedented insights into carrageenan structures.

Carrageenans are major gel forming polysaccharides in the extracellular matrix of the red macroalga Chondrus crispus. These galactans are made of linear chains of repetitive disaccharide motifs based on d-galactose residues alternately linked by β-1,4 and α-1,3 glycosidic bonds. A definite number of disaccharide motifs are known, based on their regular sulfations and the presence of a 3,6-anhydro bridge. While these motifs are variable as a function of species, life cycle phases, or seasons, our understanding of the in vivo regulation of carrageenan fine structures is still limited. Characterized hydrolytic enzymes (κ-, ι- and λ-carrageenases) are powerful tools for identifying glycan structures in extracted ECMs. Their use, combined to chromatography and high-resolution mass spectrometry, allowed us to refine our understanding of carrageenan variability in the phases of C. crispus. We provide the first demonstration that κ/ι carrabiose motifs are not limited to gametophytes, but are also present in tetrasporophytes, together with over- and under-sulfated λ-motifs. Our findings highlight a more complex carrageenan composition than previously described in this model system. These results are further discussed in the light of recent transcriptomic data and suggest that the historical hypotheses on the biosynthetic pathway of carrageenans in red algae may need revision. HYPOTHESIS: Combined characterized hydrolytic enzymes to chromatography and high-resolution mass spectrometry can refine our understanding of carrageenan variability in the phases of C. crispus.