Ulvans are not equal - Linkage and substitution patterns in ulvan polysaccharides differ with Ulva morphology

Ulva are hardy green seaweeds that contain the sulfated polysaccharide ulvan and grow in two distinct morphologies: foliose and tubular. The authors hypothesise that ulvan from tubular species are more structurally complex than ulvans from foliose species. Herein, using standardised methods, the glycosyl linkage positions and sulfate ester substitutions of constituent monosaccharides of ulvan isolated from foliose (U. lacinulata and U. stenophylloides) and tubular (U. prolifera and U. ralfsii) species of Ulva were investigated. Comparison of native ulvans with 80 and 100 degrees C desulfated counterparts indicated that 4-linked rhamnose is predominantly 3-O- sulfated in all four ulvans. Ulvans from the foliose species predominantly contained -*3,4)-Rhap-(1-*, -*4)GlcAp-(1-* and -*4)-IdoAp-(1-*, collectively accounting for 67 to 81 mol% of the total linkages. In contrast, these same linkages in ulvans from the tubular species only collectively accounted for 29 to 36 mol%. Instead, ulvan from tubular species contained a combination of -*2,3,4)-Rhap-(1-*, terminal Rhap-(1-*, -*4)-GlcAp-(1-*, -*4)Xylp-(1-*, and/or -*4)-Galp-(1-* in high proportions; some of the latter three residues were also likely O-2 sulfated. The results presented here suggest that ulvan from foliose species are predominantly unbranched polysaccharides composed of repeat disaccharides while ulvans from tubular species contain a greater diversity of branch and sulfate substitution locations.