## 2 Molecules, Properties, and Sources
### Ulvan  
Ulvan, a water-soluble anionic polysaccharide, is the short designation of a complex sulfated polymer extracted from the cell wall of _Ulva_ species [37; 38], commonly known as sea lettuce. _Ulva_ is an abundant and ubiquitous green macroalgae, proposed as a model system with phenotypic plasticity [39; 40]. The genus _Ulva_ is adapted to various environmental conditions and could be wild-harvested or cultivated regarding ulvan extraction [41; 42; 43]. The extraction of the polymer occurs from many different locations around the world, including the species: _U. rotudata_[44], _U. urzoriana_[44; 45], _U. rigida_[46; 47], _U. clathrata_[48], _U. fascita_[49; 50; 51; 52], _U. intestinalis_[53], _U. alnoi_[54; 55], _U. lactuca_[56; 57; 58], _U. pertussis_[59], and many others. Despite the dominance of red and brown seaweed production, the particular interest of _Ulva_ is due to its fast growth rates and high ulvan carbohydrate content [40; 60]. Consequently, the biomass of the _Ulva_ species is emerging as a sustainable raw material, with increasing commercial importance and diversification for the bioeconomy [43]. Ulvan is gelling water-soluble cell-wall acidic polysaccharide [61; 62], crosslinked with other components by ionic interactions [38]. While the cell wall polysaccharides represent about 38-54% of the dry algal matter [38], ulvan is the key polysaccharide present in _Ulva_ sp. ranging up to 29-36% of dry weight of the macroalgae biomass [38; 63]. It is a complex marine xyloglucuronorhaman, i.e., a heteropolysaccharide composed mainly of xylose and glucuronic acid and includes rare sugars such as rhamn and iduronic acid within its backbone, conferring a distinct character to the molecule. Variable amounts of other monomers are often reported, such as glucose, arabinose, galactose, mannose [44; 63]. The structure contains a main repeating disaccharide unit corresponding to two forms of aldobiuronic acids, named as ulvanobiuronic acid A and B for simplicity [64]; moreover, it consists of minor units such as aldobioses, named ulvanobioses or type U (Figure 4). The detailed chemical analysis shows a branch in the ulvan main chain and minor amounts of branches containing hexose and unsaturated glucuronic acid residue were found [48; 64]. Figure 4: Structure of the main repeating disaccharides units in ulvan: ulvanobiuronic acids and ulvanobiose. In contrast to land plants that do not produce sulfated polysaccharides, ulvan contains sulphate groups mainly located at C-3 of (1\(\rightarrow\)4)-linked rhamnose residues [64]. Furthermore xylose-2-sulfate are also described [38] and about 4% of the xylose moieties bear sulfate groups in ulvan extracted from _U. clathrata_[48]. The total sulfation content varies widely from 2.3 to more than 28-40% [63] and, according to [PERSON] et al. [42], ulvan from cultivated _U. fascita_ presented stronger signals of sulphate than natural ones. The sulphate group plays an important role, and the degree of sulfation is usually positively correlated with ulvan bioactivities [63]. The sulphate content is related to antioxidant activity [65], but not related to elicitor activities in suspension-cultured cells or infected _Arabidopsis thaliana_ plants [50; 53]. One of the major challenges is the determination of ulvan molecular weight (Mw). Ulvans naturally exhibit a high level of polydispersity and a molecular weight in the range 5.3 \(\times\) 10\({}^{4}\)-1.8 \(\times\) 10\({}^{6}\) g/mol is generally obtained. These values reflect the large variations on the Mw values depending on origin, mode of extraction, different methods and determination conditions. The tendency to form microaggregates contributes to the variation [66; 67]. [PERSON] and co-workers [68] studied how the stabilization treatments of the _U. rotundata_ biomass can impact the ulvan extraction. Frozen and freeze-dried samples yielded polymers with higher Mw that is positively correlated with the gel formation [44]. Molecular weight variations interfere with ulvan biological activities [69; 70].