Biomass composition of the golden tide pelagic seaweeds Sargassum fluitans and S. natans (morphotypes I and VIII)to inform valorisation pathways

Massive strandings of the pelagic brown algae Sargassumhave occurred in the Caribbean, and to a lesser extent, in western Africa, almost every year since 2011. These events have major environmental, health, and economic impacts in the affected countries. Once on the shore, Sargassum is mechanically harvested and disposed of in landfills. Existing commercial applications of other brown algae indicate that the pelagic Sargassumcould constitute a valuable feedstock for potential valorisation.However, limited data on the composition of this Sargassumbiomass was available to inform on possible application through pyrolysis or enzymatic fractionation of this feedstock. To fill this gap, we conducted a detailed comparative biochemical and elemental analysis of three pelagic Sargassum morphotypes identified so far as forming Atlantic blooms: Sargassumnatans I (SnI), S. fluitans III (Sf), and S. natans VIII (SnVIII). Our results showed that SnVIII accumulated a lower quantity of metals andmetalloids compared to SnI and Sf, but it contained higher amounts of phenolics and non-cellulosic polysaccharides. SnVIII also hadmore of the carbon storage compoundmannitol.No differences in the content and composition of the cellwall polysaccharide alginate were identified among the three morphotypes. In addition, enzymatic saccharification of SnI produced more sugars compared to SnVIII and Sf. Due to high content of arsenic, the use of pelagic Sargassum is not recommended for nutritional purposes. In addition, low yields of alginate extracted from this biomass, comparedwith brown algae used for industrial production, limit its use as viable source of commercial alginates. Further work is needed to establish routes for future valorisation of pelagic Sargassum biomass