Unusual holopelagic Sargassum mass beaching in Northwest Africa: Morphotypes, chemical composition, and potential valorisation.

The rapid proliferation of holopelagic Sargassum spp. in the tropical Atlantic Ocean presents environmental challenges and economic opportunities. In 2022, Senegal witnessed its first significant holopelagic Sargassum beaching event, triggering widespread concern and interest from civil society, industrial sectors, and government. This study represents the first analysis of stranded holopelagic Sargassum's morphotypes and chemical composition in Northwest Africa. We highlight the nature of Sargassum stranding, dominated by S. fluitans III, and describe a putative new morphotype. Compared to most of the studies in the tropical Atlantic, Senegalese Sargassum displayed lower arsenic concentrations (9-29 ppm), higher cadmium levels (9-15 ppm), and increased mercury content (0.47-0.57 ppm). In addition, Senegalese Sargassum showed higher levels of iron (237-1017 ppm) and phosphorus (1300-1772 ppm). The biochemical analysis revealed high total protein levels (15-40 % DW) in Senegalese samples, though further analysis is required to confirm this. Furthermore, variations in biochemical composition within various parts of the Sargassum thallus were observed. The low arsenic content makes the beached Senegalese Sargassum attractive for valorisation and sets it apart from holopelagic Sargassum from all other regions where it occurs. However, caution should be taken regarding the high concentrations of cadmium. Our study highlights promising applications in Senegal and neighbouring countries, particularly in animal feed and agriculture. Noteworthy is the notable palladium content (2 ppm), valuable phenolic compounds, and mannitol, which present additional opportunities for the chemical industry. Our interdisciplinary approach enhances the global scientific understanding of the Sargassum issue. With the anticipation of more frequent Sargassum beaching events and, more generally, for seaweed exploitation, we advocate for inter-governmental African organisations to establish standardised norms for their exploitation. We recommend that the Food and Agriculture Organization/World Health Organization consider incorporating more seaweed in the Codex Alimentarius to facilitate their uses particularly when states deal with algal blooms.