Supattra Maneein

Sargassum muticum is an invasive species to the coasts of the British Isles, mainland Europe and North America, with negative ecological and socioeconomic impacts. Pelagic Sargassum inundations on the beaches of the Caribbean have also been causing adverse health, ecological and economic effects. The finding of commercial uses of these biomasses may alleviate the costs of removal and control. Both pelagic Sargassum and S. muticum could be low-cost biosorbents for removing aqueous cationic dyes but may not be suitable for anionic substances without modification. This study found that a Sargassum biomass could remove up to 93% of methylene blue and that the species, concentration and treatment (CaCl₂) were all statistically highly significant factors (p < 0.001) in its removal.

The Turks and Caicos Islands (TCI) have been affected by sargassum inundations, with impacts on the economy and environment. Sargassum removal can be costly, but sargassum use and valorisation may generate income and offset environmental damage. A significant barrier to the valorisation of sargassum is insufficient knowledge of its chemical makeup, as well as its seasonal variation and decay after stranding. The chemical characterisation of mixed sargassum and its constituent species and morphotypes (S. natans I, S.natans VIII and S. fluitans) collected from TCI between September 2020 and May 2021 and changes in the composition of sargassum decaying (over 147 days) were studied. High ash (24.61–51.10% dry weight (DW)) and arsenic (49–217 mg kg⁻¹) could severely hamper the use of this seaweed for food or feed purposes. Although there was some reduction in arsenic levels in decaying sargassum, levels remained high (>49 mg kg⁻¹). Biomethane production by anaerobic digestion (AD) is a potential option. Nevertheless, the exploitation of sargassum for biogas, either fresh or as it decays on the beach, is challenging due to low methane yields (<42% of theoretical potential). Pre-treatment or co-digestion with other waste may be options to improve yield. The metal sorption ability of sargassum, which can be problematic, makes biosorption of pollutants an option for further research.

The Caribbean has been experiencing beach inundations of pelagic Sargassum, causing environmental, health and financial issues. This study showed variations in the composition and methane potential (MP) between the species of Sargassum. The MPs for S. natans VIII, S. natans I and S. fluitans (145, 66 and 113 mL CH₄ g⁻¹ Volatile Solids) were considerably below theoretical potentials, possibly due to the high levels of indigestible fibre and inhibitors. The mixed mats Sargassum composition was substantially different from the individual species, being higher in ash, calcium, iron, arsenic and phenolics. The mixed mats produced no methane, perhaps due to the high levels of phenolics. There was a strong correlation between MP and phenolic content. Heavy metals and metalloids were at levels that should not cause concern, except for arsenic (21–124 mg kg⁻¹ dry weight). Further work on the speciation of arsenic in Sargassum is required to fully determine the risk to health and agriculture. Both protein and lipid levels were low. The ‘indispensable amino acid’ profile compares favourably with that recommended by the World Health Organisation. Lipids had a high proportion of Polyunsaturated Fatty Acids. The use of Sargassum for biogas production could be challenging, and further work is required.