Selected Alien Macroalgae Species from Madeira Archipelago as a Source of Sustainable Antifungal and Elicitor Agents: A Review on Their Valorization Potential and Green Extraction Approaches

Non-indigenous or alien macroalgae are increasingly recognized as ecological threats, sources of raw material, and reservoirs of bioactive compounds for industry and agriculture. This review analyses the valorization potential of this biomass, focusing on their antifungal and elicitor activities against phytopathogenic fungi, particularly Mediterranean (De Bary) Whetzel, 1945. The literature published since 2020 was retrieved from Scopus using targeted keyword combinations. Three major topics were examined: (i) invasive and beach-cast macroalgal and their ecological context, (ii) antifungal and elicitor properties of macroalgal extracts, and (iii) the use of deep eutectic solvents (DES) for the green extraction of bioactive compounds. Species such as Asparagopsis armata, Rugulopteryx okamurae, and Sargassum muticum have shown promising antifungal and elicitor effects, frequently associated with phenolic compounds and polysaccharides. Extracts from these algae can inhibit the growth of fungi or activate plant defense pathways, providing environmentally friendly alternatives to synthetic pesticides. Moreover, DES and natural DES (NADES) offer tunable, biodegradable solvents capable of efficiently extracting these bioactive molecules while reducing the environmental impact associated with conventional organic solvents. Overall, the valorization of this biomass represents a sustainable strategy that simultaneously mitigates ecological and economic impacts and contributes to the development of sustainable inputs in agriculture.