Nutritional potential of invasive Rugulopteryx okamurae, of Gelidium corneum industrial by-product, and of five native Atlantic macroalgae as feedstocks for alternative protein production.

Macroalgae are increasingly considered sustainable resources for protein-oriented food and bioprocessing applications due to their rapid growth, wide availability, low freshwater and fertilizer requirements, and potential as natural sources of proteins and bioactive compounds. However, integrated assessments comparing native, invasive, and industrial side-stream macroalgal biomasses under the same nutritional and safety framework remain scarce, limiting the identification of suitable candidates for protein-oriented valorization. In this context, this study assessed seven Atlantic macroalgal biomasses from Spanish and Portuguese sources: the invasive brown seaweed Rugulopteryx okamurae (R. okamurae), the underutilized Cystoseira usneoides (C. usneoides), an industrial Gelidium corneum (G. corneum) by-product, and four commonly studied species, Saccharina latissima (S. latissima), Porphyra umbilicalis (P. umbilicalis), Gracilaria gracilis (G. gracilis), and Ulva rigida (U. rigida). A common analytical framework was applied to determine proximate composition, carbon-to-nitrogen ratio, amino acid and fatty acid profiles, nutritional indices, mineral and phenolic contents, heavy metals and in vitro cytotoxicity. The results showed that the red algae P. umbilicalis and G. gracilis exhibited the highest protein content among the evaluated macroalgae, with values of 30.78 and 22.69%, respectively, together with relatively low C/N ratio indicating nitrogen-rich compositions. Leucine, isoleucine, and phenylalanine were the predominant essential amino acids, whereas the G. corneum by-product displayed the highest total essential amino acid content (44.8 g/100 g protein) and essential amino acid index (141%). Most biomasses exceeded an essential amino acid index of 90%, supporting their nutritional relevance. Fatty acid analysis revealed unusually high monounsaturated fatty acids levels in R. okamurae and C. usneoides, with R. okamurae being the only brown alga containing both EPA and DHA. Moreover, in vitro cytotoxicity assays confirmed satisfactory biocompatibility and safety for most algal extracts. Overall, G. gracilis, P. umbilicalis and the G. corneum by-product emerged as the most promising candidates for protein-oriented valorization, providing a basis for the development of alternative protein ingredients and higher-value uses of macroalgal resources.