The Agronomic Potential of the Invasive Brown Seaweed Rugulopteryx okamurae: Optimisation of Alginate, Mannitol, and Phlorotannin Extraction

<i>Rugulopteryx okamurae</i> is an invasive brown macroalga that has recently proliferated in the western Mediterranean Sea, causing significant environmental challenges. This alga, however, contains valuable bioactive compounds-alginate, mannitol, and phlorotannins-that can serve as biofertilizers to promote plant growth and aid in bioremediation of degraded or contaminated soils. This study focused on optimizing the extraction of these compounds from <i>R. okamurae</i>, transforming an ecological issue into a beneficial resource. Algae samples collected from the Spanish Mediterranean coast were processed through a randomized factorial response surface design. Extraction conditions varied by time, temperature, algae-to-solvent ratio, and ethanol-to-water ratio to determine optimal yields. The highest yields achieved were 29.4, 11.9, and 0.35 g/100 g for alginate, mannitol, and phlorotannin's under extraction conditions of 6, 6, and 3 h; 58.8, 60.0, and 60.0 °C; and an algae:solvent ratio of 1:50, 2:45, and 1.40 g/mL, respectively. Characterization of the extracted sodium alginate using ¹H-NMR, FTIR, and high-resolution electron microscopy confirmed its high purity and typical morphological features. This study highlights a sustainable approach to mitigating the invasive spread of <i>R. okamurae</i> while supporting soil health and sustainable agriculture. Harnessing this invasive species' biofertilizer potential provides a dual solution, aiding marine ecosystem conservation and developing eco-friendly agricultural practices.