High-Value Brown Algae Extracts Using Deep Eutectic Solvents and Microwave-Assisted Extraction

Utilizing deep eutectic solvents (DESs) combined with microwave-assisted extraction (MAE) provides a sustainable method for extracting bioactive compounds from the macroalgae <i>Ascophyllum nodosum</i> and <i>Laminaria hyperborea</i>. Two DES formulations, choline chloride/lactic acid (ChCl/LA) and sodium acetate/lactic acid (AcNa/LA), were evaluated under varying extraction conditions. For <i>L. hyperborea</i>, ChCl/LA at 150 °C for 10 min yielded a total phenolic content (TPC) of 15.34 mg GAE/g DW, with antioxidant activities measured by DPPH (34.55 mg TE/g DW) and ABTS (27.06 mg TE/g DW). Extending the extraction to 20 min at 130 °C increased the TPC to 19.12 mg GAE/g DW<i>. A. nodosum</i> exhibited higher bioactivity, with the TPC reaching 47.51 mg GAE/g DW under the same conditions. High-performance liquid chromatography (HPLC) identified significant phenolics such as 3,4-dihydroxybenzoic acid (678.05 µg/g DW) and vanillin (6718.5 µg/g DW). Antimicrobial assays revealed strong inhibition (zones > 20 mm) against <i>Clostridium perfringens,</i> moderate activity against <i>Staphylococcus aureus,</i> and selective activity against <i>Escherichia coli</i>. FT-IR confirmed the presence of phenolics, polysaccharides, and lipids. Thermal and structural characterization revealed that <i>A. nodosum</i> residue showed an amorphous structure, while <i>L. hyperborea</i> retained crystallinity with decomposition profiles indicating potential bioenergy potential. SEM images revealed significant cell wall disruption correlating with extraction efficiency. These results demonstrate DES-MAE as an effective, green strategy for producing high-value algal extracts and valorizing residual biomass for biotechnological applications.