Seasonal and Residence-Time-Dependent Molecular Changes in Seaweed Revealed by Integrated Microwave Extraction and Multi-Analytical Profiling

Seasonal biochemical shifts in seaweed strongly influence their behavior under microwave-assisted extraction (MAE) and shape the molecular composition of both liquid extracts and solid residues. Ascophyllum nodosum (A. nodosum) harvested in winter, spring, summer, and fall, together with Saccharina latissima (S. latissima) as a structural reference, was analyzed using FT-ICR MS, solid-state NMR, and compositional profiling to determine how changes in proteins, carbohydrates, lipids, phenolics, and minerals control extraction outcomes. Winter and spring biomass contained the highest protein levels (14 and 10 wt %) and elevated acidic amino acids (glutamic acid 24–28%, aspartic acid 15–20%), while summer and fall biomass showed greater extractable carbohydrate content, reaching up to 30 wt % of glucose and fucose after MAE at 170 °C. Short residence times of 5–10 min efficiently released amorphous polysaccharides from summer and fall biomass, producing carbohydrate-rich extracts with CHO class intensities up to 68% and leaving crystalline cellulose and lignin-like domains in the solids. Longer treatment at 60 min promoted fucoidan breakdown and generated fucose (C6H12O5) and intermediates such as HMF. FT-ICR MS assigned 2500–4000 molecular formulas per extract, and multivariate analysis showed clear grouping of samples by season and extraction time, with the first and second components explaining 37.9% and 17.9% of the total molecular variation, respectively. These findings demonstrate that optimal MAE performance depends on the season-specific molecular architecture of the biomass, with late-summer and fall material supporting rapid carbohydrate extraction at short residence times and winter biomass benefiting from extended treatment to access fucoidan-derived structures, providing a season-aware framework for tuning extraction pathways in marine biorefineries.