Complementary Whole-Biomass ATR-FTIR and Ethanol-Extract Optical Spectroscopy of Invasive Rugulopteryx okamurae: Descriptive Comparison of Collection States and Drying Treatments

Collection state and post-harvest drying may influence both the structural matrix of macroalgal biomass and the compounds recovered by solvent extraction. We combined whole-biomass ATR-FTIR with UV–visible absorption, fluorescence excitation and emission spectroscopy, and excitation–emission matrices (EEMs) of ethanolic extracts to characterise invasive Rugulopteryx okamurae collected as submerged, intertidal and naturally beach-exposed biomass and, separately, to compare freeze-drying with drying at 45 °C. Each collection state was represented by one source biomass portion with three separately processed subsamples, which were used to describe within-source variability. Collection-state comparisons were therefore descriptive and specific to the material examined. The submerged and intertidal source portions showed closely similar FTIR and optical profiles, whereas the beach-exposed source portion retained a prominent carbohydrate-associated FTIR region but showed a dominant maximum near 1020 cm−1, an additional contribution near 1081 cm−1, approximately 72–73% lower absorbance at 430 nm and approximately 78% lower red fluorescence. Within the separate drying series, biomass dried at 45 °C produced stronger extract absorption and fluorescence than freeze-dried biomass without materially shifting the principal spectral features. These complementary measurements support preliminary characterisation of macroalgal biomass, while broader biological inference requires independent spatial and temporal replication.