Spectroscopic Profiling of Phlorotannins of Selected Brown Algae Species from Pemba Island Using UV-Vis and FTIR Techniques

Phlorotannins are polyphenolic compounds widely distributed in brown algae, differing with regard to species and environmental conditions. This study investigated the total phlorotannin content (TPC) and spectroscopic characteristics of phlorotannins isolated from six brown algae species (VMBK-SC, VMBK-TO, VMBK-PBY, MSH-SC, MSH-TO, and WF-PBY). Extraction was conducted using three solvents of different polarity (methanol, ethanol, and petroleum ether). Significant differences in TPC were observed among the species and solvents (p < 0.05). Polar solvents (methanol and ethanol) revealed higher TPC compared to petroleum ether. In general, the TPC extracted by ethanol was 76.26 ± 0.25 mg PGE/g DW followed by methanol, 41.13 ± 0.64 mg PGE/ g DW and the rest was petroleum ether 31.61±0.21 mg PGE/ g DW. The results indicated that methanol was more effective for Sargassum sp. whereas ethanol showed better extraction efficiency for Turbinaria sp. and Padina sp. UV–Visible spectroscopy showed characteristic absorption peaks around 300–400 nm in all extracts, confirming the presence of phenolic compounds. FTIR analysis additionally validates these results by identifying functional groups typical of phlorotannins, including O–H, C–H, C=O, C=C, and C–O stretching vibrations. Conclusively, the present study confirms that brown seaweeds are rich sources of phlorotannins and that solvent polarity plays a significant role in extraction efficiency.