Marine Bromophenols from Laminaria hyperborea’s Epiphytic Biomass: Chemical Profiling, Cytotoxicity, and Antioxidant Activity

The epiphytic community of Laminaria hyperborea, dominated by red algae, is typically discarded during industrial processing despite its potential as a source of high-value natural products. This study aims to valorize this underutilized biomass by characterizing its secondary metabolites and evaluating the biological activities of its major bromophenols. A combined chromatographic workflow enabled the isolation and structural elucidation of five bromophenols (1–5), including one previously undescribed compound (5). Among these, compound 4 exhibited the strongest cytotoxicity against the acute myeloid leukemia (AML) cell line MOLM-13 (EC50 = 6.23 μM) and induced pronounced apoptotic features. When tested on two normal cell lines (NRK and H9c2) and in zebrafish larvae, it showed moderate toxicity at higher concentrations, indicating a reasonable selectivity window. In contrast, compound 5 was more toxic to normal cells than to MOLM-13 in vitro, while showing no acute toxicity in zebrafish; however, interpretations are preliminary due to compound purity. Bromophenols 1–4 were also tested for antioxidant activity, with 4 being the most potent (ABTS EC50 = 22.1 μM), although this did not translate into protection against doxorubicin-induced cardiotoxicity. Additionally, non-targeted UHPLC-QTOF MS/MS analysis tentatively identified nine additional bromophenols and provided an estimation of their origin species within the epiphytic assemblage.