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

The epiphytic community of <i>Laminaria hyperborea</i>, 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 (<b>1</b>-<b>5</b>), including one previously undescribed compound (<b>5</b>). Among these, compound <b>4</b> 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 <b>5</b> 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 <b>1</b>-<b>4</b> were also tested for antioxidant activity, with <b>4</b> 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.