Arctic Diatoms as a Source of Antibiofilm Compounds: Identification of Methyl 3-Hydroxyoctadecanoate and Pheophorbide a

Marine diatoms are prolific producers of bioactive metabolites, but Arctic species remain underexplored as sources of antibacterial and antibiofilm agents. Here, seven species were grown in photobioreactors (PBRs) and systematically screened for antibacterial, antibiofilm, and cytotoxic activities. All strains inhibited Gram-positive bacteria, and four reduced <i>Staphylococcus epidermidis</i> biofilm formation. <i>Porosira glacialis</i> emerged as a lead species, combining potent antibiofilm activity with favourable traits for large-scale cultivation, and no detectable cytotoxicity. Bioactivity-guided fractionation of <i>P. glacialis</i> yielded two antibiofilm compounds: methyl 3-hydroxyoctadecanoate, the first time reported in diatoms and newly associated with antibiofilm bioactivity, and pheophorbide <i>a</i>, a chlorophyll degradation product. Both inhibited <i>S. epidermidis</i> biofilm formation without any observed cytotoxicity. Notably, <i>Cylindrotheca closterium</i> exhibited cultivation-dependent antibiofilm activity, underscoring the importance of growth conditions for metabolite production. These findings highlight the potential of Arctic diatoms as a sustainable source of antibiofilm agents and support further exploration of their metabolites for antimicrobial and industrial applications.