Despite the ban on polychlorinated biphenyls (PCBs) in developed countries since the 1930s, their persistence and use endured in developing countries, with reported toxic effects on various organisms. PCB congener 153 (PCB-153, 2, 2', 4, 4', 5, 5'-hexachlorobiphenyl) is notably prevalent in marine biota. However, its mode of action and impact on primary marine diatoms are not well documented. Phaeodactylum tricornutum cultures exposed to environmentally relevant PCB-153 levels (0, 1, 3, and 6 µg/L) showed proportional accumulation, impairing photochemical processes by directly affecting the structure and function of Photosystem II and hindering light harvesting and electron conversion. This impairment significantly inhibited growth (calculated IC