AbstractThe chain‐forming dinoflagellate Gymnodinium catenatum was exposed to hydrogen peroxide. Microscopical examination revealed striking dose–response alterations in chain formation above 245 μm: singlets replaced the dominance of long chain formations. These observations were valid for cells acclimated to halogen light. Under fluorescent light, cells were more resistant to modifications in chain length after H2O2 exposure. Growth along 9 h in the presence of extracellular H2O2 followed an hormesis response in both light regimes. Under halogen light conditions, alterations in chain formation and net growth were related to culture time, inocula concentration and geomagnetic activity (GMA) in the proceeding hours. Below a 16 nT threshold in GMA average growth was 0%, while above 16 nT it was circa +9%, independently if the local static magnetic field was altered by a permanent magnet or not. Mycosporine‐like amino acids that can have an antioxidant role and are easily oxidized decreased from 7.1 to 6.5 pg cell−1 (P < 0.05) under halogen light and exposure to 245 μm H2O2. GMA, as well as UV‐A, increased stress responsiveness that can momentarily protect cells from extracellular H2O2 addition. However, stress response is dependent on bio‐availability of several micronutrients and macronutrients, many found at limiting concentrations in oceanic waters.