Magnetic fields cause oxidative stress in organisms by altering energy lev­els and spin orientation of electrons and concentration and lifetime of free radicals, which change the relative probabilities of recombination of other interactions with possible biological consequences [67]. Oxidative stress due to the radical pair mechanism becomes applicable around 1 mT which can be common in industrial or laboratory settings, while the geomag­netic field intensity stays below 0.07 mT. Studies with Chlorella vulgaris demonstrated that hydroxyl ions increase in magnetically treated medium suggesting alteration of free radical levels in the medium that might hy — peractivate antioxidant defense system of the organism. This situation also affects the membrane permeability and ion transport process and might be responsible for the acceleration of chlorophyll excitation by the light [30].